key: cord-0925666-td3qrgap authors: Warren, Matthew F; Livingston, Kimberly A title: Implications of Vitamin D Research in Chickens can Advance Human Nutrition and Perspectives for the Future date: 2021-02-25 journal: Curr Dev Nutr DOI: 10.1093/cdn/nzab018 sha: 17ffe86c24366f63326c034860995643270dc633 doc_id: 925666 cord_uid: td3qrgap The risk of vitamin D insufficiency in humans is a global problem that requires improving ways to increase vitamin D intake. Supplements are a primary means for increasing vitamin D intake, but without a clear consensus on what constitutes vitamin D sufficiency, there is toxicity risk with taking supplements. Chickens have been used in many vitamin D-related research studies, especially studies involving vitamin D supplementation. Our state-of-the-art review evaluates vitamin D metabolism and how the different hydroxylated forms are synthesized. We provide an overview with how vitamin D is absorbed, transported, excreted, and what tissues in the body store vitamin D metabolites. We also discuss a number of studies involving vitamin D supplementation with broilers and laying hens. Vitamin D deficiency and toxicity are also described and how they can be caused. The vitamin D receptor (VDR) is important for vitamin D metabolism. However, there is much more that can be understood with VDR in chickens. Potential research aims involving vitamin D and chickens should explore VDR mechanisms which could lead to newer insights with VDR. Utilizing chickens in future research to help with elucidating vitamin D mechanisms has great potential to advance human nutrition. Finding ways to increase vitamin D intake will be necessary because the coronavirus 2019 disease (COVID-19) pandemic is leading to increased risk of vitamin D deficiency in many populations. Chickens can provide a dual purpose with addressing pandemic-caused vitamin D deficiency: 1) vitamin D supplementation gives chickens added value with possibly leading to vitamin D-enriched meat and egg products; and 2) chickens’ use in research provides data for translational research. Expanding vitamin D-related research in chickens to include more nutritional aims in vitamin D status has great implications with developing better strategies to improve human health. pandemic. Our state-of-the-art review aims to highlight the research and added-value that chickens can bring to further understanding vitamin D. We searched the literature for studies published in the English language using vitamin D and muscle, vitamin D and COVID-19. We did not set any exclusion criteria for studies when searching; however, we focused primarily on searching for studies involving chickens. We included studies that used different animal models (e.g., mouse, rat, pig, etc.) because it was important to include lab animal models as well as production animals with regards to this review's scope. We also included in vitro studies while searching because they could have an experimental approach with giving vitamin D to the animal before collecting their cells for in vitro experiments. There are two major classes of vitamin D which are further subdivided into various forms that exert hormonal and physiological effects or inactive forms that are excreted. Cholecalciferol (D 3 ) is a major class of vitamin D and is synthesized de novo by animals (19, 20) ( Figure 1 ). Ergocalciferol (D 2 ) is another major class of vitamin D that is primarily synthesized by microalgae and fungi (21, 22) . Animals can use both D 2 and D 3 , but D 3 has been reported to have higher binding affinity to VDR (23). Vitamin D has intact A, B, and D steroid rings because of photolysis of B ring of 7dehydrocholesterol (7-DHC or pro-vitamin D 3 ) when compared to generic steroids (24). The 7-DHC's structure allows the A ring to have the conformational capacity to undergo interconversion between two chair confirmations. D 3 's structure comprises of a saturated eight-carbon side chain, which is metabolically produced by photolysis of 7-DHC on skin surface exposed to ultraviolet irradiation (25). This review will focus on D 3 because of its bioavailability and involvement in humans and chickens. Vitamin D 3 synthesis in animals is a quick process dependent on exposure to ultraviolet B light (UVB, 290-315 nm) against skin (1) . Vitamin D synthesis begins when cholesterol is converted to 7-DHC in the skin ( Figure 2 ). UVB interacts with 7-DHC by inducing the electrolytic ring opening because of light absorption of the B ring's 5,7diene, which converts 7-DHC to pre-vitamin D 3 . Pre-vitamin D 3 can photochemically convert to lumisterol, tachysterol, or D 3 by thermal isomerization. D 3 is a major product of thermal isomerization because it requires the least energy for thermal rearrangement. D 3 on skin that is exposed too long to sunlight will be degraded to 5,6-trans-vitamin D 3 which has no calcemic effects like lumisterol or tachysterol (26) Thermal rearrangement gives pre-vitamin D 3 and D 3 a state of equilibrium and reversibility; although this equilibrium favors D 3. De novo synthesis in humans converts 10-15% of available 7-DHC to D 3 (25). Physical "sun-screen" properties of skin, such as melanin resulting in darker skin, reduces yield; environmental factors such as time of day, season, and latitude also affect yield (20, 27). The biological half-life of D 3 was suggested to be 50 h in human plasma, but radio-isotopically labelled D 3 was observed to last 4 d (28, 29). Vitamin D 3 can subsequently convert into different metabolite forms as illustrated in ( Figure 3 ). Feathers cover the skin on birds and also reflect UVB rays (30), and can likely prevent 7-DHC on skin from being converted to pre-vitamin D 3 . The earliest research on biogenic vitamin D research in chickens was in the 1920s when Hou (31) described how surgically removing the preen (or uropygial) gland can cause rickets in chicks that were exposed to UVB or sunlight. The preen gland produces preen oil that is a lipid compound that birds rub onto feathers to waterproof them (32, 33 published findings on how preen glands in ducks, geese, and chickens had no 7-DHC and concluded that preening feathers was not how birds acquired vitamin D 3 . A recent study described how laying hens fed a vitamin D 3 -deficient diet and exposed to UVB were able to lay eggs fortified with vitamin D 3 (36). Therefore, chickens would not be limited in vitamin D research with approaches involving UVB. Most vitamin D is taken up by liver and is hydroxylated at side chain C-25 to yield 25-OH-D 3 . 25-OH-D 3 is the major circulating form of vitamin D, and it is synthesized in the liver of mammals (37-39) and the intestine, liver, and kidneys of birds (40, 41). Hydroxylation of C-24 can occur with 25-OH-D 3 or with 1,25-(OH) 2 Vitamin D 3 is found and stored in multiple tissues like adipose tissue, skeletal muscle tissue, bone, liver, intestinal mucosa, (113, 114) , brain (115) , and skin (25). Adipose tissue contains the highest levels of vitamin D 3 with skeletal muscles being the second highest storage site for vitamin D 3 (113) . Vitamin D 3 is found in mammalian liver, but the liver will contain high levels of 25-OH-D 3 because the liver is a transient organ for hydroxylating vitamin D 3 to 25-OH-D 3 (116, 117) . Plasma vitamin D is mostly 25-OH-D 3 and then 24,25-(OH) 2 -D 3 (46). Variable levels of distribution of vitamin D 3 and its multiple metabolite forms denotes differences in tissue lipid content and DBP associated with tissues. It has been recently speculated that vitamin D deficiency in humans will be a global issue for addressing health because of implications related to osteomalacia (118) (119) (120) Broiler chickens (grown for meat) have been thoroughly used for vitamin Drelated studies to elucidate specific impacts of vitamin D intake with physiology and metabolism. Studies with broilers can also be designed to control for sunlight exposure or monitoring feed intake ( Table 2) . Researchers can examine nutrient effects on Table 2 ). There are studies that examined vitamin D 3 supplementation with other nutrients: strontium supplementation reduced body weight gain (132) and adding D 3 to diets with increased P and microbial phytase improved P and Ca utilization (133) ( to broilers observed increase in bone ash and plasma Ca and more effective with reducing incidence of rickets (139-141). 1α-OH-D 3 has been observed to have better bioefficacy than 25-OH-D 3 when used as an additive with broiler feed (142) and it is about 8x more effective than D 3 (143) ( Table 2) . Broiler chicks fed diets with higher levels of Ca and supplemented with 1α-OH-D 3 had lower plasma 25-OH-D 3 concentration (144) . It has also been suggested that 1α-OH-D 3 's high bioavailability in a synthetic mixed micelle with human intestinal cells (Caco-2) that 1α-OH-D 3 can be used to treat severe vitamin D deficiency (145) . Laying hens provide eggs for consumption and their production performance is an accessible trait to measure for quantifying dietary or supplementary effects ( Table 3 ). There was improved bone structure in laying hens that were fed vitamin D 3 , 25-OH-D 3 , or 1,25-(OH) 2 -D 3 (146, 147) . However, Ca intake is more important for egg production and quality (148, 149) . While studies involving vitamin D 3 with laying hens focused on egg quality ( breeder diets with supplemented vitamin A, the vitamin A concentration in egg yolk was greatly increased (153). Increasing levels of vitamin E in a laying hen diet also increases vitamin E content in egg yolk (154) ( Table 3) . It should be noted that competitive antagonism for intestinal absorption can occur between fat-soluble vitamins when dietary supplemental levels are provided to hens (155, 156). However, there are possibilities of creating specific value-added eggs to address specific vitamin needs by feeding hens with supplemental levels of a particular vitamin in their diet (151). Egg yolk vitamin D 3 content can reach a level that will meet daily vitamin D demands if a hen is fed a diet with high levels of vitamin D 3 (157) ( Table 3) . Mattila et al. Even though vitamin D is stored in tissues, vitamin D deficiency can occur with specific conditions. An animal consuming a vitamin D-poor diet and not exposed to sunlight will eventually express signs of deficiency such as rickets for young, growing animals and an increased risk of bone fractures, or muscle weakness (163-165). Elderly men and women are most susceptible to vitamin D deficiency (166, 167) ( Table 1) Table 1) . When taken into consideration with what was observed with rats and chickens, excessive vitamin D 3 intake may not be as toxic as suspected, especially with regulatory feedback mechanisms. Increasing intake of 25-OH-D 3 may be more effective for eliciting vitamin D pathways for increasing Ca absorption, but there are risks associated with toxicity. Chicks fed diets containing 100 mg of 25-OH-D 3 / kg of feed led to emaciation and deaths as a result of vitamin D intoxication (176) . In young chicks, toxicity caused kidney lesions and mineralization and fragile bones (176) . In rats, excessive levels of VDR has also been found in human brain along with 1α-hydroxylase, suggesting that vitamin D has autocrine/paracrine potential in the brain (191) ( Table 1) . Aging is also connected to reducing VDR expression in human muscle tissues (192). An importance of VDR's expression in most tissues in the body is its connection to immune function (193) . Immune cells like macrophages express VDR (194) and when 1,25-(OH) 2 -D 3 binds to VDR, then it causes signal transduction with immune activity (195) . VDR is connected to anti-inflammatory elements and modulating antiinflammatory cytokine activity while inhibiting pro-inflammatory cytokine production (196, 197) . Research in mice has shown that lacking or having defective VDR led to increased inflammation (197, 198) . Reduced or no signaling from VDR also highlights why vitamin D deficiency is correlated to increased inflammation (199). VDR's connection to immune function will be a significant hotbed area of research in the future because of the global need to address vitamin D insufficiency. There is a lack of the research into the VDR in chickens that will be necessary for illustrating vitamin D mechanisms. However, the chicken genome has been sequenced (200, 201) , so VDR has been annotated. There is some research that explored VDR as a biological candidate gene for improving production (growth rate for broilers, egg production and egg quality for laying hens) (202) (203) (204) . VDR has also been shown to be a mediator for muscle tissue in broilers and in vitro (205 Hypothesis testing for vitamin D studies in chickens can increase their impact by considering VDR levels. VDR has been identified to be present in almost all tissues in mammalian models, but does the same principle apply to the avian model which has a distinctly different kidney structure (206) that is also very efficient? Birds evolved to mobilize Ca quickly from their bones for eggshell formation and this Ca may also be resorbed from kidneys, which is also a site for VDR (207, 208) . If chickens were fed exorbitant levels of 25-OH-D 3 and were at risk of renal calcification, then VDR expression with 1α-OHase should be greatly increased to explain that effect. However, the alternative hypothesis would be explained by a decrease in 24-OHase which can also explain why renal calcification would be possible because the chicken had higher Ca absorption without feedback inhibition. By understanding how VDR is affected by experimental treatments in chickens, future researchers will be able to draw conclusions that can connect the biological effects of vitamin D. Such knowledge can be translated towards human nutrition to identify ways to address vitamin D insufficiency while reducing risk of vitamin D toxicity. Future research with vitamin D in chickens can help elucidate vitamin D deficiency-related issues to help address high-impact areas of human nutrition The COVID-19 pandemic led to a significant reduction in sunlight exposure for many people, as a result of lockdown and stay-home orders, inevitably increasing risk of vitamin D deficiency (209, 210) . Vitamin D deficiency is linked to a higher risk of COVID-19 infection because 1,25-(OH) 2 -D 3 needs to bind to VDR for signal transduction pathways related to innate immunity (211) . Vitamin D deficiency leads to increased Vitamin D content in egg yolk achieved a peak (30 µg D 3 / 100 g yolk weight) around 8 -13 d from the start of the experimental high D 3 diets; feeding 1708.7 µg D 3 /kg diet did not affect eggshell strength or harm the hens Lower calcium levels led to increased feed intake, body weight, liver weight, and fat pad; although, there was no effect on egg size or production . Biochemical reactions of 7-dehydrocholesterol (7-DHC) that leads to synthesis of vitamin D 3 and potential noncalcemic metabolites. When ultraviolet B (UVB) rays or sunlight hits 7-dehydrocholesterol (7-DHC) on skin, then 7-DHC is converted to pre-vitamin D 3 which is then converted to vitamin D 3 , lumisterol-3, or tachysterol-3 by thermal isomerization. D 3 enters blood circulation to be hydroxylated to its more active forms. Vitamin D deficiency Vitamin D: dietary requirements and food fortification as a means of helping achieve adequate vitamin D status Regulation of vitamin D metabolism and function Overview of general physiologic features and functions of vitamin D Calcitroic acid: Biological activity and tissue distribution studies The distribution and storage of vitamin D and its metabolites in human tissues Tissue Distribution of Cholecalciferol and 25-hydroxycholecalciferol in Normal and Obese Mice Fed Different Levels of Vitamin D (P24-003-19) Associations of Vitamin D and Vitamin K and Their Metabolites Across Four Regions of the Human Brain: The Memory and Aging Project (MAP) (FS05-02-19) The role of the liver in the metabolism of vitamin D In vitro production of 25-hydroxycholecalciferol Vitamin D deficiency: a global perspective Efficacy of 1-α-Hydroxycholecalciferol Supplementation in Young Broiler Feed Suggests Reducing Calcium Levels at Grower Phase Comparison of the micellar incorporation and the intestinal cell uptake of cholecalciferol, 25-hydroxycholecalciferol and 1-α-hydroxycholecalciferol Influence of vitamin D3, 1α-Hydroxyvitamin D3, and 1, 25-Dihydroxyvitamin D3 on eggshell quality, tibia strength, and various production parameters in commercial laying hens Effects of additional dosage of vitamin D3, vitamin D2, and 25-hydroxyvitamin D3 on calcium and phosphorus utilization, egg quality and bone mineralization in laying hens Calcium and its relationship to excess feed consumption, body weight, egg size, fat deposition, shell quality, and fatty liver hemorrhagic syndrome Eggshell quality as influenced by sodium bicarbonate, calcium source, and photoperiod Vitamin D3 requirement of young chicks receiving diets varying in calcium and available phosphorus Relative Toxicity and Metabolic Effects of Cholecalciferol and 25-Hydroxycholecalciferol in Chicks Plasma concentrations of vitamin D3 and its metabolites in the rat as influenced by vitamin D3 or 25-hydroxyvitamin D3 intakes Safety of 25-hydroxyvitamin D3 as a source of vitamin D3 in layer poultry feed Safety of 25-hydroxycholecalciferol as a source of cholecalciferol in poultry rations Acute Administration of 25-Hydroxycholecalciferol in Man Biological Activity of 25-Hydroxyergocalciferol in Rats Vitamin D receptor: molecular signaling and actions of nutritional ligands in disease prevention Vitamin D receptor (VDR)-mediated actions of 1α, 25 (OH) 2vitamin D3: genomic and non-genomic mechanisms. Best practice & research RXRβ: a coregulator that enhances binding of retinoic acid, thyroid hormone, and vitamin D receptors to their cognate response elements Vitamin D and Human Health: Lessons from Vitamin D Receptor Null Mice Cloning of a functional vitamin D receptor from the lamprey (Petromyzon marinus), an ancient vertebrate lacking a calcified skeleton and teeth Sequence and comparative analysis of the chicken genome provide unique perspectives on vertebrate evolution Thibaud-Nissen F. A new chicken genome assembly provides insight into avian genome structure Polymorphisms in vitamin D receptor, osteopontin, insulin-like growth factor 1 and insulin, and their associations with bone, egg and growth traits in a layer-broiler cross in chickens Age, phosphorus, and 25-hydroxycholecalciferol regulate mRNA expression of vitamin D receptor and sodium-phosphate cotransporter in the small intestine of broiler chickens Effects of Presence of an Egg and Calcium Deposition in the Shell Gland on Levels of Messenger Ribonucleic Acid of CaBP-D28K and of Vitamin D3 Receptor in the Shell Gland of the Laying Hen 25-hydroxycholecalciferol enhances male broiler breast meat yield through the mTOR pathway Structure and concentrating ability in the avian kidney Novel N-Terminal Variant of Human VDR Altered gene expression profile in the kidney of vitamin D receptor knockout mice Evidence that vitamin D supplementation could reduce risk of influenza and COVID-19 infections and deaths The COVID-19 pandemic: how to maintain a healthy immune system during the lockdown-a multidisciplinary approach with special focus on athletes Mechanisms in endocrinology: Vitamin D and COVID-19 25-Hydroxyvitamin D Concentrations Are Lower in Patients with Positive PCR for SARS-CoV-2 Oral vitamin D3 and calcium for secondary prevention of low-trauma fractures in elderly people (Randomised Evaluation of Calcium Or vitamin D, RECORD): a randomised placebo-controlled trial Short and Long-Term Variations in Serum Calciotropic Hormones after a Single Very Large Dose of Ergocalciferol (Vitamin D2) or Cholecalciferol (Vitamin D3) in the Elderly Vitamin D 3 and 25-hydroxyvitamin D 3 in pork and their relationship to vitamin D status in pigs The authors would like to thank Gavin Conant, Peter Ferket, Andrew Hardwick, Matt Koci, Catherine Lopez, and Shannon Madden who gave feedback for the initial manuscript and revision drafts. The authors' responsibilities were as follows-MFW: drafted the initial manuscript; MFW and KAL: critically revised the manuscript for important intellectual content; and both authors: read and approved the final manuscript.