Inhibition of Niemann-Pick C1-like protein 1 by ezetimibe reduces uptake of deuterium-labeled vitamin D in mice

Vitamin D beyond the blood: Tissue distribution of vitamin D metabolites after supplementation

Vitamin D3's role in mineral homeostasis through its endocrine function, associated with the main circulating metabolite 25-hydroxyvitamin D3, is well characterized. However, the increasing recognition of vitamin D3's paracrine and autocrine functions-such as cell growth, immune function, and hormone regulation-necessitates examining vitamin D3 levels across different tissues post-supplementation. Hence, this review explores the biodistribution of vitamin D3 in blood and key tissues following oral supplementation in humans and animal models, highlighting the biologically active metabolite, 1,25-dihydroxyvitamin D3, and the primary clearance metabolite, 24,25-dihydroxyvitamin D3. While our findings indicate significant progress in understanding how circulating metabolite levels respond to supplementation, comprehensive insight into their tissue concentrations remains limited. The gap is particularly significant during pregnancy, a period of drastically increased vitamin D3 needs and metabolic alterations, where data remains sparse. Within the examined dosage ranges, both human and animal studies indicate that vitamin D3 and its metabolites are retained in tissues selectively. Notably, vitamin D3 concentrations in tissues show greater variability in response to administered doses. In contrast, its metabolites maintain a more consistent concentration range, albeit different among tissues, reflecting their tighter regulatory mechanisms following supplementation. These observations suggest that serum 25-hydroxyvitamin D3 levels may not adequately reflect vitamin D3 and its metabolite concentrations in different tissues. Therefore, future research should aim to generate robust human data on the tissue distribution of vitamin D3 and its principal metabolites post-supplementation. Relating this data to clinically appropriate exposure metrics will enhance our understanding of vitamin D3's cellular effects and guide refinement of clinical trial methodologies.

Dietary cholesterol increases body levels of oral administered vitamin D3 in mice

Vitamin D and cholesterol share the same intestinal transporters. Thus, it was hypothesized that dietary cholesterol adversely affects vitamin D uptake. The current studies investigated the influence of cholesterol on the availability of oral vitamin D. First, 42 wild-type mice received a diet with 25 µg/kg labelled vitamin D3 (vitamin D3-d3), supplemented with either 0% (control), 0.2%, 0.4%, 0.6%, 0.8%, 1.0% or 2.0% cholesterol for four weeks to investigate vitamin D uptake. In a second study, 10 wild-type mice received diets containing 0% (control) or 1% cholesterol over four weeks to determine cholesterol-induced changes in bile acids. Finally, we investigated the impact of cholesterol versus bile acids on vitamin D uptake in Caco-2 cells. Surprisingly, dietary cholesterol intake was associated with 40% higher serum levels of vitamin D3-d3 and 2.3-fold higher vitamin D3-d3 concentrations in the liver compared to controls. The second study showed that cholesterol intake resulted in higher concentrations of faecal bile acids (control: 3.55 ± 1.71 mg/g dry matter; 1% dietary cholesterol: 8.95 ± 3.69 mg/g dry matter; P < 0.05) and changes in the bile acid profile with lower contents of muricholic acids (P < 0.1) and higher contents of taurodeoxycholic acid (P < 0.01) compared to controls. In-vitro analyses revealed that taurocholic acid (P < 0.001) but not cholesterol increased the cellular uptake of vitamin D by Caco-2 cells. To conclude, dietary cholesterol seems to improve the bioavailability of oral vitamin D by stimulating the release of bile acids and increasing the hydrophobicity of bile.

UV light exposure versus vitamin D supplementation: A comparison of health benefits and vitamin D metabolism in a pig model

There is limited data on the effect of UV light exposure versus orally ingested vitamin D3 on vitamin D metabolism and health. A 4-week study with 16 pigs (as a model for human physiology) was conducted. The pigs were either supplemented with 20 µg/d vitamin D3 or exposed to UV light for 19 min/d to standardize plasma 25-hydroxyvitamin D3 levels. Important differences were higher levels of stored vitamin D3 in skin and subcutaneous fat, higher plasma concentrations of 3-epi-25-hydroxyvitamin D3 and increases of cutaneous lumisterol3 in UV-exposed pigs compared to supplemented pigs. UV light exposure compared to vitamin D3 supplementation resulted in lower hepatic cholesterol, higher circulating plasma nitrite, a marker of the blood pressure-lowering nitric oxide, and a reduction in the release of pro- and anti-inflammatory cytokines from stimulated peripheral blood mononuclear cells. However, plasma metabolome and stool microbiome analyses did not reveal any differences between the two groups. To conclude, the current data show important health relevant differences between oral vitamin D3 supplementation and UV light exposure. The findings may also partly explain the different vitamin D effects on health parameters obtained from association and intervention studies.

Gastrointestinal genetic reprogramming of vitamin A metabolic pathways in response of Roux-en-Y gastric bypass

Roux-en-Y gastric bypass (RYGB) is one of the most performed bariatric surgical techniques. However, RYGB commonly results, as side effects, in nutritional deficiencies. This study aimed to examine changes in the expression of vitamin A pathway encoding genes in the gastrointestinal tract (GI) and to evaluate the potential mechanisms associated with hypovitaminosis A after RYGB. Intestinal biopsies were obtained through double-balloon endoscopy in 20 women with obesity (age 46.9±6.2 years; body mass index [BMI] 46.5±5.3 kg/m2 [mean±SD]) before and three months after RYGB (BMI, 38.2±4.2 kg/m2). Intestinal mucosal gene microarray analyses were performed in samples using a Human GeneChip 1.0 ST array (Affymetrix). Vitamin A intake was assessed from 7-day food records and serum retinol levels were evaluated by electrochemiluminescence immunoassay. Our results showed the following genes with significant downregulation (p≤0.05): LIPF (-0.60), NPC1L1 (-0.71), BCO1 (-0.45), and RBP4 (-0.13) in the duodenum; CD36 (-0.33), and ISX (-0.43) in the jejunum and BCO1 (-0.29) in the ileum. No significant changes in vitamin A intake were found (784±694 retinol equivalents [RE] pre-operative vs. 809±753 RE post-operative [mean±SD]). Although patients were routinely supplemented with 3500 international units IU/day (equivalent to 1050 μg RE/day) of oral retinol palmitate, serum concentrations were lower in the post-operative when compared to pre-operative period (0.35±0.14 μg/L vs. 0.52±0.33 μg/L, respectively - P=0.07), both within the normal range. After RYGB, the simultaneous change in expression of GI genes, may impair carotenoid metabolism in the enterocytes, formation of nascent chylomicrons and transport of retinol, resulting in lower availability of vitamin A.

Microalgae-derived sterols do not reduce the bioavailability of oral vitamin D3 in mice

Microalgae have drawn increasing attention as sustainable food sources, also because of their lipid-lowering phytosterols. As phytosterols are also discussed critically regarding their effect on the availability of fat-soluble vitamins, this study aimed to investigate microalgae-derived phytosterols and their effect on vitamin D status. GC-MS analysis showed large variations in the phytosterol profiles of microalgal species. The most frequent sterols were β-sitosterol and stigmasterol. To investigate their effects on vitamin D status, 40 mice were randomized to four groups and fed a vitamin D3-adequate (25 μg/kg) Western-style diet with 0% phytosterols (control) or 1% ergosterol (a fungal sterol not typical for microalgae), β-sitosterol or stigmasterol for four weeks. Contrary to the hypothesis that phytosterols adversely affect vitamin D uptake, mice fed β-sitosterol had significantly higher concentrations of vitamin D3 in plasma (3.15-fold, p<0.01), liver (3.15-fold, p<0.05), and skin (4.12-fold, p<0.005) than the control group. Small increases in vitamin D3 in plasma and skin were also observed in mice fed stigmasterol. In contrast, vitamin D3 levels in the ergosterol and control groups did not differ. The increased tissue levels of vitamin D3 in mice fed β-sitosterol and stigmasterol were not attributable to the observed reduction in liver triglycerides in these groups. The data rather suggest that changes in bile acid profiles were responsible for the beneficial effect of microalgae sterols on the bioavailability of vitamin D3. In conclusion, consumption of microalgae might not adversely affect vitamin D status.

Intestinal Absorption and Anti-Inflammatory Effects of Siphonein, a Siphonaxanthin Fatty Acid Ester from Green Algae

Siphonein is a C19 acylated siphonaxanthin found in some edible green algae (e.g., Codium fragile and Caulerpa lentillifera). Although the content of siphonein in these green algae is similar to or higher than that of siphonaxanthin, studies of health-related biological activity of siphonein are much less than those of siphonaxanthin. Given the difference in the position of the acyl chain, one cannot infer intestinal absorption of siphonein from other general carotenoid fatty acid esters. In this study, we first investigated the intestinal absorption of siphonein using mouse and cell culture models. A small amount of siphonein was detected in the plasma of treated mice, and its concentration was higher than that of siphonaxanthin (i.e., the hydrolyzed product of ingested siphonein) from 1 to 6 h after administration. Pharmacological inhibition tests with differentiated Caco-2 cells showed that Nieman-Pick C1-like 1-mediated facilitated diffusion was involved in the cellular uptake of siphonein. These results indicate that, unlike general carotenoid fatty acid esters, siphonein can be absorbed without hydrolysis. We also evaluated the anti-inflammatory effect of siphonein in differentiated Caco-2 cells. Siphonein pretreatment modulated lipopolysaccharide-induced cellular lipidome alterations and suppressed mRNA expression of proinflammatory chemokines, CXCL8 protein release, and activation of NF-κB. This study provides new insights into the absorption processes of carotenoids and shows the anti-inflammatory effect of siphonein for the first time.

Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade

It is now well established that vitamins D, E, and K and carotenoids are not absorbed solely through passive diffusion. Broad-specificity membrane transporters such as SR-BI (scavenger receptor class B type I), CD36 (CD36 molecule), NPC1L1 (Niemann Pick C1-like 1) or ABCA1 (ATP-binding cassette A1) are involved in the uptake of these micronutrients from the lumen to the enterocyte cytosol and in their secretion into the bloodstream. Recently, the existence of efflux pathways from the enterocyte back to the lumen or from the bloodstream to the lumen, involving ABCB1 (P-glycoprotein/MDR1) or the ABCG5/ABCG8 complex, has also been evidenced for vitamins D and K. Surprisingly, no membrane proteins have been involved in dietary vitamin A uptake so far. After an overview of the metabolism of fat-soluble vitamins and carotenoids along the gastrointestinal tract (from the mouth to the colon where interactions with microbiota may occur), a focus is placed on the identified and candidate proteins participating in the apical uptake, intracellular transport, basolateral secretion and efflux back to the lumen of fat-soluble vitamins and carotenoids in enterocytes. This review also highlights the mechanisms that remain to be identified to fully unravel the pathways involved in fat-soluble vitamin and carotenoid intestinal absorption.

Tachysterol2 increases the synthesis of fibroblast growth factor 23 in bone cells

Tachysterol₂ (T₂) is a photoisomer of the previtamin D₂ found in UV-B-irradiated foods such as mushrooms or baker’s yeast. Due to its structural similarity to vitamin D, we hypothesized that T₂ can affect vitamin D metabolism and in turn, fibroblast growth factor 23 (FGF23), a bone-derived phosphaturic hormone that is transcriptionally regulated by the vitamin D receptor (VDR). Initially, a mouse study was conducted to investigate the bioavailability of T₂ and its impact on vitamin D metabolism and Fgf23 expression. UMR106 and IDG-SW3 bone cell lines were used to elucidate the effect of T₂ on FGF23 synthesis and the corresponding mechanisms. LC-MS/MS analysis found high concentrations of T₂ in tissues and plasma of mice fed 4 vs. 0 mg/kg T₂ for 2 weeks, accompanied by a significant decrease in plasma 1,25(OH)₂D and increased renal Cyp24a1 mRNA abundance. The Fgf23 mRNA abundance in bones of mice fed T₂ was moderately higher than that in control mice. The expression of Fgf23 strongly increased in UMR106 cells treated with T₂. After Vdr silencing, the T₂ effect on Fgf23 diminished. This effect is presumably mediated by single-hydroxylated T₂-derivatives, since siRNA-mediated silencing of Cyp27a1, but not Cyp27b1, resulted in a marked reduction in T₂-induced Fgf23 gene expression. To conclude, T₂ is a potent regulator of Fgf23 synthesis in bone and activates Vdr. This effect depends, at least in part, on the action of Cyp27a1. The potential of oral T₂ to modulate vitamin D metabolism and FGF23 synthesis raises questions about the safety of UV-B-treated foods.

UVB-exposed wheat germ oil increases serum 25-hydroxyvitamin D2 without improving overall vitamin D status: a randomized controlled trial

PURPOSE

This study investigated whether UVB-exposed wheat germ oil (WGO) is capable to improving the vitamin D status in healthy volunteers.

METHODS

A randomized controlled human-intervention trial in parallel design was conducted in Jena (Germany) between February and April. Ultimately, 46 healthy males and females with low mean 25-hydroxyvitamin D (25(OH)D) levels (34.9 ± 10.6 nmol/L) were randomized into three groups receiving either no WGO oil (control, n = 14), 10 g non-exposed WGO per day (- UVB WGO, n = 16) or 10 g WGO, which was exposed for 10 min to ultraviolet B-light (UVB, intensity 500-630 µW/cm2) and provided 23.7 µg vitamin D (22.9 µg vitamin D2 and 0.89 µg vitamin D3) (+ UVB WGO, n = 16) for 6 weeks. Blood was obtained at baseline, after 3 and 6 weeks and analyzed for serum vitamin D-metabolite concentrations via LC-MS/MS.

RESULTS

Participants who received the UVB-exposed WGO were characterized by an increase of circulating 25(OH)D2 after 3 and 6 weeks of intervention. However, the 25(OH)D3 concentrations decreased in the + UVB WGO group, while they increased in the control groups. Finally, the total 25(OH)D concentration (25(OH)D2 + 25(OH)D3) in the + UVB WGO group was lower than that of the non-WGO receiving control group after 6 weeks of treatment. In contrast, circulating vitamin D (vitamin D2 + vitamin D3) was higher in the + UVB WGO group than in the control group receiving no WGO.

CONCLUSION

UVB-exposed WGO containing 23.7 µg vitamin D can increase 25(OH)D2 levels but do no improve total serum levels of 25(OH)D of vitamin D-insufficient subjects.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT03499327 (registered, April 13, 2018).

Bone Health in Patients with Dyslipidemias: An Underestimated Aspect

Beyond being aging-related diseases, atherosclerosis and osteoporosis share common pathogenetic pathways implicated in bone and vascular mineralization. However, the contributory role of dyslipidemia in this interplay is less documented. The purpose of this narrative review is to provide epidemiological evidence regarding the prevalence of bone disease (osteoporosis, fracture risk) in patients with dyslipidemias and to discuss potential common pathophysiological mechanisms linking osteoporosis and atherosclerosis. The effect of hypolipidemic therapy on bone metabolism is also discussed. Despite the high data heterogeneity and the variable quality of studies, dyslipidemia, mainly elevated total and low-density lipoprotein cholesterol concentrations, is associated with low bone mass and increased fracture risk. This effect may be mediated directly by the increased oxidative stress and systemic inflammation associated with dyslipidemia, leading to increased osteoclastic activity and reduced bone formation. Moreover, factors such as estrogen, vitamin D and K deficiency, and increased concentrations of parathyroid hormone, homocysteine and lipid oxidation products, can also contribute. Regarding the effect of hypolipidemic medications on bone metabolism, statins may slightly increase BMD and reduce fracture risk, although the evidence is not robust, as it is for omega-3 fatty acids. No evidence exists for the effects of ezetimibe, fibrates, and niacin. In any case, more prospective studies are needed further to elucidate the association between lipids and bone strength.

Safety Assessment of Vitamin D and Its Photo-Isomers in UV-Irradiated Baker's Yeast

Vitamin D deficiency due to, e.g., nutritional and life style reasons is a health concern that is gaining increasing attention over the last two decades. Vitamin D₃, the most common isoform of vitamin D, is only available in food derived from animal sources. However, mushrooms and yeast are rich in ergosterol. This compound can be converted into vitamin D₂ by UV-light, and therefore act as a precursor for vitamin D. Vitamin D₂ from UV-irradiated mushrooms has become an alternative source of vitamin D, especially for persons pursuing a vegan diet. UV-irradiated baker’s yeast (Saccharomyces cerevisiae) for the production of fortified yeast-leavened bread and baked goods was approved as a Novel Food Ingredient in the European Union, according to Regulation (EC) No. 258/97. The Scientific Opinion provided by the European Food Safety Authority Panel on Dietetic Products, Nutrition, and Allergies has assessed this Novel Food Ingredient as safe under the intended nutritional use. However, recent findings on the formation of side products during UV-irradiation, e.g., the photoproducts tachysterol and lumisterol which are compounds with no adequate risk assessment performed, have only been marginally considered for this EFSA opinion. Furthermore, proceedings in analytics can provide additional insights, which might open up new perspectives, also regarding the bioavailability and potential health benefits of vitamin D-fortified mushrooms and yeast. Therefore, this review is intended to give an overview on the current status of UV irradiation in mushrooms and yeast in general and provide a detailed assessment on the potential health effects of UV-irradiated baker’s yeast.

Molecular cloning and characterization of NPC1L1 in the Chinese tree shrew (Tupaia belangeri chinensis)

BACKGROUND

The Niemann-Pick C1-Like 1 protein, a multi-transmembrane domain molecule, is critical for intestinal cholesterol absorption, and is the entry factor for hepatitis C virus (HCV). The Chinese tree shrew (Tupaia belangeri chinensis) is closer to primates in terms of genetic evolution than rodents. Previous studies indicated that the tree shrew was suitable for HCV research; however, little is known about tree shrew NPC1L1.

METHODS AND RESULTS

TsNPC1L1 cDNA was amplified by rapid amplification of cDNA ends (RACE) technology. The cDNA sequence, its encoded protein structure, and expression profile were analyzed. Results indicated that the tsNPC1L1 mRNA is 4948 bp in length and encodes a 1326 amino acid protein. TsNPC1L1 possesses 84.97% identity in homology to human NPC1L1 which is higher than both mouse (80.37%) and rat (81.80%). The protein structure was also similar to human with 13 conserved transmembrane helices, and a sterol-sensing domain (SSD). Like human NPC1L1, the tsNPC1L1 mRNA transcript is highly expressed in small intestine, but it was also well-expressed in the lung and pancreas of the tree shrew.

CONCLUSION

The homology of tree shrew NPC1L1 was closer to human than that of rodent NPC1L1. The expression of tsNPC1L1 was the highest in small intestine, and was detectable in lung and pancreas. These results may be useful in the study of tsNPC1L1 function in cholesterol absorption and HCV infection.

The Complex ABCG5/ABCG8 Regulates Vitamin D Absorption Rate and Contributes to its Efflux from the Intestine

SCOPE

Most people are vitamin D insufficient around the world. Vitamin D intestinal absorption should thus be optimized. The role of the ATP-binging cassette G5/G8 (ABCG5/G8) heterodimer in vitamin D intestinal efflux is investigated.

METHODS AND RESULTS

Both cholecalciferol and 25-hydroxycholecalciferol apical effluxes are increased by ABCG5/G8 overexpression in human Griptite cells. Mice deficient in ABCG5/G8 at the intestinal level (I-Abcg5/g8^-/- mice) display an accumulation of cholecalciferol in plasma in females and in liver in males compared to control animals. I-Abcg5/g8^{- / -} mice display a delay in cholecalciferol postprandial response after gavage compared with controls. 25-Hydroxycholecalciferol transfer from plasma to lumen is observed in vivo in intestine-perfused mice, and the lack of intestinal ABCG5/G8 complex induces a decrease in this efflux, while vitamin D bile excretion remains unchanged.

CONCLUSION

Overall, it is showed for the first time that the ABCG5/G8 heterodimer regulates the kinetics of absorption of dietary vitamin D by contributing to its efflux back to the lumen, and that it also participates in vitamin D transintestinal efflux.

Oral Intake of Lumisterol Affects the Metabolism of Vitamin D

SCOPE

The treatment of food with ultraviolet-B (UV-B) light to increase the vitamin D content is accompanied by the formation of photoisomers, such as lumisterol2 . The physiological impact of photoisomers is largely unknown.

METHODS AND RESULTS

Three groups of C57Bl/6 mice are fed diets containing 50 µg kg-1 deuterated vitamin D3 with 0, 50 (moderate-dose) or 2000 µg kg-1 (high-dose) lumisterol2 for four weeks. Considerable quantities of lumisterol2 and vitamin D2 are found in the plasma and tissues of mice fed with 2000 µg kg-1 lumisterol2 but not in those fed 0 or 50 µg kg-1 lumisterol2 . Mice fed with 2000 µg kg-1 lumisterol2 showed strongly reduced deuterated 25-hydroxyvitamin D3 (-50%) and calcitriol (-80%) levels in plasma, accompanied by downregulated mRNA abundance of cytochrom P450 (Cyp)27b1 and upregulated Cyp24a1 in the kidneys. Increased tissue levels of vitamin D2 were also seen in mice in a second study that are kept on a diet with 0.2% UV-B exposed yeast versus those fed 0.2% untreated yeast containing iso-amounts of vitamin D2 .

CONCLUSION

High doses of lumisterol2 can enter the body, induce the formation of vitamin D2 , reduce the levels of 25(OH)D3 and calcitriol and strongly impact the expression of genes involved in the degradation and synthesis of bioactive vitamin D.

Active vitamin D is cardioprotective in experimental uraemia but not in children with CKD Stages 3-5

BACKGROUND

Uraemic cardiac remodelling is associated with vitamin D and Klotho deficiency, elevated fibroblast growth factor 23 (FGF23) and activation of the renin-angiotensin system (RAS). The cardioprotective properties of active vitamin D analogues in this setting are unclear.

METHODS

In rats with 5/6 nephrectomy (5/6Nx) treated with calcitriol, the cardiac phenotype and local RAS activation were investigated compared with controls. A nested case-control study was performed within the Cardiovascular Comorbidity in Children with Chronic Kidney Disease (4C) study, including children with chronic kidney disease (CKD) Stages 3-5 [estimated glomerular filtration rate (eGFR) 25 mL/min/1.73 m2] treated with and without active vitamin D. Echocardiograms, plasma FGF23 and soluble Klotho (sKlotho) were assessed at baseline and after 9 months.

RESULTS

In rats with 5/6Nx, left ventricular (LV) hypertrophy, LV fibrosis and upregulated cardiac RAS were dose-dependently attenuated by calcitriol. Calcitriol further stimulated FGF23 synthesis in bone but not in the heart, and normalized suppressed renal Klotho expression. In the 4C study cohort, treatment over a mean period of 9 months with active vitamin D was associated with increased FGF23 and phosphate and decreased sKlotho and eGFR compared with vitamin D naïve controls, whereas LV mass index did not differ between groups.

CONCLUSIONS

Active vitamin D ameliorates cardiac remodelling and normalizes renal Klotho expression in 5/6Nx rats but does not improve the cardiac phenotype in children with CKD Stages 3-5. This discrepancy may be due to further enhancement of circulating FGF23 and faster progression of CKD associated with reduced sKlotho and higher serum phosphate in vitamin D-treated patients.

Vitamin D Status of Mice Deficient in Scavenger Receptor Class B Type 1, Cluster Determinant 36 and ATP-Binding Cassette Proteins G5/G8

Classical lipid transporters are suggested to modulate cellular vitamin D uptake. This study investigated the vitamin D levels in serum and tissues of mice deficient in SR-B1 (Srb1^-/-), CD36 (Cd36^-/-) and ABC-G5/G8 (Abcg5/g8^-/-) and compared them with corresponding wild-type (WT) mice. All mice received triple-deuterated vitamin D₃ (vitamin D₃-d₃) for six weeks. All knockout mice vs. WT mice showed specific alterations in their vitamin D concentrations. Srb1^-/- mice had higher levels of vitamin D₃-d₃ in the serum, adipose tissue, kidney and heart, whereas liver levels of vitamin D₃-d₃ remained unaffected. Additionally, Srb1^-/- mice had lower levels of deuterated 25-hydroxyvitamin D₃ (25(OH)D₃-d₃) in the serum, liver and kidney compared to WT mice. In contrast, Cd36^-/- and WT mice did not differ in the serum and tissue levels of vitamin D₃-d₃, but Cd36^-/- vs. WT mice were characterized by lower levels of 25(OH)D₃-d₃ in the serum, liver and kidney. Finally, Abcg5/g8^-/- mice tended to have higher levels of vitamin D₃-d₃ in the serum and liver. Major alterations in Abcg5/g8^-/- mice were notably higher levels of 25(OH)D₃-d₃ in the serum and kidney, accompanied by a higher hepatic mRNA abundance of Cyp27a1 hydroxylase. To conclude, the current data emphasize the significant role of lipid transporters in the uptake, tissue distribution and activation of vitamin D.

Markers Indicating Body Vitamin D Stores and Responses of Liver and Adipose Tissues to Changes in Vitamin D Intake in Male Mice

Circulating 25-hydroxyvitamin D (25(OH)D) is regarded as the most reliable biomarker of vitamin D status. However, limited data exist concerning the suitability of 25(OH)D as an indicator of body vitamin D stores and the ability of adipose tissue to mobilize vitamin D. In the first study, in which male mice received different vitamin D₃ doses for three weeks, we found strong linear response relationships between vitamin D₃ intake and levels of vitamin D₃ in the plasma (p < 0.001), liver (p < 0.001) and adipose tissues (p < 0.001), and strong positive correlations between plasma and tissue stores of vitamin D₃ (p < 0.001). Plasma levels of 25(OH)D₃ and 24,25-dihydroxyvitamin D₃ (24,25(OH)₂D₃) showed weak or no correlations with tissue vitamin D₃ stores. Data from a second study demonstrate a strong and rapid response of plasma 25(OH)D₃ in vitamin D₃-treated mice with a low vitamin D status. Additionally, mice fed a vitamin D-free diet showed a strong and rapid decline in vitamin D₃ in the liver, whereas the decline in different adipose tissues was distinctly lower than that in the liver. To conclude, tissue stores of vitamin D₃ were best reflected by plasma vitamin D₃. In contrast to the liver, adipose tissues responded less sensitively to an absence of vitamin D intake.