key: cord-0876166-xhgxjaq2 authors: Hribar, Maša; Benedik, Evgen; Gregorič, Matej; Blaznik, Urška; Kukec, Andreja; Hristov, Hristo; Žmitek, Katja; Pravst, Igor title: A Systematic Review of Vitamin D Status and Dietary Intake in Various Slovenian Populations date: 2021-12-27 journal: Zdr Varst DOI: 10.2478/sjph-2022-0009 sha: d2e10cac24d2b495008bae2598682788e745a865 doc_id: 876166 cord_uid: xhgxjaq2 AIM: Vitamin D (VitD) is involved in calcium and phosphate homeostasis, bone health, and normal functioning of the immune system. VitD status is monitored using serum 25-hydroxy-vitamin D (25(OH)D) as a biomarker. Serum 25(OH)D concentrations below 30 nmol/L indicate VitD deficiency and below 50 nmol/L indicate insufficiency. VitD can be synthesised endogenously in human skin when exposed to ultraviolet B (UVB) radiation. In the absence of sufficient UVB-light exposure, VitD intake becomes the main source of VitD, with a recommended daily intake of 20 μg. The aim of this study was to conduct a review and meta-analysis on the abovementioned topics, focusing on scientific studies in various Slovenian populations. METHODS: We conducted a systematic review and meta-analysis of published scientific papers, academic theses, or conference contributions reporting serum 25(OH)D status and VitD intake across various Slovenian populations. A search was carried out using Web of Science, Scopus, Medline, and the Slovenian library database. RESULTS: We identified 43 pertinent studies that addressed 25(OH)D status and 16 that addressed VitD intake. Serum 25(OH)D status was generally low across all populations, and notable seasonal variability was observed. VitD intakes were below 5 μg in all studies. CONCLUSIONS: A general observation is that various population groups across Slovenia are at high risk of vitamin D insufficiency and deficiency, particularly during wintertime. Regarding vitamin D intake, all included studies reported daily intakes below the recommended level. We also identified key research gaps that need to be addressed to support further public health decision-making. Vitamin D (VitD) participates in numerous functions in the human body, including calcium and phosphate homeostasis, bone health, and the normal functioning of the immune system (1, 2) . Deficient 25-hydroxy-vitamin D (25(OH)D) status causes rickets and osteomalacia (depending on age) (3, 4) , contributes to the development of osteoporosis and myopathy, and has been linked to various non-skeletal disorders, such as acute and chronic respiratory infections, and risks of death due to cardiovascular disease, cancer, and other causes (5, 6) . While numerous reports indicate a high prevalence of 25(OH)D deficiency, we should note that different thresholds are used in different studies. VitD status is determined considering the level of serum 25(OH)D, usually reported in both nanomoles per litre (nmol/L) and nanograms per millilitre (ng/mL; 1 ng/mL equals 2.5 nmol/L) (7) . Thresholds for risk of 25(OH) deficiency and insufficiency are typically defined with serum concentrations below 30 and 50 nmol/L, respectively (3, 8, 9) , while the Endocrine Society's guidelines highlight that optimal levels are above 75 nmol/L (3). VitD can be synthesised endogenously in human skin when exposed to ultraviolet B(UVB) light. Considering the geographical location of Slovenia (latitudes 45-46° north), the intensity of UVB light is sufficient for VitD endogenous synthesis only from April to October, but not during wintertime (10) . Therefore, seasonal variation in 25(OH)D status is expected in the population. In the absence of UVB-induced endogenous synthesis, sufficient dietary intake of VitD is needed. Recommendations for VitD intake are not yet standardised. While the World Health Organization (WHO) recommends 10 µg VitD per day for adults below 65 years and 15 µg for the elderly population (11) , the European Food Safety Authority (EFSA) and the nutrition societies of Germany, Austria, and Switzerland (D-A-CH) recommend a VitD intake of 15 and 20 µg per day, respectively for all above the age of 1 year (12) . It should be noted that these are recommendations for dietary intake in the absence of endogenous UVB-induced VitD synthesis. However, the usual daily dietary VitD intake in adults is below 10 µg in most European countries (13) . This is because only a few foods (i.e., fatty fish) are naturally rich in VitD (14) , and those are not common in the European diet. Some countries, therefore, have introduced the fortification of particular foods with VitD (15) or prescribed VitD supplementation to certain subpopulation groups. While several studies have investigated dietary intake or 25(OH)D status in various Slovenian populations, this topic has not yet been systematically reviewed. The aim of this study was thus to conduct a review and meta-analysis on the abovementioned topics, focusing on studies published either as scientific papers, academic thesis, or conference contributions. We focused on two topics: A) serum 25(OH)D levels as a biomarker for VitD status, and B) VitD intake in various population groups in Slovenia. The main literature search was carried out in September 2020. The following four databases were searched (Box 1): • Medline, Scopus, and Web of Science (WoS) -with the searching syntax: (TITLE-ABS-KEY ((("vitamin D*") OR (cholecalciferol)) AND sloven*), and • COBISS -Slovenian library database (Co-operative Online Bibliographic System & Services), with the searching syntax: kw=("vitamin d" or cholecalciferol). For this review, we included original scientific articles, academic thesis (diploma, masters, doctoral, and others), and conference contributions in the Slovene or English language. We included publications which provided data on either serum 25(OH)D levels or dietary VitD intake for the defined population. We included publications with original results, which were available as full texts or comprehensive abstracts, either in digital or hard copy. The literature search was conducted without any time limits, and therefore the identified results correspond to periods covered in the abovementioned databases. In addition to the described literature search, we also asked members of the national expert working group on guidelines for sufficient vitamin D levels in the Slovenian population to provide information on the references with novel data on dietary VitD intake and 25(OH)D status in Slovenia that they are aware of. These members are experts and researchers in the field, and therefore well informed about nationally available data. The identified relevant publications, which were not already found in the literature search, were counted as additional records identified through other sources. The methods used in the present review follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement (16 (3, 8) , we presented the prevalence of serum 25(OH)D levels below 30/50 nmol/L (VitD deficiency/insufficiency) and those above 50/75 nmol/L (sufficient/optimal level) separately. Statistical analyses were carried out using Stata Statistical Software, Release 15 (StataCorp LLC, College Station, TX, USA). The proportions of deficiency/optimal vitamin D status were analysed by applying the STATA Metaprop module for pooled meta-analyses of proportions. Analyses were carried out considering studies with healthy individuals, where seasonal proportions of deficiency/ optimal 25(OH)D status were provided for a sample of at least 50 subjects. The literature search resulted in 562 records. After the removal of duplicates (n=69) and the addition of records identified through other sources (n=9), a total of 502 records were screened for relevance. After the exclusion of 186 non-relevant records, we started collecting the full texts of 268 publications. Although we made a large effort to obtain full-text access to all of these records (we contacted the authors of publications, searched for records on various internet sources, and ordered the remaining missing publications through the Central Technical Library at the University of Ljubljana), we were unable to obtain full texts for 24 records. Therefore, the literature review was conducted using 58 relevant publications ( Figure 1 ). Finally, we separated identified publications into two sections, depending on the reported study outcomes: a) serum 25(OH)D levels (25(OH)D status) and b) dietary intake of VitD. One publication (17) was used in both sections. In the section of 25(OH)D status, we included 43 texts and allocated them into 30 research clusters, and in the section of dietary VitD intake, we included 16 texts. The first reported research is dated as from 1993 (18) and the last from 2021 (19) . The literature review of 25(OH)D included 43 publications (Supplementary table 1) , comprising 20 scientific journal articles, 11 conference papers, and 12 academic theses (details provided in Table 1 ). Altogether, we identified 30 research clusters, the majority referring to the adult population (n=21), one referred to older adults and adults, with an additional two for older adults and two for pregnant women. Two-thirds of the research clusters (n=21) reported data from patients (in-or out-patients), thirteen from the general population, and four reported separately for healthy controls and patient populations. Therefore, we considered these clusters twice, once in the healthy and once in the patient population. Four research clusters referred to children and one additional to newborn infants, giving five in total. In the meta-analyses, we included four studies that reported seasonal proportions of deficiency/optimal 25(OH)D status in a sample of at least 50 subjects. Among the healthy adult population, three research clusters (20) (21) (22) were composed of healthy individuals, one cluster of presumably healthy individuals (23) (as the data collection was carried out retrospectively in a laboratory with self-pay service), and another three research clusters (24-27) of healthy individuals from control groups. A nationally representative study sample (adults 18-64 years) was only used in one study (20) , The review covered two research clusters of healthy pregnant women. One included data collected in the 'My Milk' research project ('The role of human milk in the development of breastfed child's intestinal microbiota') (17, 31, 32) . We only included publications with a full study sample (17) , as two references refer to a smaller subsample size (31, 32) . The second cluster is described in six different publications (33) (34) (35) (36) (37) (38) . Both research clusters reported serum 25(OH)D levels throughout the year, with mean levels 43.4±23.8 nmol/L and 74.7±27.5 nmol/L for the first and second clusters, respectively. One research cluster included data on new-born infants (33) (34) (35) (36) (37) (38) ; one was with healthy young athletes (16.9±4.4 years) (39) , and three clusters included paediatric patients (3.8-18 years) (40) (41) (42) (43) (44) ; one of these included a healthy control group (40) (41) (42) . In infants, mean serum 25(OH) D concentrations were 55.2±30.9 nmol/L and varied notably throughout the year. The lowest was observed in March (36.4±22.6 nmol/L), and highest in September (72.9±31.7 nmol/L) and the concentrations were approximately 10 nmol/L higher than in mothers (33) (34) (35) (36) (37) (38) . In young athletes (39), serum 25(OH)D was measured in samples collected in April (67.6±36.2 nmol/L); there was no significant difference between a group of swimmers and artistic gymnasts. Eighteen research clusters included data from adult patient populations: five research clusters included chronic kidney disease patients (24, 25, (45) (46) (47) (48) (49) (50) , three research clusters included type 2 diabetics (26, 51, 52), two clusters included patients with bone metabolism disorder or trauma (53, 54) , two clusters included mixed data from University Medical Centre Ljubljana laboratories (55, 56) , and single-cluster studies included patients after heart transplant (57), HIV patients (58), chronic intestinal failure patients on long-term home parenteral nutrition (59) , and patients treated with antiepileptic drugs (27). The population of healthy older adults in our review included three research clusters. The first cluster (28, 29) represented 80 adults aged between 51 and 93 years, living at home or institutionalised; the second cluster (20) included nationally representative data of 155 people aged 65 to 75 years, and the third cluster (30) Six clusters reported data for 25(OH)D insufficiency (<50 nmol/L) in eight different (general), populations including pregnant women and newborns (17, 20, 21, 27, (33) (34) (35) (36) (37) (38) (39) . The prevalence of 25(OH)D insufficiency in yearround samples varied from 14% in pregnant women (17) to 63% in a nationally representative sample of the elderly (20) . However, due to notable seasonal differences, we focused on studies that reported seasonal results. Four clusters were included in meta-analyses, since they reported winter-and summertime insufficient serum 25(OH)D levels (<50 nmol/L) in at least 50 healthy subjects ( Figure 3 ). (20)). Nevertheless, considerable seasonal variation in 25(OH)D insufficiency was observed, with much higher prevalence rates during wintertime (p<0.001; z=5.62). Seven research clusters (general populations) reported on the prevalence of non-optimal 25(OH)D concentrations using thresholds comparable with the Endocrine Society's guidelines (75 nmol/L) (3). Four clusters with at least 50 healthy subjects were used for meta-analyses. As seen in Figure In total, sixteen publications reported dietary VitD intakes ( Figure 1 ); of these, ten were journal articles and six were academic theses (details provided in Supplementary table 2). Most (n=12) publications referred to a healthy population (17, 19, (60) (61) (62) (63) (64) (65) (66) (67) (68) (69) (70) (71) , while two were on patient populations (72, 73) . When considering the age and pregnancy status of the participants, one publication referred to children (<10 years) (60), six to adolescents (10-19 years) (17, 19, (61) (62) (63) (64) (65) , eight to adults (>19 years) (19, (64) (65) (66) (67) (68) (69) (70) (71) (72) (73) , and one publication to pregnant women (17) . One journal article covered both adolescents and adults (65), and one adolescents, adults, and the elderly (19) . Two studies reported VitD intake separately -for food intake, and intake with food and supplements (66, 71) . Data on intake of VitD was collected using different methods: a) 24-hour dietary recalls (24h-recall) (n=4), b) different kinds of food frequency questionnaires (FFQ) (n=4), and c) dietary record (DR) ranging from 3 to 7 days (n=6). Two studies investigated VitD intake using both FFQ and 24h-recall (Supplementary table 2 ). In the adult population, mean daily VitD intake ranged from 1.1 µg in older adults living in residential home (69, 70) (65-74 years) . VitD intakes were estimated using a combination of two 24h-recalls adjusted with a food propensity questionnaire. Mean daily VitD intakes for adolescents, adults and the elderly were 2.7 µg, 2.9 µg, and 2.5 µg, respectively, and we should also note that the amount of VitD supplements was not recorded. Energy intake was a significant factor across all age groups, sex was determined as a significant factor, affecting VitD intakes in adolescents and adults, with lower VitD intakes observed in females. The most important dietary sources of VitD were eggs, fish, and fish products, and meat and meat products (19) . (74) is related to a growing body of evidence that connects sufficient 25(OH)D status with better health outcomes (75) (76) (77) (78) , while highlighting the high prevalence of VitD deficiency in different populations (13) . This topic has recently gained even more attention due to the possible associations of VitD status with health outcomes of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infected patients (78) (79) (80) (81) (82) . In Slovenia, there is currently no official policy on food fortification and supplementation of diets with VitD in the general population. To support further public health decision-making, the Ministry of Health appointed a national expert working group on guidelines for sufficient vitamin D levels in the Slovenian population, under the umbrella of the National Institute of Public Health. To support this activity, this work aimed to review scientific publications, reporting dietary intake and/or 25(OH)D status in various Slovenian population subgroups. We reviewed all available publications with novel research data collected in Slovenia, including original scientific articles, conference papers, and academic theses. We used reported data on serum 25(OH)D levels as a biomarker of 25(OH)D status across various populations in Slovenia, but due to methodological and analytical differences in different studies, data should be interpreted with caution. For example, in different studies, blood samples were collected in different seasons, or nonevenly distributed across the calendar year. We should also mention that different assays were used for the quantification of 25(OH)D in different studies. Only one study reported nationally representative data on 25(OH)D concentrations in adults (18-64 years) and the elderly (65-75 years) (20) . The prevalence of insufficiency was 58.2% and 62.9% for adults and the elderly, respectively, and 83.3% and 84.4% of adults and the elderly, respectively, had 25(OH)D values below the optimal level. The levels of 25(OH)D were influenced by sex and physical activity, and most importantly by calendar season. In a much smaller cluster (n=22), 59.1% were insufficient; 90.1% were below the optimal status (27) , and in a cluster referring to self-paying subjects from the laboratory information system (n=73), 58.9% were below optimal status (23). There were notable differences in the reported prevalence of 25(OH)D insufficiency (<50 nmol/L) and below optimal status (<75 nmol/L) in different studies, as these were conducted on different population groups and in different seasons. We should also note seasonal variations, which were reported in a nationally representative Slovenian study (20) . (90) , seasonal variation in insufficiency was even more pronounced, and it varied from 4.4% in summer (August) to 70% in winter (February). In the present review, seasonal variation was also noted in pregnant women (17, (33) (34) (35) (36) (37) (38) and across other populations (22, 23, 30, 43, 54) . The abovementioned seasonal variability in 25(OH)D status showed that in Slovenia (45-46°-N), endogenous synthesis of VitD is not sufficient during wintertime, as was also observed in other regions (10) . In such cases, dietary intake of VitD becomes very important. However, very few foods are rich in VitD (e.g., fatty fish), and these are not common in the Slovenian diet. Achieving adequate VitD intake is therefore challenging without the use of fortified foods and/or VitD dietary supplementation. In a recent Slovenian study (91) it was established that the 33% of population was taking VitD supplements (before the SARS-CoV-2 epidemic), and we should consider this when interpreting the data. In Slovenia, we follow the D-A-CH reference values for VitD intake, which recommend a daily intake of 20 µg VitD, for all above the age of 1 year in the absence of endogenous synthesis (12, 92, 93) . Various dietary intake assessment tools have been used (FFQ, DR, and 24h-recall) to estimate dietary intake levels, and we should therefore compare the results with some caution. At the same time, the food database is often incomplete or lacking compositional data regarding VitD content in a certain food/dish, which may result in underestimation of dietary intake of VitD (94) . We reviewed sixteen publications reporting dietary intake of VitD in different Slovenian populations, one of which was nationally representative for adolescents, adults, and the elderly (19) . All studies reported VitD intakes well below reference values across all the populations. In the adult and elderly population, daily mean VitD intakes ranged from 1.1 µg to 14.2 µg, the latter with supplements and plant-based meal replacements. A study by Lichthammer et al. (65) (FFQ) included populations from various Central-Eastern European countries (Austria, Poland, Hungary, and Slovenia). The mean daily VitD intake was very low in all countries, and was the lowest in Austria with 2.2 µg and then immediately followed by Slovenia with 2.6 µg. A similar intake for the Austrian population was found in another study (2.1 µg per day; n=4,972; various dietary intake tools) (95) . In a Slovenian nationally representative study, where the amount of supplementation was not included, mean daily VitD intakes were 2.7 µg, 2.9 µg, and 2.5 µg in adolescents, adults, and the elderly, respectively (19) . A review of European studies on VitD intake showed a mean VitD intake of 3 to 5 µg per day (13, (96) (97) (98) (99) , with higher intakes in Northern Europe (up to 11 µg/day) and lower in Southern Europe (13, 84, 97, 99, 100) . This can be explained by the high consumption of sea fish and VitD enriched foods in Northern Europe. Our review also covered six children or adolescent populations. In teenage populations, VitD intake varied from 1.0 µg in teenage girls (2x24h-recall) (64) to 4.0 µg in teenage boys (FFQ, portion size-adjusted with 3-day DR) (61) . These results are comparable to an Austrian study (95) and other European studies (96, 98, 101, 102) . A European nutrition and health report (100) also highlighted that VitD intake in children was generally very low, with the exception of some Nordic countries due to the general vitamin D food fortification policy. This review has identified several studies reporting dietary intake and/or 25(OH)D status in various Slovenian populations, but the majority were conducted using the convenience sampling method and are therefore not nationally representative. Nevertheless, the reviewed results showed considerable seasonal variability in serum 25(OH)D levels in different populations, with a particularly high prevalence of 25(OH)D deficiency during the wintertime. Only one study investigated 25(OH)D status in a nationally representative sample of adults and reported insufficient wintertime 25(OH)D levels in about 80% of adults and 25(OH)D deficiency in about 40%. The study sample also included those who reported vitamin D supplementation. The majority of other studies also showed that various population groups in Slovenia are mostly at risk in wintertime with regard to insufficient 25(OH)D status according to the WHO criteria (103) . This literature review has also identified some important research gaps. The SARS-CoV-2 pandemic has significantly affected the dietary habits of Slovenians, and increased the consumption of food supplements and medicines with VitD. This might have affected 25(OH)D status in some populations studied in the last year. In addition, some population groups have not been sufficiently investigated. There is very limited data on the 25(OH)D status in healthy children and adolescents, and a lack of data on institutionalised populations, such as those living in elderly care institutions, who are rarely exposed to sunlight and therefore at an even higher risk of 25(OH)D deficiency. We also reviewed the data on dietary VitD intake in Slovenia. Several studies were identified that estimated VitD intakes in different populations using a variety of study designs and methods. One study reported nationally representative data. All studies reported very low VitD intakes. While the recommended daily intake of VitD in adults is 20 µg in the absence of endogenous synthesis, most studies reported dietary intakes below 5 µg. The majority of the studies did not estimate intakes of VitD with medicines and food supplements or fortified foods, though the latter practice has been negligible in Slovenia. VitD food supplementation practices should be further investigated in the future. The general observation of our review is that various population groups across Slovenia are at high risk of 25(OH)D insufficiency and deficiency, particularly during the wintertime. Regarding VitD intake, the majority of included studies reported daily intakes below 5 µg. We also identified key research gaps that need to be addressed to support further public health decision-making. We acknowledge the support of the members of the national expert working group on guidelines for sufficient vitamin D levels in the Slovenian population, particularly Marija Pfeifer, Andraž Dovnik, and Aneta Soltirovska Šalamon. We also acknowledge the support of Joško Osredkar in data collection. The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. I.P. has led and participated in various other research projects in nutrition, public health, and food technology that were (co)funded by the Slovenian Research Agency, Ministry of Health of the Republic of Slovenia, the Ministry of Agriculture, Forestry, and Food of the Republic of Slovenia, and, for specific applied research projects, also by food businesses. I.P., K.Ž, and E.B. are members of a national expert working group on guidelines for sufficient vitamin D levels in the Slovenian population. The study was conducted within the national research program P3-0395 (Nutrition and Public Health) and project L7-1849 (Challenges in achieving adequate vitamin D status in the adult population) and supported by the Slovenian Research Agency and Ministry of Health of the Republic of Slovenia. This is a review article. No ethical approval is needed. Blazina Notes: M -males, F -females, DDR -day dietary record, FFQ -food frequency questionnaire, FPQ -food propensity questionnaire, sqsemi-quantitative; Publication type (A -scientific paper, B -conference paper, C -academic thesis; 1 -doctoral, 2 -master, 3 -diploma, 4 -other) Vitamin D deficiency Vitamin D: physiology, dietary sources and requirements Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline Vitamin D insufficiency, deficiency, and bone health Vitamin D and risk of cause specific death: systematic review and meta-analysis of observational cohort and randomised intervention studies Vitamin D and health -the missing vitamin in humans Existing and potentially novel functional markers of vitamin D status: a systematic review Committee to Review Dietary Reference Intakes for Calcium and Vitamin D. 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AG: 35.3% <75 nmol/L Adults: 83.3% Adults winter: 98.0% Adults summer: 62.6% Elderly: 84.4% Elderly winter: 92.2% Elderly