Biological activity of vitamin D metabolites and analogs: dose-response study of 45Ca transport in an isolated chick duodenum perfusion system

We have previously reported that vascular perfusion of the normal vitamin D3-replete chick duodenum with physiological amounts of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] increases the unidirectional movement of 45Ca from the lumen to the venous effluent under conditions of normal (0.9 mM) Ca2+ concentrations in both the lumen and vascular perfusate [Endocrinology 115: 1476 1984)]. The purpose of the present study was to determine the dose responsivity of this perfused intestinal calcium transport system for 1,25(OH)2D3 and some structurally related congeners. The dose-response curve was biphasic for all compounds studied; for 1,25(OH)2D3 initial stimulation of transport was detected at only 30 pM [the plasma concentration of 1,25(OH)2D3 is normally 125 pM] while maximal stimulation was 154% above control at a concentration of 650 pM. Above 650 pM 1,25(OH)2D3 the stimulation fell off sharply and transport had returned to basal levels by 1.3 nM. The relative potency of the D homologs tested was respectively 1,25(OH)2D3: 10,000; 1-alpha-hydroxyvitamin D3: 400; 25-hydroxyvitamin D3: 200; 24R,25-dihydroxy-vitamin D3: 137; vitamin D3: 34; 5,6-trans-25-hydroxyvitamin D3: 3. These results establish the usefulness of the perfused intestinal calcium transport system to study the nongenomic actions of 1,25(OH)2D3 on intestinal calcium transport.     

Effect of ascorbic acid on 25-hydroxyvitamin D3 metabolism in the kidneys and 1,25-dihydroxyvitamin D3 reception in the small intestine mucosa of guinea pigs

Ascorbic acid deficiency in vitamin D-supplied guinea pigs caused a moderate decrease of Ca in the blood and osseous tissue, a 1.5-fold decrease of 2.5-hydroxyvitamin D (25-OH D) in blood serum, a 2-fold decrease of the 25-OH D 1-hydroxylase activity in kidneys and a 1.6-fold increase of the 24-hydroxylase activity. The concentration of 1.25-dihydroxyvitamin D3 (1.25-(OH)2D3) nuclear receptors in small intestinal mucosa diminished by 20-30%; in this case the percentage of occupied hormone receptors reduced from 11.8 to 8.6%. The affinity of receptors for 1.25-(OH)2D3 did not change thereby (Kd = 0.25-0.26 nM; Kd2 = 0.06-0.10 nM). At the same time the value of cooperativity coefficient showed a decrease-from 1.7 to 1.4, which was accompanied by a reduction of the maximum capacity of receptors (1.2-1.5-fold). Vitamin C depletion augmented the manifestation of vitamin D deficiency in guinea pigs and impeded their correction after administration of cholecalciferol. This markedly retarded the restoration of the 25-OH D level in the blood as well as the number of occupied and unoccupied nuclear receptors for 1.25-(OH)2D3. The experimental results illustrate the effects of ascorbic acid on the vitamin D hormonal system function, which is manifested both at the level of 1.25-(OH)2D3 synthesis in the kidneys and of its receptor binding in target tissues.     

Up-regulation of the intestinal 1,25-dihydroxyvitamin D receptor during hypervitaminosis D: a comparison between vitamin D2 and vitamin D3

Concentrations of intestinal 1,25-dihydroxyvitamin D receptor were measured in rats receiving pharmacological amounts (25,000 IU/rat daily for 6 days) of either vitamin D2 or vitamin D3. The data showed that both hypervitaminosis D2 and hypervitaminosis D3 resulted in significant up-regulation of intestinal 1,25-dihydroxyvitamin D receptor (fmol/mg protein) relative to controls (409 +/- 24, vitamin D2-treated; 525 +/- 41, vitamin D3-treated; and 249 +/- 19, control). The 1,25-dihydroxyvitamin D receptor enhancement also was accompanied by elevated plasma 25-hydroxyvitamin D and hypercalcemia. These data suggest that increased target-tissue 1,25-dihydroxyvitamin D receptor may play a role in enhancing target-tissue responsiveness and, thus, have a significant role in mediating the toxic effects of hypervitaminosis D.     

Effect of age and the X-linked Hyp mutation on renal adaptation to vitamin D and calcium deficiency

The age (4 weeks vs 5 weeks vs 6.5 weeks) at which dietary restriction of vitamin D and calcium is initiated has a profound effect on the resulting concentration of serum calcium, urinary cAMP and on renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) activity in normal (+/Y) mice; no such age relationship is apparent in Hyp/Y littermates. After 40 days on the restrictive diet, it was found that the younger the +/Y mice at the time of diet initiation, the lower the resulting serum calcium (4 weeks less than 5 weeks less than 6.5 weeks) and the higher the urinary cAMP and 1-hydroxylase activity (4 weeks greater than 5 weeks greater than 6.5 weeks). Age on diet has no effect on serum phosphate and fractional excretion index of phosphate in +/Y and Hyp/Y littermates. Renal 1-hydroxylase activity is significantly lower than normal in the younger groups of Hyp/Y mice whereas 24-hydroxylase (25-hydroxyvitamin D3-24-hydroxylase) activity is higher than normal in all groups of Hyp/Y mice.     

Renal parathyroid hormone-dependent adenylate cyclase in vitamin D-deficient rats. Inhibition by hydroxylated vitamin D3 metabolites

The adenylate cyclase activation by bovine synthetic parathyroid hormone (bPTH) (1-34) was studied in vitro in kidney plasma membranes from D-deficient (D-Mb) or normal (D+Mb) rats. In D-Mb, the apparent affinity of parathyroid hormone (PTH) for membranes (170 +/- 30 nM) was significantly higher than that measured in D+Mb (55 +/- 5 nM). The maximum velocity of the PTH-stimulated adenylate cyclase was significantly higher in D+Mb than in D-Mb (163.0 +/- 13.7 and 93.4 +/- 6.7 pmol of cAMP/mg of protein/min, respectively). The action of vitamin D metabolites on the adenylate cyclase stimulation by PTH was then studied in vitro in D-Mb and D+Mb. In D-Mb, 25-hydroxyvitamin D3, 24,25-, and 1, 25-dihydroxyvitamin D3 significantly inhibited cAMP production in the presence of 0.87 microM of bPTH. Vitamin D3 had no effect. Maximal inhibition (86%) was observed for 1,25-dihydroxyvitamin D3. 1,25-Dihydroxyvitamin D3 decreased the maximum velocity of PTH-stimulated adenylate cyclase but did not modify the bPTH apparent affinity for D-Mb. The vitamin D3 metabolites tested did not modify the cyclase stimulation by isoproterenol, sodium fluoride, or 5'-guanylylimidodiphosphate. The presence of 1,25-dihydroxyvitamin D3 or 25-hydroxyvitamin D3 did not increase the (Na-K)-ATPase or the phosphodiesterase activities. In the presence of 1,25-dihydroxyvitamin D3 and bPTH, the apparent affinity of ATP for the catalytic moiety was not modified. The maximum velocity was decreased. These results suggest an in vitro interaction between hydroxylated vitamin D metabolites and kidney membranes PTH receptor.     

Synthesis, biologic activity, and protein binding characteristics of a new vitamin D analog, 22-hydroxyvitamin D3

We synthesized a novel vitamin D analog, 22-hydroxyvitamin D3 9 and tested its biologic activity (and antivitamin properties) in vivo in vitamin D-deficient rats, and in vitro in the chick embryonic duodenum. We examined its ability to bind to the sterol carrier protein, vitamin D binding protein and the chick intestinal cytosol receptor for 1,25-dihydroxyvitamin D3. The new vitamin 9 was synthesized from 3 beta-hydroxy-22,23-dinorcholenic acid 1 in 12 steps. The vitamin 9 displayed no vitamin D agonist activity in the intestine or in bone in vivo and did not block the activity of vitamin D3 or 25-hydroxyvitamin D3. It was a weak vitamin D3 agonist in the chick embryonal duodenum in vitro. It did not antagonize the activity of 1,25-dihydroxyvitamin D3. Vitamin 9 bound to the chick intestinal cytosol receptor with low affinity. 22-Hydroxyvitamin D3 and various vitamin D sterols were bound to vitamin D binding protein in the following order: 25-hydroxyvitamin D3. (24R)-24,25-dihydroxyvitamin D3, and (25S)-25,26-dihydroxyvitamin D3 greater than 22-hydroxyvitamin D3 greater than 11 alpha-hydroxyvitamin D3 greater than 1,25-dihydroxyvitamin D3 greater than vitamin D3. We conclude that the introduction of a hydroxyl group at C-22 in the side chain of the vitamin D3 molecule decreases its biological activity.     

Vitamin D3 and 1,25-dihydroxyvitamin D3 stimulate the skeletal muscle-calcium mobilization in rachitic chicks

The Ca content in skeletal muscle relative to vitamin D3 intake was studied in chicks. It was found that the Ca content in rachitic chick muscle was significantly higher than normal and it decreased with vitamin D3 treatment. In 4-week-old chicks fed a vitamin D-deficient diet, the Ca content in leg muscle reached 9.86 +/- 1.07 mg/100 g wet wt, although in chicks receiving vitamin D3 in doses of 100 and 500 IU/kg diet, it was 7.80 +/- 0.72 and 6.08 +/- 0.61 mg/100 g wet wt, respectively. A single i.m. dose of 0.50 micrograms of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or vitamin D3 caused a dramatic decrease in the muscle Ca content by 3 to 6 h after the injection. A simultaneous rise in the Ca level in blood serum was observed. However, at this time the Ca binding protein content in duodenal mucosa and the stimulation of Ca absorption were negligible. These findings allow the conclusion that the vitamin D deficiency in chicks leads to a surplus Ca accumulation in skeletal muscle. The administration of vitamin D3 or its metabolites causes rapid Ca release during the first 6 h. This may be the source of the Ca level increase in blood serum. In this respect 1,25(OH)2D3 was much more effective than vitamin D3.     

25-Hydroxyvitamin D3: evidence of an enterohepatic circulation in man.

Within 24 hr after intravenous administration of isotopic 25-hydroxyvitamin D3 to three normal adults for kinetic studies, one-third of the radioactivity was secreted into the lumen of the duodenum, probably with the bile. The subsequent intestinal reabsorption of over 85% of secreted radioactivity suggests that this major metabolite of vitamin D has a hitherto unrecognized enterohepatic circulation. Our observation of a dynamic hepatic secretion and intestinal reabsorption of radioactivity administered as 3H-labeled 25-hydroxyvitamin D3 to vitamin D-replete man is indicative of an enterohepatic circulation that may be of physiologic importance. It is conceivable that interruption in the recycling of 25-OH-D3 may be an important mechanism of acquired deficiency of vitamin D in gastrointestinal disease.     

Antiproteinuric treatment reduces urinary loss of vitamin D-binding protein but does not affect vitamin D status in patients with chronic kidney disease

Vitamin D deficiency is common in chronic kidney disease (CKD). Increased urinary loss of vitamin D binding protein (VDBP), the main transporter of 25-hydroxyvitamin D(3) in the circulation, has been postulated to contribute to vitamin D deficiency in proteinuria. To test this hypothesis we analyzed urinary and plasma levels of VDBP, 25-hydroxyvitamin D(3) and 1,25-dihydroxyvitamin D(3) from proteinuric patients, before and after antiproteinuric interventions. We performed a post-hoc analysis of a clinical trial in CKD patients (n=13, creatinine clearance median 60 (range 25-177) ml/min) subjected to the following study periods: washout (no antiproteinuric treatment, 4 weeks), lisinopril 40mg QD (ACEi, 6 weeks), or indomethacin 75mg BID (NSAID, 4 weeks) in randomized sequence. Healthy subjects screened for donation (n=10) served as controls. Plasma and urine VDBP levels were measured by ELISA, 25-hydroxyvitamin D(3) levels by LC-MS and 1,25-dihydroxyvitamin D(3) levels by radioimmunoassay. In CKD patients urinary VDBP excretion was strongly increased (median (range) 5413 (155-211,027) μg/24h) as compared to healthy controls (64 (23-111) μg/24h, p<0.001). Both NSAID and ACEi significantly decreased urinary VDBP excretion, in proportion to proteinuria reduction. Plasma VDBP, 25-hydroxyvitamin D(3) and 1,25-dihydroxyvitamin D(3) levels, however, were similar between patients and controls and not affected by antiproteinuric intervention. Urinary VDBP excretion is markedly increased in proteinuria and responds to antiproteinuric treatment. Urinary VDBP loss is not associated with plasma VDBP or vitamin D(3) levels, suggesting that urinary loss of VDBP does not affect vitamin D status.     

6-Fluoro-vitamin D3: a new antagonist of the biological actions of vitamin D3 and its metabolites which interacts with the intestinal receptor for 1 alpha,25(OH)2-vitamin D3

The biological activity and the binding affinity for the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] intestinal receptor of a new fluorine-containing vitamin D compound, namely 6-fluoro-vitamin D3 (6-F-D3), is reported. A significant interaction of 6-F-D3 with the 1,25(OH)2D3 receptor was found, with a relative competitive index (RCI) of 0.26 +/- 0.04, which is intermediate between 25-hydroxyvitamin D3 (0.14 +/- 0.01) and 1 alpha-hydroxyvitamin D3 (0.46 +/- 0.08), where the RCI of 1,25(OH)2D3 is defined to be 100. In contrast, vitamin D3 was unable to interact with the 1,25(OH)2D3 receptor. Also, the biological activity of 6-F-D3 was assessed in vivo in the vitamin D-deficient chick. 6-F-D3 at doses up to 130 nmol displayed no biological action on either intestinal calcium absorption (ICA) or bone calcium mobilization (BCM) over the time interval of 14-48 h after dosing. However, when 130 nmol 6-F-D3 was given 2 h before and 6 h after vitamin D3 (1.62 nmol), a significant inhibition of vitamin D-mediated ICA was noted. Also, a dose of 130 nmol 6-F-D3 given 2 h before and 6 h after 1,25(OH)2D3 (0.26 nmol) significantly inhibited ICA, as measured at 12 h. 6-F-D3 is the first vitamin D analog found which has an ability to both bind to the 1,25(OH)2D3 receptor and to antagonize the production of biological responses by 1,25(OH)2D3.     

Metabolism of 25-hydroxyvitamin D3 and its regulation in rats during hypokinesis

The influence of short-(7 days) and long-term (28 days) hypokinesia on 25-hydroxyvitamin D3 metabolism was investigated in rats fed on a normal calcium (0.6%), normal phosphorus (0.6%), vitamin D-supplemented diet. The animals were given a single intraperitoneal dose of tritiated [26,27-3H]25(OH)D3 (200 pmol) eighteen hours before sacrifice. [3H]Labelled vitamin D3 metabolites were separated by high performance liquid chromatographic procedure, and their radioactivity levels in serum, kidney, intestinal mucosa and femoral bone were measured. Long-term hypokinesia resulted in decreased levels of [3H]1.25(OH)2D3 and increased levels of [3H]24.25(OH)2D3 in serum and kidney (3.15 +/- 0.62 vs. 4.33 +/- 0.41% and 5.34 +/- 0.69 vs. 3.76 +/- 0.29% for [3H]1.25(OH)2D3 and [3H]24.25(OH)2D3 in serum; 7.52 +/- 0.69 vs. 11.6 +/- 0.79% and 9.33 +/- 0.55 vs. 5.94 +/- 0.24% for those in kidney). The levels of [3H]1.25(OH)2D3 as well as of [3H] 24.25(OH)2D3 were decreased in intestinal mucosa and bone (21.5 +/- 1.46 vs. 30.1 +/- 3.04% and 7.30 +/- 0.58 vs. 9.18 +/- 0.78% for [3H]1.25(OH)2D3 and [3H]24.25(OH)2D3 in intestinal mucosa; 6.39 +/- 06.5 vs. 11.5 +/- 1.64% and 7.78 +/- 0.71 vs. 13.9 +/- 1.28% for those in bone). The data obtained suggest a suppressed synthesis of 1.25(OH)2D3 and enhanced production of 24.25(OH)2D3 in kidney as well as a diminished binding of 24.25(OH)2D3 in intestinal mucosa and bone in hypothetic rats. Possible causes of variations in biosynthesis of vitamin D3 active metabolites, and role of these variations in the disorders of calcium metabolism and bone state during hypokinesia are discussed.     

On the specificity of vitamin D compounds binding to chick pig intestinal 1,25-dihydroxyvitamin D3 receptor

The binding activity of four vitamin D metabolites and/or analogs for the intestinal 1,25-dihydroxyvitamin D3 receptor was evaluated after incubation at 25 degrees C for 1 h or at 0-4 degrees C for 18 h. The incubation conditions, which had no effect on the binding of 1,25-dihydroxyvitamin D3, had a dramatic effect on the binding of 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3 and a small but reproducible effect on 24,25-dihydroxyvitamin D3 binding to receptor. Affinities 10- to 20-fold higher were obtained for 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3, and affinities 3-fold higher were obtained for 24,25-dihydroxyvitamin D3 at the 0-4 degrees C/18-h incubation. A comparison of intestinal receptor from chick and pig with nine vitamin D compounds showed no major differences between the two species. The relative affinity of the vitamin D analogs to compete with tritiated 1,25-dihydroxyvitamin D3 for the receptor in pig nuclear extract, expressed as ratios of the molar concentration required for 50% binding of the tritiated 1,25-dihydroxyvitamin D3 compared to nonradioactive 1,25-dihydroxyvitamin D3, are as follows: 1,25-dihydroxyvitamin D3 (1) = 1,25-dihydroxyvitamin D2 = 24-homo-1,25-dihydroxyvitamin D3 greater than 1,24,25-trihydroxyvitamin D3 (4) greater than 25-hydroxyvitamin D3 (21) = 10-oxo-19-nor-25-hydroxyvitamin D3 = 1 alpha-hydroxyvitamin D3 (37) greater than 24,25-dihydroxyvitamin D2 (257) much much greater than vitamin D3 (greater than 10(6)).     

1 alpha-hydroxylation of 24-hydroxyvitamin D2 represents a minor physiological pathway for the activation of vitamin D2 in mammals

C24-Hydroxylation was evaluated as a possible activation pathway for vitamin D2 and vitamin D3. Routine assays showed that 24-hydroxyvitamin D2 and 1,24-dihydroxyvitamin D2 could be detected in rats receiving physiological doses (100 IU/day) of vitamin D2; however, 24-hydroxyvitamin D3 could not be detected in rats receiving similar doses of vitamin D3. In rats, 24-hydroxyvitamin D2 was very similar to 25-hydroxyvitamin D2 at stimulating intestinal calcium transport and bone calcium resorption. The biological activity of 24-hydroxyvitamin D2 was eliminated by nephrectomy, suggesting that 24-hydroxyvitamin D2 must undergo 1 alpha-hydroxylation to be active at physiological doses. In vivo experiments suggested that when given individually to vitamin D deficient rats, 24-hydroxyvitamin D2, 25-hydroxyvitamin D2, and 25-hydroxyvitamin D3 were 1 alpha-hydroxylated with the same efficiency. However, when presented simultaneously, 24-hydroxyvitamin D2 was less efficiently 1 alpha-hydroxylated than either 25-hydroxyvitamin D3 or 25-hydroxyvitamin D2. 1,24-Dihydroxyvitamin D2 was also approximately 2-fold less competitive than either 1,25-dihydroxyvitamin D2 or 1,25-dihydroxyvitamin D3 for binding sites on the bovine thymus 1,25-dihydroxyvitamin D receptor. These results demonstrate that 24-hydroxylation followed by 1 alpha-hydroxylation of vitamin D2 represents a minor activation pathway for vitamin D2 but not vitamin D3.     

Photoaffinity labeling of serum vitamin D binding protein by 3-deoxy-3-azido-25-hydroxyvitamin D3

3-Deoxy-3-azido-25-hydroxyvitamin D3 was covalently incorporated in the 25-hydroxyvitamin D3 binding site of purified human plasma vitamin D binding protein. Competition experiments showed that 3-deoxy-3-azido-25-hydroxyvitamin D3 and 25-hydroxyvitamin D3 bind at the same site on the protein. Tritiated 3-deoxy-3-azido-25-hydroxyvitamin D3 was synthesized from tritiated 25-hydroxyvitamin D3, retaining the high specific activity of the parent compound. The tritiated azido label bound reversibly to human vitamin D binding protein in the dark and covalently to human vitamin D binding protein after exposure to ultraviolet light. Reversible binding of tritiated 3-deoxy-3-azido-25-hydroxyvitamin D3 was compared to tritiated 25-hydroxyvitamin D3 binding to human vitamin D binding protein. Scatchard analysis of the data indicated equivalent maximum density binding sites with a KD,app of 0.21 nM for 25-hydroxyvitamin D3 and a KD,app of 1.3 nM for the azido derivative. Covalent binding was observed only after exposure to ultraviolet irradiation, with an average of 3% of the reversibly bound label becoming covalently bound to vitamin D binding protein. The covalent binding was reduced 70-80% when 25-hydroxyvitamin D3 was present, indicating strong covalent binding at the vitamin D binding site of the protein. When tritiated 3-deoxy-3-azido-25-hydroxyvitamin D3 was incubated with human plasma in the absence and presence of 25-hydroxyvitamin D3, 12% of the azido derivative was reversibly bound to vitamin D binding protein. After ultraviolet irradiation, four plasma proteins covalently bound the azido label, but vitamin D binding protein was the only protein of the four that was unlabeled in the presence of 25-hydroxyvitamin D3.     

Severe Vitamin D Deficiency in Arab-American Women Living in Dearborn,Michigan

ObjectiveTo determine the prevalence and degree of 25-hydroxyvitamin D deficiency in a group of Arab- American women in the largest, most-concentrated Arab- American settlement in the United States and to search for correlations with dress, diet, and use of vitamin D–fortified foods and vitamin supplements.MethodsIn this cross-sectional study, Arab-American women, 18 years and older, who attended an ethnic market on April 7 or 14, 2007, were recruited. Participants were interviewed by bilingual English- and Arabic-speaking investigators using a semi-structured interview to assess dress; demographic variables; medical history; medication use; clinical symptoms associated with vitamin D deficiency (eg, joint or bone pain, muscle weakness); and dietary intake of vitamin D from fortified orange juice, milk, and vitamin supplementation. Blood samples were drawn to measure concentrations of serum calcium, creatinine, phosphorus, alkaline phosphatase, parathyroid hormone, and 25-hydroxyvitamin D. Participants were initially divided into 2 groups based on whether the woman was veiled and further subdivided into 3 groups on the basis of vitamin D intake from supplemented food sources (milk or vitamin D–fortified orange juice) and vitamin pills: unveiled, veiled and taking supplements, and veiled and taking no supplements.ResultsEighty-seven women participated. Serum 25-hydroxyvitamin D levels were uniformly low, with the highest levels in the unveiled group (median [interquartile range]) (8.5 ng/mL [5.75-13.5 ng/mL]) followed by the veiled, supplemented group (7 ng/mL [4-11.5 ng/mL]) and the veiled, unsupplemented group (4 ng/mL [2-6.8 ng/ mL]). 25-Hydroxyvitamin D levels were lower in women with less experience in the United States and in those with less education. Vitamin D–fortified orange juice consumption had a greater positive predictive effect on serum 25- hydroxyvitamin D levels than either milk or vitamin pills and may possibly serve as a surrogate marker for vitamin D awareness.ConclusionsVitamin D deficiency, as assessed by 25-hydroxyvitamin D concentrations, is endemic in a sample of Arab-American women living in Dearborn, Michigan. These findings potentially identify an important health problem in the largest, most-concentrated Arab- American population in the United States. (Endocr Pract. 2009;15:35-40)     

25-Azavitamin D3, an inhibitor of vitamin D metabolism and action.

25-Azavitamin D3 inhibited both the bone calcium mobilization and intestinal calcium transport responses of rats to vitamin D3 but not to 25-hydroxyvitamin D3. Although 25-azavitamin D3 had no effect on the response of bone to 1alpha,25-dihydroxyvitamin D3, it did diminish the response of the intestine to that metabolite. 25-Azavitamin D3 increased liver vitamin D content and reduced the concentration of 25-hydroxyvitamin D3 required to inhibit the metabolism of vitamin D3 (75 and 200 microgram) were similar to the doses of 25-azavitamin D3 required to inhibit the action of vitamin D3 in vivo (50 and 150 microgram). 25-Azavitamin D3 is thus a vitamin D antagonist, acting for the most part via inhibition of the liver 25-hydroxylation of vitamin D3.     

Effects of vitamin D supplementation on calcium balance and bone growth in young rats fed normal or low calcium diet

OBJECTIVE: We examined the effect of vitamin D supplementation on bone growth in young rats fed a normal or low calcium diet. METHODS: Fifty female Sprague-Dawley rats, 6 weeks of age, were randomized by stratified weight method into five groups with 10 rats in each group: baseline control, 0.5% (normal) or 0.1% (low) calcium diet, and 0.5 or 0.1% calcium diet + vitamin D (25 microg/100 g, food intake). Duration of the experiment was 10 weeks. RESULTS: Vitamin D supplementation stimulated intestinal calcium absorption and increased urinary calcium excretion in rats fed a low or normal calcium diet. Vitamin D supplementation prevented the reduction in periosteal bone gain but enhanced enlargement of the marrow cavity and reduced the maturation-related cancellous bone gain in rats fed a low calcium diet, and increased the maturation-related cancellous and cortical bone gains in rats fed a normal calcium diet. CONCLUSION: This study shows the differential effects of vitamin D supplementation on born growth in young rats fed a normal or low calcium diet.     

Vitamin D deficiency as a risk factor for urinary tract infection in women at reproductive age

Vitamin D deficiency is a pandemic problem and an ever-increasing issue in human nutrition and health. Vitamin D (serum 25-hydroxyvitamin D) deficiency causes many health problems such as autoimmune diseases, Crohn’s disease, diabetes, inflammation, asthma, hypertension, and cancer. Vitamin D3 (cholecalciferol) deficiency has been documented as a persistent problem among adults, children, and elderly persons in most of the countries. Our main objective of this study was to determine the hypothesis that the vitamin D deficiency among women can lead to them developing frequent urinary tract infections. Vitamin D has a potential role in immune regulation and it prevents infections especially urinary tract infections (UTI). Therefore it has positive regulatory role in both acute and recurrent infections especially in women of reproductive ages. As women at this age group have specific differences in their urinary tract and the reproductive organ anatomy, make them more prone for micro-organisms' invasion, The present study was carried out to ascertain certain relation between serum 25-hydroxyvitamin D levels and UTI in women while contemplating the significance of knowing the risk factors associated with UTI and also finding ways to avoid serious complications. 75 women with (case group) UTI were differentiated with 35 healthy with no UTI (control group) and 40 women with UTI and their serum 25-hydroxyvitamin D levels were checked in a case control study. The women were between at 17–52 years of age. Using ELISA, Serum 25-hydroxyvitamin D levels were measured. Analysis and comparison of the results were done among the two groups. Vitamin D mean levels in the case group was considerably lower when in comparison with the control group (11.09 ± 7.571 ng/mL vs. 24.08 ± 11.95 ng/mL, P < 0.001).     

Circulating vitamin D3 and 25-hydroxyvitamin D in humans: An important tool to define adequate nutritional vitamin D status

Circulating 25-hydroxyvitamin D [25(OH)D] is generally considered the means by which we define nutritional vitamin D status. There is much debate, however, with respect to what a healthy minimum level of circulation 25(OH)D should be. Recent data using various biomarkers such as intact parathyroid hormone (PTH), intestinal calcium absorption, and skeletal density measurements suggest this minimum level to be 80 nmol (32 ng/mL). Surprisingly, the relationship between circulating vitamin D₃ and its metabolic product—25(OH)D₃ has not been studied. We investigated this relationship in two separate populations: the first, individuals from Hawaii who received significant sun exposure; the second, subjects from a lactation study who received up to 6400 IU vitamin D₃/day for 6 months.

Results (1) the relationship between circulating vitamin D₃ and 25(OH)D in both groups was not linear, but appeared saturable and controlled; (2) optimal nutritional vitamin D status appeared to occur when molar ratios of circulating vitamin D₃ and 25(OH)D exceeded 0.3; at this point, the V_max of the 25-hydroxylase appeared to be achieved. This was achieved when circulating 25(OH)D exceeded 100 nmol.

We hypothesize that as humans live today, the 25-hydroxylase operates well below its V_max because of chronic substrate deficiency, namely vitamin D₃. When humans are sun (or dietary) replete, the vitamin D endocrine system will function in a fashion as do these other steroid synthetic pathways, not limited by substrate. Thus, the relationship between circulating vitamin D and 25(OH)D may represent what “normal” vitamin D status should be.     

Parathyroid hormone and 25-hydroxyvitamin D concentrations in sick and normal elderly people.

Serum 25-hydroxyvitamin D and immunoreactive parathyroid hormone concentrations were measured in normal elderly subjects living at home and in sick elderly patients in hospital. Normal old people tended to have high 25-hydroxyvitamin D and low parathyroid hormone concentrations; in the sick elderly this pattern was reversed. The raised serum parathyroid hormone concentrations in the sick elderly were not due to poor renal function and may have been a response to vitamin D deficiency. A high serum parathyroid hormone concentration in an elderly patient must be interpreted in the light of the patient''s general health and nutritional state. Caution is needed in diagnosing primary hyperparathyroidism in this age group.