key: cord-0985683-tloq4oq7
authors: Biesalski, Hans K.
title: Vitamin D deficiency and co-morbidities in COVID-19 patients – A fatal relationship?
date: 2020-06-07
journal: nan
DOI: 10.1016/j.nfs.2020.06.001
sha: 1ea12ecb197ac67c817b6c9c309a23de0fb61970
doc_id: 985683
cord_uid: tloq4oq7

nan

negatively 15 . The non-genomic effects involve the activation of a variety of signaling molecules that interact with Vitamin D responsive element (VDRE) in the promoter regions of vitamin D dependent genes 16 . Vitamins A and D are also important to reinforce mucous membranes in the respiratory tract 17 , 18 .

Vitamin D is of great importance for the immune system. 19 . Vitamin D interferes with the majority of the immune systems cells such as macrophages, B and T lymphocytes, neutrophils and dendritic cells, which express VDR (for details 20 and figure 3 ). Cathelicidin, a peptide formed by vitamin D stimulated expression, has shown antimicrobial activity against bacteria, fungi and enveloped viruses, such as corona viruses 21 , 22 . Furthermore Vitamin D inhibits the production of pro-inflammatory cytokines and increases the production of antiinflammatory cytokines 23 .

The active metabolite of vitamin D in macrophages and dendritic cells , derived from the precursor 25(OH)D, leads to the activation of VDR, which, after RXR heterodimerization, results in the expression of various proteins of the innate and adaptive immune system (Treg cells, cytokines, defensins, pattern recognition receptors etc.) 24 . Vitamin D exerts opposite effects on the adaptive (inhibition) and innate (promotion) immunsystem This correlates with an anti-inflammatory response and balances the immune response 25 The active metabolite of vitamin D, 1,25(OH)2D3 can be formed in T and B lymphocytes and inhibits T cell proliferation and activation 26 . This way, vitamin D may suppress T-cell mediated inflammation and stimulate Treg cells proliferation, by increasing IL-10 formation in DC cells, and thus enhance their suppressive effect 27,28 .

There are only few dietary sources of vitamin D (cod liver oil, fat fish) that could satisfy the recommended daily allowance (15-20ug/day for adults). To reach such amount besides availability of dietary sources, vitamin D skin synthesis, which contributes to 80% in healthy individuals up to the age of 65, is important.

With the exception of mushrooms there are no plant sources of vitamin D. In particular wild mushrooms, which are grown in light. Sun-dried but not fresh mushrooms can contain between 7-25ug/100g of vitamin D2 29 , which is an important source 30 with a good shelf life 31 and comparable bioavailability to vitamin D3 32 . .Vitamin D status can be significantly improved by fortified foods, as was shown in a meta-analysis 33 .

Insufficient levels of vitamin D are caused by two main physiological causes: Low UVB exposure, especially in northern regions during the winter season 34 and in case of strong pigmentation, as well as decreased vitamin synthesis in the skin with aging 35 . In addition a poor diet, low in fish and fortified food (if available) are the major reason for deficiency in old age and people living in poverty. Major risk groups, 36 besides pregnant women and children under 5, include elderly, over 65 years, those with little or no sun exposure (full body coverage, little contact with the outside world) as well as people with dark skin, especially in Europe and the USA.

The vitamin D deficiency is a worldwide problem, which is not only observed in the northern countries, but increasingly also in the south. While in Europe, for example, deficits (< 30nmol) are between 20 -60% in all age groups, in Asia the figure for children is 61% (Pakistan, India) and 86% (Iran) 37 , 38 .

Particularly critical is the number of migrants from Southern countries with insufficient vitamin D status (< 25nmol/L) 39 : e.g. Netherlands 51%, Germany 44% (in summer), UK 31% (end of summer) and 34% (autumn). In India, the number of adults with values < 25nmol/L ranges from 20% to 96% depending on the region.

The half-life of 25(OH)D 3 is about 15 days and that of 25(OH)D 2 is between 13 and 15 days, due to the weaker affinity to the vitamin D binding protein 40 .. Consequently, longer periods of time indoor, e.g. in care homes or longer time in quarantine, pose risk for developing vitamin D deficiency.

Older age and co-morbidities are linked to an insufficient vitamin D supply. Over 60 years of age, a reduction in the synthesis of vitamin D in the skin becomes apparent, which further increases getting older 41 . The precursor of vitamin D, 7-dehydrocholesterol in the skin declines about 50% from age 20 to 80 42 , and the elevation of cholecalciferol levels in serum following UVB radiation of the skin shows more than a 4-fold difference in individuals aged 62-80 yrs compared with controls (20-30 yrs) 43 RAS plays an important role in maintaining vascular resistance and extracellular fluid homoeostasis. Figure 1 summarizes the essential steps of this system.

Mainly in the juxtaglomerular apparatus of the kidney, but also in other tissues and cells, renin is formed, which cleaves the angiotensinogen secreted from the liver very selectively to the inactive form angiotensin I (Ang I). This decapeptide is then cleaved by a further protease the angiotensin-converting-enzyme (ACE) on the surface of the endothelial cells to the active angiotensin II (Ang II), which can bind to two different receptors AT1R or AT2R. Synthesis and secretion of renin in the kidney, as rate limiting enzyme of RAS, is stimulated by fluid volume, reduction of the perfusion pressure or salt concentration and by the sympathetic nervous system activity.

Renin synthesis and secretion is inhibited with increasing Ang II via an AT1R mediated effect and stimulated with decreasing Ang II 44 . The stimulating effect on renin synthesis and secretion due to either low levels of Ang II or Ang II converting inhibitors (ACEI) or Ang II receptor blockers (ARB) is mediated through ligands that activate cAMP/PKA (Protein Kinase A) pathways (e.g. catecholamines, prostaglandins and nitric oxide) 45 , 46 .

Ang II leads to the release of catecholamines and vasoconstriction. Via AT1R, Ang II increases aldosterone release and sodium reabsorption. Furthermore, binding to AT1R has proinflammatory and pro-oxidative effects and inhibits the action of insulin in endothelial and muscle cells. The latter can lead to a decrease in NO production in endothelial cells and thus will further increase vasoconstriction. 47 .

With the discovery of ACE2, a novel homologue of ACE, a transmembrane metallopeptidase with an extracellular ectodomain, the understanding of RAS manifold regulatory function was deepened (Review 48 ). ACE2, a monocarboxypeptidase has been shown to cleave Ang I to Ang 1-9, and Ang II to Ang 1-7. This degradation can weaken the effect of Ang II at AT1R and thus counteract the pathological changes. While Ang 1-9 exerts a cardioprotective effect via AT2R 49 , Ang 1-7 acts via the Mas Oncogene receptor. This counterbalances the effect of ANG II at AT1R and subsequently the "overstimulation" of the RAS and its pathological consequences 50 . ACE2 is expressed in many organs, especially kidney and lung, and in the cardiovascular system in cardiomyocytes, cardiac fibroblasts, vascular smooth muscle and endothelial cells. It can counteract the effects of RAS, such as inflammation, vasoconstriction, hypertrophy and fibrosis, by degrading Ang I and Ang II, thus making them less available for the ACE/AngII/AT1 axis. At the same time ACE2 can strengthen the ACE2/Ang 1-7/Mas axis which attenuates the proinflammatory RAS activation.

Infection with SARS-CoV-2 causes the virus spike protein to come into contact with ACE2 on the cell surface and thus to be transported into the cell. This endocytosis causes upregulation of a metallopeptidase (ADAM17), which releases ACE2 from the membrane, resulting in a loss of the counter regulatory activity to RAS 51 . As a result, proinflammatory cytokines are released extensively into the circulation. This leads to a series of vascular changes, especially in the case of preexisting lesions, which can promote further progression of cardiovascular pathologies.

SARS-CoV-2 not only reduces the ACE2 expression, but also leads to further limitation of the ACE2/Ang 1-7/Mas axis via ADAM17 activation, which in turn promotes the absorption of the virus. This results in an increase in Ang II, which further upregulates ADAM 17. Thus a vicious circle is established turning into a constantly self-generating and progressive process. This process may contribute not only to lung damage (Acute respiratory distress syndrome -ARDS), but also to heart injury and vessels damage, observed in COVID-19 patients. Thus, previous lesions of the cardiovascular system represent a risk factor, since coexisting pathologies can progress as a result of the virus infection 52 , 53 .

Several studies have shown increased plasma renin activity, higher Ang II concentrations and higher RAS activity as a consequence of low vitamin status 54 , 55 . The same applies to the decreasing Renin activity with increasing vitamin D levels 56 . There is an inverse relationship between circulating 25(OH)D and renin, which is explained by the fact that vitamin D is a negative regulator of renin expression and reduces renin expression by suppressing transcriptional activity in the renin gene promoter, thus acting as a negative RAS regulator to prevent overreaction In VDR knock out mice 57 , 58 . The 1,25(OH)2D induced repression of the renin gene expression is independent from Ang II feedback regulation.

Permanent increase of the renin levels with an increased Ang II formation has been described, suggesting that in vitamin D deficiency the expression and secretion of renin is increased at an early stage 59 , 60 . This results in increased fluid and salt intake and rise in blood pressure, that has been explained by an increase in renin and consecutive upregulation of the RAS in the brain 61 . Figure 2 gives a short description of the impact of vitamin D on RAS.

In a small (open-label, blinded endpoint) study with 101 participants who received 2,000 IU vitamin D3 or placebo over 6 weeks, a significant decrease in plasma renin activity and concentration was described 62 .

The EVITA study examined the effect of vitamin D supplementation (4,000 IU/day) over 36 months 63 . No relationship was found between blood levels of 1,25(OH)2D and various parameters of the RAS (renin, aldosterone) and vitamin D plasma levels increase. Rather, vitamin D supplementation led to an increase in renin in a subgroup that initially had an J o u r n a l P r e -p r o o f Journal Pre-proof initial deficiency of vitamin D. The 25(OH)D value in these subgroups increased from 20.4nmol/L to 83.7nmol/L after 36 months. Renin from 859 mIU/L to 1656mIU/L. It cannot be excluded that these were rather toxic effects of a dose in the upper level range. However, the fact that blood levels increase naturally reduced the renin concentration become clear when looking at the placebo group with initial hypovitaminosis D (21.3 nmol/L) with a strong increase after 36 months (45.6nmol/L). Renin decreases from the initial value of 507 to 430mIU/L after 36 months. According to this, a moderate suppressive effect of vitamin D is conceivable under physiological conditions and in particular in participants with a compensated vitamin D deficiency. The plasma level of renin and 1,25(OH)2D show a significant inverse correlation in hypertensive individuals 64 . In a study on 184 normotensive participants, higher circulating Ang II levels were associated with decreasing 25(OH)D blood levels. After infusion of Ang II there was a blunted renal blood flow, both effects were considered RAS activation in the setting of lower plasma 25(OH)D 65 .

Vitamin D supplementation leads to a reduction in blood pressure in patients with essential hypertension 66 , 67 , and to a reduction in blood pressure, plasma renin activity and angiotensin II levels in patients with hyperparathyroidism 68 , 69 . Low vitamin D status may contribute to increased activity of the RAS and subsequent higher blood pressure. An inverse relationship between the concentration of the active metabolite 1,25(OH)2D3 and blood pressure has been described in hypertensive as well as normotensive individuals 70 , 71 . In a study using the mendelian randomization approach in 35 trials (146,581 participants) with four SNPs (Single Nucleotidf Polymorphism), a causal relationship was shown between increasing 25(OH)D levels and decreased risk of hypertension in individuals with genetic variants leading to low Vitamin D plasma levels 72 Depending on the study, the number of COVID-19 patients affected with hypertension was between 20 and 30% and the proportion of diabetics between 15 and 22% 73 . Data from 5 studies in Wuhan (n:1458) reported 55.3% and 30.6% cases respectively of hypertension and of diabetes 74 . 49% of the 1591 patients in ICUs in Italy (Lombardy), 1287 of whom needed respirators, had hypertension and were older than the normotens ive ones. 75 , Hypertension, followed by diabetes (16.2%), was the most frequent concomitant morbidity in patients with severe course disease 76 , 77 . 78 .

Vitamin D has multiple functions in the cardiovascular system and thus represents an important protective factor of endothelial, vascular muscle, and cardiac muscle cells 79 . In a meta-analysis of 65,994 participants an inverse relationship between 25(OH)D vitamin D plasma levels (below 60nmol/L) and cardiovascular events was shown 80 . These findings have been confirmed by the Framingham and NHANES data 81 , 82 . As for the positive effects on respiratory diseases shown by vitamin D supplementation, also for cardiovascular disease positive effect was reported only if there was a deficit before supplementation.

In a large cohort of patients (n=3296) referred to coronary angiography, a significant increase in plasma renin and angiotensin II was observed with decreased 25(OH)D and J o u r n a l P r e -p r o o f Journal Pre-proof 1,25(OH)2D levels, but not with circulating aldosterone levels 83 . Vitamin D plasma levels are an independent risk factor for CVD mortality. 92% of 1801 patients with metabolic syndrome, had a low vitamin D status (22.2% were severely deficient (25(OH)D < 25nmol). CVD mortality and total mortality were reduced respectively by 69% and 75% in those with highest 25(OH)D levels (> 75nmol/L) 84 .

CVD is considered an independent risk factor for fatal outcome in COVID-19 patients. The proportion of survivors with CVD was 10.8%, among non-survivors 20% 85 . Disturbed coagulation, endothelial dysfunction and proinflammatory stimuli described as a result of a viral infection are considered to be among the major causes 86 .

Obesity (BMI > 30kg/m2) is often associated with low 25(OH)D plasma level 87 , 88 . Using a bidirectional genetic approach, 26 studies (42,024 participants -Caucasians from Northern Europe and America), including 12 SNPs, showed that higher BMI (Body Mass Index) leads to lower 25(OH)D plasma levels. The repeatedly discussed hypothesis that low 25(OH)D level leads to increased BMI could not be verified 89 . Obesity is therefore another risk factor for an insufficient vitamin D status independent from age 90 .

Low 25(OH)D plasma values are also found in diabetes II 91 , 92 . This is often associated with an increased risk of metabolic syndrome, hypertension and cardiovascular diseases 93 , 94 . One of the main causes could be insulin resistance, often found in connection with low vitamin D levels 95 . This is well documented by the evaluation of observational and intervention studies using metabolic indicators. 10 out of 14 intervention studies showed a positive effect of Vitamin D on metabolic indicators 96 . Vitamin D deficiency is therefore also considered to be a potential link between obesity and diabetes type II 97 .

Via a short-loop feedback Ang II inhibits the further release of renin via AT1R. If the renin secretion is not sufficiently inhibited, an overreaction of the RAS can lead to a further increase in blood pressure, increased sodium reabsorption, increased aldosterone secretion and thus increased insulin resistance 98 . This overreaction is considered to be a major cause of the development of hypertension, diabetes and cardiovascular disease, especially in people with high BMI, since adipose tissue contributes to an overreaction of the RAS 99 . Adiponectin synthesis in adipocytes counteracts most of these effects, however circulating levels are inversely related to BMI 100 , 101 . Vitamin D can regulate the formation and release of adiponectin 102 , 103 . Obese people often have low adiponectin and vitamin D levels and an inverse relationship between fat mass and vitamin D levels has been described 104 . Therefore, vitamin D deficiency might explain RAS overreaction and following consequences 105 .

In a small study on 124 IUC patients with SARS-CoV-2 it was found that obesity (BMI > 35kg/m 2 ) occurred in 47.6% of the cases and severe obesity (BMI > 35kg/m 2 ) in 28.2% 106 . In the latter case, 85.7% had to be mechanically ventilated invasively, 60 patients (50%) had hypertension, 48 of these (80%) had to be ventilated invasively. A study from Shenzhen, China also confirmed that obesity is a risk factor for severe course of disease. In a cohort of 383 patients with COVID-19, overweight patients (BMI 24-27.9) had 86% higher risk of J o u r n a l P r e -p r o o f Journal Pre-proof developing pneumonia and obese patients (BMI > 28) had 142% higher risk of developing pneumonia compared to normal weight patients. 107 .

The main cause of death in COVID-19 patients is ARDS. Patients (without COVID-19) (mean age 62 Y) with ARDS (n:52) and those at high risk of ARDS (n:57) (esophagectomy) had low (27.6 nmol/L) to very low (13.7 nmol/L) 25(OH)D blood levels as a sign for severe vitamin D deficiency 108 .

ACE2 exerts a counter-regulation of the harmful effect of ACE. Ultimately, it would then be the balance between ACE and ACE2 that explains the reaction of the RAS. The ACE2 effect on the RAS is shown in experimental studies in which ACE2 knock out mice developed severe lung disease with increased vascular permeability and pulmonary edema 109 . Over-expression or the use of recombinant ACE2 improves blood flow and oxygenation and inhibits the development of ARDS after LPS-induced lung damage 110 , 111 .

The development of ARDS shows typical changes in membrane permeability of the alveolar capillary, progressive edema, severe arterial hypoxemia and pulmonary hypertension 112 . The same changes can be achieved in animal experiments by injection of lipopolysaccharides (LPS) 113 . Vitamin D significantly attenuates the lung damage caused by LPS. LPS exposure leads to a significant increase in the pulmonary expression of renin and ANGII. This promotes the pro-inflammatory effects of the conversion of AngII via AT1R and suppresses ACE2 expression. The administration of vitamin D was able to reduce the increased renin and AngII expression and thus significantly lower the lung damage. The authors conclude that this may have been due to the reduction of the renin and ACE/AngII/AT1R cascade and the promotion of ACE2/Ang1-7 activity by vitamin D through its influence on renin synthesis.

Increased ACE and ANGII expression and reduced ACE2/Ang1-7 expression in lung tissue favors lung damage induced by ischemia reperfusion in mice 114 . The ACE/Ang1-7 expression and the amount of circulating Ang 1-7 was increased at the onset of ischemia and then decreased rapidly in contrast to the tissue concentration, while AngII increased. This suggests a dysregulation of local and systemic RAS. The application of recombinant ACE2 was able to correct the dysregulation and attenuate the lung damage, while ACE2 knock out increased the imbalance and was associated with more severe damage. Inhibition of the ACE/AngII/AT1R pathway or activation of the ACE2/Ang1-7 pathway have therefore been proposed as therapeutic options.

In rats with LPS-induced acute lung injury (ALI), the administration of vitamin D (calcitriol) was associated with a significant reduction in clinical symptoms of ALI. C alcitriol treatment led to a significant increase in the expression of VDR mRNA and ACE2 mRNA. VDR expression may have resulted in a reduction of angiotensin II, ACE2 expression in increased antiinflammatory effects 115 .

J o u r n a l P r e -p r o o f VDR is not only a negative regulator of renin, but also of NFkB 116 , leading both to an increase in Ang II formation 117 , which in turn promotes pro-inflammatory cascades. Furthermore SARS-CoV-2 infects T-lymphocytes 118 and the Covid-19 disease severity seems to be related to lymphopenia 119 , which occurs in 83,2% of COVID-19 patients at hospital admission 120 . Indeed, in a recent meta-analysis on 53.000 COVID-19 patients decreased lymphocyte count and increased CRP were highly associated with severity 121 .

Regulatory T cells (Treg) play an important role in the development of ARDS 122 . They can attenuate the pro-inflammatory effects of the activated immune system. Vitamin D increases the expression of Treg cells and supplementation of healthy volunteers results in a significant increase in Tregs 123 . Vitamin D causes a reduction in pro-inflammatory cytokines by inhibiting B-and T-cell proliferation 124 , 125 . Inflammatory processes also play an important role in the development of hypertension and CVD 126 , 127 . Here, an interesting but so far not proven connection between vitamin D and RAS is found. T-cells have a RAS system, which contributes to the generation of reactive oxygen species (ROS) and the development of high blood pressure through the formation of Ang II 128 . To what extent vitamin D in T cells is also a negative regulator of renin is not known, but could be one of the reasons for the antiinflammatory effect 129 .

In patients with a severe disease course (ARDS) a cytokine storm is assumed to be the underlying cause 130 . SARS CoV-2 can lead to a downregulation of ACE2 in the lungs and to a shedding of the ectodomain of ACE2. This soluble sACE2 shows enzymatic activity, but the biological role is unclear. The soluble form is believed to exert systemic influence on angiotensin II 131 ; since SARS-CoV-2 induces shedding, it is assumed that sACE2 is directly related to the virus-induced inflammatory response 132 .

Downregulation of ACE2 expression by SARS-CoV infection is associated with acute lung damage (edema, increased vascular permeability, reduced lung function) 133 and with RAS dysregulation leading to increased inflammation and vascular permeability. Inflammatory cytokines such as TACE (TNF-a-converting enzyme) induce increase shedding 134 , which in turn can be also caused by spike protein of the virus, promoting virus uptake by ACE2 135 .

Comparative studies on mortality rates in different countries and analysis of the relationship between vitamin D and CRP (as a marker of cytokine storm) plasma levels, concluded that risk factors for severity of the clinical course, predicted by high CRP and low vitamin D ( < 25nmol) levels, were reduced by by 15.6% following vitamin D status normalization (>75nmol) 136 . It is interesting to note that calmodulin kinase IV (CaMK IV) stimulates vitamin D receptor (VDR) transcription and interaction with co-activator SRC (steroid receptor coactivator) 137 KD is an acute vasculitis which can lead to aneurysms of the coronary arteries and is considered the leading cause of acquired heart disease in children 142 . A number of cases have been observed in recent weeks suggesting a relationship between Kawasaki syndrome and COVID-19 143 .

One reason probably relies upon ACE gene polymorphisms 144 . In these polymorphisms there is a strong increase in ACE without affecting AngII plasma levels 145 . There is a direct relationship between ACE polymorphism (with high ACE plasma levels) and the occurrence of KD, according to a recent meta-analysis 146 .

Irrespective of this, the disease occurs seasonally during the winter months in extratropical northern atmosphere and is often associated to respiratory tract infections 147 . A KD associated Antigen was found in proximal bronchial epithelium in 10 out of 13 patients with acute KD and in a subset of macrophages of inflamed tissues 148 . That strengthens the hypothesis that an infectious agent entering the respiratory tract, might be the cause of KD. Indeed, it was reported that children with KD were affected by respiratory diseases with HCoV: New Haven coronavirus 149 . The authors concluded that there was a significant association between KD and HCoV-NH infection.

Just like current evidence suggest that vitamin D-deficiency is associated with increased risk of CVD, including hypertension, heart failure, and ischemic heart disease, patients with KD also show very low vitamin D levels. Children with KD (79) had significantly lower 25(OH)D levels (9.17 vs 23.3 ng/ml) compared to healthy children of the same age 150 .

Intravenous immunoglobulin (IVIG) has become the standard therapy for KD 151 , with a good therapeutic response from young patients, of which only 10-20% need additional antiinflammatory medication 152 . In a study on 91 KD children, 39 of them with very low plasma vitamin D levels (< 20ng/ml), showed immunoglobulin resistance compared to the rest of the children (n= 52) children with higher levels (>20ng/ml) 153 . Children with immunoglobulin resistance also have a higher incidence of coronary artery complications 154 , 155 .

The relationship between ACE polymorphism and peripheral vascular disease is observed in Asians but not in Caucasians 156 , 157 . Furthermore the prevalence of KD in Japan (240/100,000) is 10 times higher than in North America (20/100,000) 158 , 159 . During February and April 2020, 10 cases of COVID-19 and KD were reported in Bergamo, Italy, corresponding to 30 times higher rate than the last 5 years incidence 160 162 . It is conceivable that Vitamin D deficiency which activates the RAS, promotes the development and course of KD.

The aim of a therapy with vitamin D should be a normalization of the vitamin D status, preferably > 75nmol/L. Basically, it can be assumed that a vitamin in physiological doses can do little more than remedy the symptoms or secondary manifestations of a deficiency. Vitamin D is a prohormone. Therefore, the question of correcting the status should be treated in the same way as for other hormones (e.g. thyroid hormone). Before starting therapy, the plasma level should be determined. This allows a dosage and therapy to be initiated that corresponds to the respective status. The analysis should be carried out especially in risk groups (table 2) in order to be able to react adequately, especially in acute cases. The general recommendation to supplement with a recommended daily dose (800IU) may apply to people who do not belong to a risk group, are healthy and free of symptoms. The vitamin D status is the basis for treatment with vitamin D. There are indeed, risk groups were a poor status can be expected. As it is known that the amount of 25(OH)D circulating in the blood and less the active metabolite 1,25(OH)2D is a better indicator for a deficit, threshold values have been set here. A vitamin D status below 20ng/ml or < 50nmol/L should be treated to achieve a minimum level of 30ng/ml (75nmol/L). Values around 75nmol/L are considered optimal, with respect to the skeletal activities 167 . Particularly in countries where vitamin D fortified foods are not available, the importance of an adequate supply should be emphasized. A sufficient vitamin D status can be achieved in the healthy populations following the recommendations and the thresholds of the plasma levels. In case of comorbidities related to the clinical development of COVID-19 there might be a higher need and therefore it is discussed to choose other recommendations for the adequate care of persons with chronic diseases 168 , 169 .

A recent meta analysis related to vitamin D and respiratory tract infections showed that a daily or weekly Vitamin D dose between 20ug and 50ug resulted in a significant reduction of infections 170 . An isolated or added bolus with high doses (2.5mg once or monthly) did not reduce risk. One study supplemented adults with high risk for ARDS with a 100ug/daily for one year 171 . The overall infection score was significantly reduced in the treated group. Those with an initial vitamin D deficit showed the greatest benefit of the supplementation. With respect to COVID-19 a recommendation for primary prevention of vitamin D deficiency seems meaningful. Whether this will be prevention against COVID related diseases remains speculative. If a patient belonging to a risk group is delivered to the hospital, vitamin D status should be immediately assessed and in case of insufficiency (< 50nmol/L) or deficiency (< 25nmol/L) higher doses might be needed as recommended by the NHS 172 . 

VDRA are discussed to counteract the effect of imbalanced immune response and have suppressant effects on the RAS. Since VDRA have been observed to contribute to a significant reduction of inflammatory processes, they are increasingly used in immunosuppressive therapy to control TH1-related overreactions via interaction of VDRA with the chemokine CXCL10, a T cell chemoattractant chemokine 174 . The induction of CXCL10 is an important step against bacterial and virus infections. However, sustained CXCL10 induction leads to amplified neuroinflammation in Coronavirus (JHMV) induced neurologic infection 175 . CXCL10 is also considered a critical factor in ARDS. H5N1 influenza infection in mice resulted in increased CXCL10 secretion with a consequent inflamed neutrophils massive chemotaxis and a subsequent pulmonary inflammation 176 . Following SARS-CoV-2 infection, CXCL10 and other chemo-and cytokines are upregulated 177 . Anti CXCL10 antibodies have shown ARDS improvement following LPS induced lung injury with high CXCL10 levels 178 Additionally evidence from animal models (diabetic nephropathy) has shown that VDRA block TGFß system in the glomerulus and thus abolish interstitial fibrosis 179 . It is assumed that VDRA modulates increased RAS activity. Indeed, a clinical study on 281 patients (type II diabetes with albuminuria) revealed that VDR activator paricalcitol (19-nor-1,15dihydroxyvitamin D 2 ) led to a significant albuminuria reduction as well as a decrease in blood pressure despite increased salt intake, as a sign of decreased RAS activity 180 ; effect that could not be achieved with losartan (ANG II receptor antagonist) 181 .

Morphine medication is an essential part of treatment for COVID patients with severe ARDS. it is used early for dyspnea or pain and for shivers 182 . Morphine, at doses similar to those used in humans, can lead to downregulation of VDR in human T cells and activation of RAS with renin upregulation and a threefold increase in Ang II production, resulting in increased reactive oxygen species (ROS) responsible for DNA damage and T cells apoptosis . VDR agonist (EB1089) inhibits VDR downregulation, leading to RAS decreased activity, inhibition of morphine induced ANG II production, reduced ROS formation and lower DNA damage, thus inhibiting T-cell apoptosis 183 . In addition, if Jurkat cells were pretreated with EB 1089 and Losartan, an Angiotensin II receptor antagonist (ARB) before incubation with morphine. The combination of the Vitamin D Receptor agonist and Losartan attenuated the morphine-induced ROS formation. Indeed, as an example ARB increase ACE2 expression 184 and Ang 1-7/Mas axis activation reduced ROS formation 185 .

Spermidine is a metabolite of polyamines which are delivered through the diet and partially metabolized by colon bacteria from undigested proteins. Polyamines can influence macrophages development into pro-inflammatory or anti-inflammatory type by altering cellular metabolism and triggering mito-and autophagy 186 . The capacity of spermidine to ensure proteostasis through the stimulation of the cytoprotective autophagy is acknowledged as one of its main features.

Recently, the effect of spermidine on autophagy in SARS-CoV-2 infected cells which results in inhibition of autophagy has been described 187 . Since spermidine promotes autophagy, spermidine and other agents may be a therapeutic approach to SARS-CoV-2 infection.

With regard to the specific risk of elderly to develop severe course of SARS-CoV-2 infection, it is interesting to note that spermidine concentrations in organs and cells decline with age and decrease autophagy 188 . Consumption of LKM512 yogurt increases spermidine synthesis in the gut in elderly 189 . Whether that has any impact on supply of spermidine to enterocytes or other tissues remains to be elucidated. Spermin and spermidine but not putrescine another polyamine metabolite can activate VDR in vitro within their physiological intracellular concentrations 190 . Vitamin D and VDR play an important role in autophagy. Vitamin D can induce autophagy similar to spermidine by inhibiting mTORC1 complex activation 191 and by increasing Beclin-1 expression, similar to spermidine 192 .

Limitations: A major limitation of al studies dealing with low levels of vitamin D and disease is the fact that there are only few studies, which show a causal relationship. Most studies show associations.

Conclusion: An inadequate supply of vitamin D has a variety of skeletal and non-skeletal effects. There is ample evidence that various non-communicable diseases (hypertension, diabetes, CVD, metabolic syndrome) are associated with low vitamin D plasma levels. These comorbidities, together with the often concomitant vitamin D deficiency, increase the risk of severe COVID-19 events. Much more attention should be paid to the importance of vitamin D status for the development and course of the disease. Particularly in the methods used to control the pandemic (lockdown), the skin's natural vitamin D synthesis is reduced when people have few opportunities to be exposed to the sun. The short half-lives of the vitamin therefore make an increasing vitamin D deficiency more likely. Specific dietary advice, moderate supplementation or fortified foods can help prevent this deficiency. In the event of hospitalisation, the status should be urgently reviewed and, if possible, improved.

In the meantime, 8 studies have started to test the effect of supplementing vitamin D in different dosages (up to 200,000 IU) on the course of the COVID-19 disease. The aim is to clarify whether supplementation with vitamin D in different dosages has an influence on the course of the disease or, in particular, on the immune response, or whether it can prevent the development of ARDS or thromboses 193 .

In the classical RAS pathway Renin, expressed from the renin gene induces cleavage of Angiotensinogen to Angiotensin I which is converted to Angiotensin II via Angiotensin converting enzyme (ACE). Ang II activates the Angiotensin 1 receptor which results in an increase of blood pressure and further effects on the vascular system. In addition, Ang II suppresses renin synthesis via AT1R. To keep the system in balance a counter regulatory pathway exists. This pathway is activated through cleavage of Ang I to Ang1-9 via ACE2 or AT2R activation or Ang II to Ang1-7 which counter regulates via Mas receptor. This helps the system to stay within a homoeostatic balance, as long as the RAS activity is controlled. Figure 2 If the system is dysbalanced this may result in a rising formation of Ang II and a higher renin synthesis which at least increases inflammatory responses. This is important in cases of a poor vitamin D status because vitamin D (1,25(OH) 2 D) can counteract the disbalance via negative expression of the renin gen which results in lower renin synthesis independent from Ang II. An increase of aldosterone will block the activities of the ACE2 and as a consequence attenuate the counter regulatory balance. If the counter regulatory circle is disrupted via ACE2 dysfunction due to SARS-CoV2 infection an uncontrolled classical pathway will run out of control and increase proinflammatory reactions and blood pressure and contribute to a couple of problems (e.g. cardiovascular, ARDS, Kawasaki disease). Ang II activates NFB through AT1 receptors 194 . This and further interactions of the RAS with inflammatory stimuli results in an increasing and less controlled inflammatory reaction. Beside its effect on renin expression vitamin D can effectively inhibit NFB activation 195 . This is especially efficient when the VDR is upregulated, which also plays an important role in other processes in the immune system through vitamin D activity.

Ang II leads to a series of pro-inflammatory stimuli in the immune system via the activation of AT1R. These include an increase in the expression of MCP-1 as well as the chemokine receptor CCR2, which lead to a massive infiltration of the endothelium with macrophages. The same applies to the activation, migration and maturation of dendritic cells (DC) and the antigen (Ag) presentation. The negative effect on T lymphocytes as well as on T regulatory cells further promotes a pro-inflammatory state. A number of other proinflammatory processes are triggered by AT1R and favor the development of inflammation, hypertension and diabetes. Vitamin D is considered to counteract this reaction by contributing to a normalization of immune function through a variety of processes. However, it should not be overlooked that most processes in the immune system initiated by vitamin D occur together with vitamin A 196 .

J o u r n a l P r e -p r o o f

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Worldwide Status of Vitamin D

The global epidemiology of vitamin D status

Evidence that loss of function filaggrin mutations evolved in northern Europeans to favor intracutaneous vitamin D3 production

Vitamin D and inflammation: potential impli cations for severity of Covid-19

Temperature, humidity, and latitude analysis to predict potential spread and seasonality for COVID-19

Hopsitalization and mortality among black patients and white patients with COVID-19

Avoidance of vitamin D deficienc to slow the COVID-19 pandemic

Editorial: low population mortality from COVID -19 in countries south of latitude 35 degrees North supports vitamin D as a factor determined severity

Skeletal and extraskeletal actions of vitamin D: current evidence and outstanding questions

Fundamentals of vitamin D hormone regulated gene expression

The non-genomic actions of vitamin D

Vitamin D, vitamin D receptor and tissue barriers . issue Barriers

New aspects in vitamin a metabolism: the role of retinyl esters as systemic and local sources for retinol in mucous epithelia

Vitamin D and i mmune function

Vitamin D3: a helpful immune-modulator

Vitamin D-directed rheostatic regulation of monocyte antibacterial responses

Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response

A eview of micronutrients and the immune system-working in harmonyto reduce the risk of infection

Impact of vitamin D on immune function: lessons learned from genome-wide analysis

Vitamin D: modulator oft he immune system

Vitamin D2, ergosterol, and vitamin B2 content in commercially dried mushrooms marketed in China and increased vitamin D2 content following UV-C irradiation

Vitamin D2 enrichment in mushrooms by natural or artificial UV-light during drying

Temperature stability of vitamin D2 and color changes during drying of UVB-treated mushrooms

Bioavailability of vitamin D2 from UV-B-irradiated button mushrooms in healthy adults deficient in serum 25-hydroxyvitamin D: A randomized controlled trial

An updated systematic review and meta-analysis of the efficacy of vitamin D food fortification

The relationship between ultraviolet radiation exposure and vitamin D status

Aging decreases the capacity of human skin to produce vitamin D3

Vitamin D: implementation of existing guidance to prevent deficiency

Current vitamin D status in European and middle east countries and strategies to prevent vitamin D deficiency: a position statement of the European calcified tissue society

Global prevalence and disease burden of vitamin D deficiency: a roadmap for action in low-and middle-income countries

Prevalence of vitamin D deficiency among Turkish, Moroccan, Indian and sub-Sahara African populations in Europe and their countries of origin: an overview

25(OH)D2 half life is shorter than 25(OH)D3 half -life and is influenced by DBP concentrations and genotype

Barriers to optimizing vitamin D3 intake for elderly

Aging decreases the capacity of human skin to produce vitamin D3

Age, vitamin D, and solar ultraviolet

Morphology, physiology, and molecular biology of renin secretion

Stimulation of renin secretion by angiotensin II blockade is Gsa-depoendent

Morphology, physiology, and molecular biology of renin secretion

Insulin resistance and hypertension

Role of the ACE2/Angiotensin 1-7 axis of the renin-angiotensin system in heart failure

Angiotensin 1-9 regulates cardiac hypertrophy in vitro and in vivo

Counter-regulatory renin-angiotensin system in cardiovascular disease

Angiotensin converting enzyme 2: A double-edged sword

SARS-coronavirus modulation of mayocardial ACE2 expression and inflammation of patients with SARS

25-hydroxyvitamin D is associated with plasma renin activity and the pressor response to dietary sodium intake in Caucasians

Interaction of 1,25-dihydroxyvitamin D and plasma renin activity in high renin essential hypertension

Vitamin D: a negative endocrine regulator of the renin-angiotensin system and blood pressure

1,25-Dihydroxyvitamin D3 suppresses renin gene transcription by blocking the activity of the cyclic AMP response element in the renin gene promoter

1,25-Dihydroxyvitamin D3 is a negative endocrine regulator of the reninangiotensin system

1,25-hydroxyvitamin D3 suppresses renin gene transcription by blocking the activity of the cyclic AMP response element in the renin gene promoter

Loss of vitamin D receptor producespolyuria by increasing thirst

Short term vitamin D3 supplementation lowers plasma renin activity in patients with stable chronic heart failure: an open-label, blinded-endpoint, randomized prospective trial (VitD-CHF trial)

Effects of vita min D supplementation on renine and aldosterone concentrations in patients with advanced heart fa ilure: The EVITA trial

Cal cium-regulating hormones in essential hypertension: relation to plasma renin activity and sodium metabolis

Plasma 25-hydroxyvitamin D and regulation of the Renin-Angiotensin system in humans

Reduction of blood pressure during long-term treatment with active vitamin D (alphacalcidol) is dependent on plasma renin activity and calcium status. A double blind, placebocontrolled study

Effects of short term vitamin D(3) and calcium supplementation on blood pressure and parathyroid hormone levels in elderly women

Effectiveness of 1,25-dihydroxyvitamin D supplementation on blood pressure reduction in pseudohyperparathyreoidism patient with high renin activity

Intravenous calcitriol regresses myocardial hypertrophy in in patients with secondary hyperparathyreoidism

Association of calcitriol blood pressure in normotensive men

Vitamin D is related to blood pressure and other cardiovascular risk factors in middle-aged men

Association of vitamin D status with arterial blood pressure and hypertension risk: a mendelian randomisation study

Are patients with hypertension and diabetes mellitus at increased risk for COVID-19 infection?

A comprehensive literature review on the clinical presentation, and management of the pandemic coronavirus disease 2019 (COVID-19)

Baseline chracteristics and outcome of 1591 patients infected with SARS-CoV-2 admitted to ICUs of the Lombardy region

Clinical characteristics of patients who died of coronavirus disease 2019 in China

Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirusinfected pneumonia in Wuhan, China

Clinical chracteristics of coronavirus dis ease in China

Prevalence of hypovitaminosis D in cardiovascular diseases (from the National Health and Nutrition Examination Survey

Vitamin D deficiency and risk of cardiovascular disease

Independent association between 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D and the renin-angiotensin system: the Ludwigshafen Risk and Cardiovascular Health (LURIC) Study

Vitamin D levels predict all -cause and cardiovascular disease mortality in subjects with the metabolic syndrome

Cardiovascular disease, drug therapy, and mortality in Covid-19

Seasonal influenza infectios and cardiovascular disease mortality

Vitamin D deficiency and type 2 diabetes

Vitamin D in obesity

Causal relationship between obesity and vitamin D-status: Bidirectional mendelian randomization analysis of multiple cohoirts

Obesity should shift severe COVID-19 disease to younger age

Predicted 25 -hydroxyvitamin D score and incident type 2 diabetes in the Framingham Offspring Study

Vitamin D and diabetes

Concentrations of serum vitamin D and the metabolic syndrome among U.S. adults. Diabetes Care

Adiposity, cardiometabolic risk, and vitamin D status: the Framingham Heart Study

Vitamin D deficiency causes insulin resistance by provoking oxidative stress in hepatocytes

Analysis of association between vitamin D deficiency and Insulin resitence

Vitamin D deficiency, obesity and diabetes

Aldosterone suppresses insulin signaling via the downregulation of insulin receptor substrate-1 in vascular smooth muscle cells

Renin-Angiotensin-Aldosterone System in diabetes and hypertension

Pediatric obesity and vitamin D deficiency: a proteomic approach identifies multimeric adiponectin as a key link between these conditions

Globular adiponectin protected ob/ob mice from diabetes and ApoEdeficient mice from atherosclerosis

Pediatric obesity and vitamin D deficiency: a proteomic approach identifies multimeric adiponectin as a key link between these conditions

Physiological functions of vita min D in adipose tissue

SARS-CoV-2) requiring invasive mechanical ventilation

Obesity and Covid-19 severity in a designated hospital in Shenzhen

Vitamin D deficiency contributes directly to the acute respiratory distress syndrome (ARDS)

Angiotensin converting encyme 2 protects from severe acute lung failure

Recombinant angiotensin-converting encyme2 improves pulmonary blood flow and oxygenation in lipopolysaccharide-induced lung injury in piglets

Angiotensin-converting encyme 2prevents lipopolysaccharide induced acute lung injury via suppressing the ERK1/2 and NF-kB pathways

The acute respiratory distress syndrome

Vitamin D alleviates lipopolysaccharide induced acute lung injury via regulation of the renin-angiotensin system

Dysregulated renin-angiotensin system contributes to acute lung injury caused by hind-limb ischemia-reperfusion in mice

Effects of vitamin D on ACE2 and vitamin D receptor expression in rats with LPS induced acute lung injury

Vitamin D: a negative endocrine regulator of the renin -angiotensin system and blood pressure

Human T and natural killer cell s possess a functional reninangiotensin system. Further mechanisms of angiotensin II -induced inflammation

SARS-CoV-2 infects T lymphocytes through its spike protein-mediated membrane fusion

Lymphopenia predicts disease severity of COVID-19: a descriptive and predictive study

Clinical Characteristics of Coronavirus Disease 2019 in China

Incidence, clinical characteristics and prognostic factors of patients with COVID-19: a systematic review and meta analysis

Regulatory T cells and acute lung injury: Cytokines, uncontrolled inflammation and therapeutic implications

Vitamin D supplementation and regulatory T cells in apparently healthy subjects: Vitamin D treatment for autoimmune diseases?

Vitamin D and the immune system

Vitamin D and 1,25(OH)2 D regulation of T-cells

Inflammatory processes in cardiovascular di sease a route to targeted therapies

Oxidative stress, inflammation and vascular aging in hypertension

Regulation of T-cell function by endogenously produced angiotensin II

Osthole: A review on its bioactivities, pharmacological properties, and potential as alternative medicine. Evidence-based

Clinical features of patients infected with 2019 novel coronavirus in Wuhan

Maintenance of normal blood pressure and renal functions are independent effects of angiotensin-converting enzyme

Understanding SARS-CoV-2 mediated inflammatory responses: From mechanisms to potential therapeutic tools

Angiotensin-converting enzyme 2 protects from severe acute J o u r n a l P r e -p r o o f lung failure

Tumor necrosis factor-alpha convertase (ADAM17) mediates regulated ectodomain shedding of the severe-acute respiratory syndrome-coronavirus (SARS-CoV) receptor, angiotensin-converting enzyme-2 (ACE2)

Modulation of TNF-a-converting encyme by the spike protein of the SARS-CoV and ACE2 induces TBF-a production and facilitates viral entry

The possible role of vitamin D in suppressing cytokine storm and associated mortality in COVID-19 patients

Cal modulin dependent kinase IV stimulates vitamin D receptormediated transcription

The identifiction of a calmodulin-binding domaine within the cytoplasmatic tail of angiotensin-converting enzyme-2

Calmodulin interacts with angiotensin -converting enzyme-2 (ACE2) and inhibits shedding of its ectodomain

Membrane-mediated actions of 1,25-Dihydroxy vitamin D3: A review of the roles of phospholipase A2 activating protein and Ca2+/calmodulin-dependat protrein kinae II

Outbreak of Kawasaki disease in children during COVID-19 pandemic: a prospective observational study

Kawasaki -like disease: emerging complications during the COVID-19 pandemic

Insertioin/deletion polymorphism of angiotensin converting enzyme gene in Kawasaki disease

Associations of ACE I/D polymorphism with the levels of ACE, kallikrein, angiotensin II and interleukin-6 in STEMI patients

Angiotensin-converting enzyme insertion/deletion polymorphism and susceptibility to Kawasaki disease: a met analysis

Kawasaki Disease Global Climate Consortium. Seasonality of Kawasaki disease: a global perspective

Detection of antigen in bronchial epithelium and macrophages in acute Kawasaki disease by use of synthetic antibody

Association between a novel human coronavirus and Kawasaki disease

Severe vitamin D deficiency in patients with Kawasaki disease: a potential role in the risk to devlop heart vascular abnormalities

Shulman ST Intravenous immunoglobulin for the treatment of Kawasaki disease

Diagnosis, treatment , and long term management of Kawasaki disease

Relationship between vitamin D levels and intravenous immunoglobulin resistance in Kawasaki disease

Medical therapies

Initial intravenous gammaglobulin treatment failure in Kawasaki disease

Epidemiology of Kawsaki disease i n Asia, Europe, and the United States

Ethnic differences in the association between angiotensin converting enzyme gene insertion/deletion polymorphism and peripheral vascular disease: a meta analysis

Racial/ethnic differences in the incidence of Kawasaki syndrome among children in Hawaii

Hospitalizations for Kawasaki syndrome among children in the United States

An outbreak of severe Kawasaki -like disease at the Italian epicenter of SARS-CoV-2 epidemic: an observational study

Kawasaki Disease Hospitalizations in a Predominantly African-American Population

IOM Committee Members Respond to Endocrine Society Vitamin D Guideline

Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline

Why the minimum desirable serum 25-hydroxyvitamin D level should be 75nmol/L (30ng/ml)

Milestones in DRI Development: What Does the Future Hold?

Options for basing Dietary Reference Intakes (DRIs) on chronic disease endpoints: report from a joint US-/Canadian-sponsored working group

Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data

Vitamin D3 supplementation in patients with frequent respiratory tract infections: a randomized and double-blind intervention study

Patterns of COVID-19 mortality and vitamin D: An Indonesian study

Vitamin D receptor agonists target CXCL10: new therapeutic tools for resolution and inflammation

Chemokine CXCL10 and coronavirus -induced neurologic disease

CXCL10-CXCR3 enhances the development of neutrophil -mediated fulminant lung injury of viral and nonviral origin

The Cytokine storm in COVID-19: An overview of the involvement of the chemokine/chemokine-receptor system

CXCL10/IP-10 Neutralization Can Ameliorate Lipopolysaccharide-Induced Acute Respiratory Distress Syndrome in Rats

Are vitamin D-recetor agonists like angiotensin converting enzyme inhibitors without side effects?

Selective vitamin D receptor acti vation with paricalcitol for reduction of albuminuria in patients with type 2 iabetes (VITAL study): a randomized controlled trial

Effects of dietary sodium and hydrochlorothiazide on the antiproteinuric efficacy of losartan

Conservative management of Covid-19 patientsemergency palliative care in action

Vitamin D-receptor activation and downregulation of renin angiotensin system attenuate morphine induced T cell apoptosis

Upregulation of Angiotensin converting enzyme 2 after myorcardial infarction by blockade of angiotensin II receptor

Angiotensin-(1-7)-ACE2 attenuates reactive oxygen species formation to Angiotensin II within the cell nucleus

Polyamines and eif5a hypusination modulate mitochondrial respiration and macrophage activation

Analysis of SARS-CoV-2 controlled autophagy reveals spermidine, MK2206, and niclosamide as putativeantivirl therapies

Induction of autophagy by spermidine promotes longevity

Impact of LKM512 yogurt on improvement of intestinal environment of the elderly

The involvement of polyamines in the activation of vitamin D receptor from porcine intestinal mucosa

Vitamin D3 induces autophagy of human myeloid leukemia cells

Vitamin D3 induces autophagy i n human monocytes/macorphages via cathelicidin

Angiotensin II, via angiotensin receptor type1/nuclear factor-kB activation, causes a synergistic effect on interleukin-I-ß-induced inflammatory responses in cultured mesangial cells

Vitamin D decreases NFB activity by increasing IB levels

Acknowledgement: My sincere thanks to Hellas Cena, University Pavia, Italy, for the critical reading of my manuscript and the excellent hints for strengthening the information contained therein. Ute Gola, Institute for nutrition and prevention, Berlin, Germany for valuable suggestions and advice.