1,25 dihydroxycholecalciferol-mediated calcium absorption and gene expression are higher in female than in male mice

Recent advances in bone and calcium (Ca) metabolism have relied upon genetically modified mice. However, although human studies have identified gender as an important modulator of Ca metabolism, its effect on Ca metabolism has not been examined in mice. Here we examined basal and vitamin D-regulated Ca absorption (in situ ligated loops) and mRNA levels for the apical membrane calcium channel, TRPV6, and the calcium binding protein, calbindin D(9k) (CaBP) mRNA levels (real-time PCR) in duodenum of female and male mice. At 2 mo of age, females fed a 5 g Ca/kg diet had higher Ca absorption (62.3 +/- 4.8 vs. 47 +/- 3.6%) and TRPV6 mRNA levels than males even though plasma 1,25 dihydroxyvitamin D [1,25(OH)(2) D] was not different. In mice fed high (20 g/kg), normal (5 g/kg), or low (0.2 g/kg) Ca diets for 7 d to alter plasma 1,25(OH)(2) D (91 +/- 12, 322 +/- 25, and 587 +/- 43 pmol/L, respectively), the relation between Ca absorption (slope = 0.116 vs. 0.084, P = 0.021) or duodenum TRPV6 mRNA (slope = 0.042 vs. 0.025, P = 0.034) and circulating 1,25(OH)(2) D was steeper in females. After a single 1,25(OH)(2) D injection (200 ng/100 g body weight), peak induction of TRPV6 mRNA was 2-fold greater (at 6 h) and CaBP mRNA was 20% higher in females (at 16 h). Duodenal vitamin D receptor mRNA levels did not differ between genders. Our data indicate that female mice are more sensitive to changes in serum 1,25(OH)(2) D levels than males and that this must be considered when using mice to study calcium and bone biology.     

Ascorbic acid effects on vitamin D hormone metabolism and binding in guinea pigs

Ascorbic acid deficiency in guinea pigs fed a vitamin D-replete diet caused a moderate reduction of Ca level in serum and bone; 25-hydroxy-cholecalciferol or 25-hydroxyergocalciferol (25-OHD) serum concentration tended to decline; renal 25-hydroxycholecalciferol-1-hydroxylase (1-OHase) activity decreased 50%; and 25-hydroxycholecalciferol-24-hydroxylase activity increased 1.6-fold. Chromatin 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] receptor concentration in the intestinal mucosa decreased 20-30%, and the percentage of occupied receptors decreased from 12-15% to 6-8%. Receptor affinity for 1,25-(OH)2D3 did not change (Kd = 0.24-0.26 nmol/L, Kd2 = 0.06-0.10 nmol/L), but the cooperativity coefficient decreased from 1.7 to 1.4. Vitamin C deficiency potentiated effects of vitamin D deprivation and impaired a restorative action of vitamin D. It was accompanied by a marked delay in the elevation of 25-OHD concentration in serum as well as decreased 1-OHase activity in kidneys and a lower concentration of occupied 1,25-(OH)2D3 receptors in the intestinal mucosa. The data demonstrate a critical role for ascorbic acid in vitamin D metabolism and binding.     

Age-related alteration of vitamin D metabolism in response to low-phosphate diet in rats

The responses of renal vitamin D metabolism to its major stimuli alter with age. Previous studies showed that the increase in circulating 1,25-dihydroxyvitamin D (1,25(OH)2D3) as well as renal 25-hydroxyvitamin D3 1-alpha hydroxylase (1-OHase) activity in response to dietary Ca or P restriction reduced with age in rats. We hypothesized that the mechanism involved in increasing circulating 1,25(OH)2D3 in response to mineral deficiency alters with age. In the present study, we tested the hypothesis by studying the expression of genes involved in renal vitamin D metabolism (renal 1-OHase, 25-hydroxyvitamin D 24-hydroxylase (24-OHase) and vitamin D receptor (VDR)) in young (1-month-old) and adult (6-month-old) rats in response to low-phosphate diet (LPD). As expected, serum 1,25(OH)2D3 increased in both young and adult rats upon LPD treatment and the increase was much higher in younger rats. In young rats, LPD treatment decreased renal 24-OHase (days 1-7, P<0.01) and increased renal 1-OHase mRNA expression (days 1-5, P<0.01). LPD treatment failed to increase renal 1-OHase but did suppress 24-OHase mRNA expression (P<0.01) within 7 d of LPD treatment in adult rats. Renal expression of VDR mRNA decreased with age (P<0.001) and was suppressed by LPD treatment in both age groups (P<0.05). Feeding of adult rats with 10 d of LPD increased 1-OHase (P<0.05) and suppressed 24-OHase (P<0.001) as well as VDR (P<0.05) mRNA expression. These results indicate that the increase in serum 1,25(OH)2D3 level in adult rats during short-term LPD treatment is likely to be mediated by a decrease in metabolic clearance via the down-regulation of both renal 24-OHase and VDR expression. The induction of renal 1-OHase mRNA expression in adult rats requires longer duration of LPD treatment than in younger rats.     

Vitamin D status in kidney transplant patients: need for intensified routine supplementation

BACKGROUND: A high prevalence of vitamin D insufficiency has been found in the general population and in patients with chronic kidney disease. OBJECTIVE: The aim was to examine vitamin D status and determinants and metabolic correlates of serum 25-hydroxyvitamin D in a population of adult Danish kidney transplant patients. DESIGN: This was a cross-sectional study of 173 adult kidney transplant patients with a mean (+/-SD) age of 53.4 +/- 11.7 y and a median graft age of 7.4 y (interquartile range: 3.3-12.7 y). Serum concentrations of intact parathyroid hormone (S-PTH), 25-hydroxyvitamin D [S-25(OH)D], and 1,25-dihydroxyvitamin D [S-1,25(OH)(2)D] were measured. Dietary and supplementary intake of vitamin D, avoidance of solar ultraviolet B exposure, and selected lifestyle factors were assessed in a subgroup (n = 97). RESULTS: Fifty-one percent of the patients had vitamin D insufficiency [S-25(OH)D 40-75 nmol/L], and an additional 29% had moderate-to-severe vitamin D deficiency [S-25(OH)D < or = 39 nmol/L]. In multiple regression analysis, sun avoidance (negative association) and vitamin D supplementation (positive association) were independent determinants of S-25(OH)D concentrations. Low S-25(OH)D concentrations were associated with 1) increased S-PTH concentrations (P = 0.0002), independently of S-1,25(OH)(2)D concentrations, and 2) decreased S-1,25(OH)(2)D concentrations (P = 0.002), independently of graft function. CONCLUSIONS: Hypovitaminosis D is common among Danish kidney transplant patients and is associated with reduced concentrations of S-1,25(OH)(2)D and increased S-PTH concentrations. Sun avoidance and vitamin D supplementation are important determinants of vitamin D status. The observed hypovitaminosis D might be corrected by intensified routine vitamin D supplementation as opposed to the current supplementation practice.     

Vitamin D status: effects on parathyroid hormone and 1, 25-dihydroxyvitamin D in postmenopausal women

BACKGROUND: Low serum 25-hydroxyvitamin D ?25(OH)D concentrations are commonly found in the elderly and are associated with hip fracture. Treatment with vitamin D and calcium can reduce the risk of fracture. The relation between the rise in parathyroid hormone (PTH) with age and the decrease in 25(OH)D is not clear. Neither is there any consensus on the serum concentration of 25(OH)D required for bone health. OBJECTIVE: Our objective was to study the relations between serum PTH, serum vitamin D metabolites, and other calcium-related variables in postmenopausal women. DESIGN: This was a cross-sectional study of 496 postmenopausal women without vertebral fractures attending our menopausal osteoporosis clinics. RESULTS: PTH was significantly positively related to age and serum 1, 25-dihydroxyvitamin D ?1,25(OH)(2)D and inversely related to 25(OH)D and plasma ionized calcium. There was a step-like increase in PTH as serum 25(OH)D fell below 40 nmol/L. In women with 25(OH)D concentrations >40 nmol/L, 1,25(OH)(2)D was positively related to 25(OH)D; in women with 25(OH)D concentrations 40 nmol/L, 1,25(OH)(2)D was most closely (inversely) related to plasma creatinine. Therefore, with serum 25(OH)D concentrations increasingly <40 nmol/L, serum 1,25(OH)(2)D becomes critically dependent on rising concentrations of PTH. CONCLUSION: The data suggest that aging women should maintain 25(OH)D concentrations >40 nmol/L (which is the lower limit of our normal range for healthy young subjects) for optimal bone health.     

Efficacy and safety of vitamin D3 intake exceeding the lowest observed adverse effect level

BACKGROUND: The Food and Nutrition Board of the National Academy of Sciences states that 95 microg vitamin D/d is the lowest observed adverse effect level (LOAEL). OBJECTIVE: Our objective was to assess the efficacy and safety of prolonged vitamin D3 intakes of 25 and 100 microg (1000 and 4000 IU)/d. Efficacy was based on the lowest serum 25-hydroxyvitamin D [25(OH)D] concentration achieved by subjects taking vitamin D3; potential toxicity was monitored by measuring serum calcium concentrations and by calculating urinary calcium-creatinine ratios. DESIGN: Healthy men and women (n = 61) aged 41 +/- 9 y (mean +/- SD) were randomly assigned to receive either 25 or 100 microg vitamin D3/d for 2-5 mo, starting between January and February. Serum 25(OH)D was measured by radioimmunoassay. RESULTS: Baseline serum 25(OH)D was 40.7 +/- 15.4 nmol/L (mean +/- SD). From 3 mo on, serum 25(OH)D plateaued at 68.7 +/- 16.9 nmol/L in the 25-microg/d group and at 96.4 +/- 14.6 nmol/L in the 100-microg/d group. Summertime serum 25(OH)D concentrations in 25 comparable subjects not taking vitamin D3 were 46.7 +/- 17.8 nmol/L. The minimum and maximum plateau serum 25(OH)D concentrations in subjects taking 25 and 100 microg vitamin D3/d were 40 and 100 nmol/L and 69 and 125 nmol/L, respectively. Serum calcium and urinary calcium excretion did not change significantly at either dosage during the study. CONCLUSIONS: The 100-microg/d dosage of vitamin D3 effectively increased 25(OH)D to high-normal concentrations in practically all adults and serum 25(OH)D remained within the physiologic range; therefore, we consider 100 microg vitamin D3/d to be a safe intake.     

Serum 25-hydroxyvitamin D is a predictor of serum 1,25-dihydroxyvitamin D in overweight and obese patients

Recent research suggests that 1,25-dihydroxyvitamin D [1,25(OH)(2)D], a steroid hormone that regulates calcium homeostasis, may also play a role in the development and progression of cancer, multiple sclerosis, cardiovascular, and other diseases. Decreased serum 1,25(OH)(2)D concentrations are often observed in overweight and obese patients. However, little is known about the factors that may influence 1,25(OH)(2)D renal synthesis, because it is generally accepted that serum 1,25(OH)(2)D concentration is strictly regulated by parathyroid hormone and serum concentrations of calcium and phosphorus. In this study, the associations among serum 1,25(OH)(2)D, serum 25-hydroxyvitamin D [25(OH)D], and body composition were analyzed in 1779 patients with excess body weight registered in a Metabolic and Medical Lifestyle Management Clinic in Oslo, Norway. According to our results, serum 25(OH)D, adiposity, age, season of blood sampling, and gender directly influence serum 1,25(OH)(2)D (r = 0.33; P < 0.001), with serum 25(OH)D being the strongest predictor for serum 1,25(OH)(2)D. The 1,25(OH)(2)D concentrations were 25.4 pmol/L (95% Cl: 19.3-31.5; P < 0.001) lower in the lowest 25(OH)D quartile to compared with highest quartile. A seasonal variation was observed for both vitamin D metabolites. Thus, our results suggest that in patients with excess body weight, serum 1,25(OH)(2)D concentrations were associated with 25(OH)D and varied during the year. Therefore, it may also be valuable to measure both serum 25(OH)D and 1,25(OH)(2)D for the evaluation of vitamin D status in overweight and obese persons.     

Vitamin D deficiency enhances the growth of MC-26 colon cancer xenografts in Balb/c mice

Vitamin D deficiency has been associated with increased risk of colon cancer in epidemiologic and prospective clinical studies. In vitro and in vivo studies demonstrated that 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and its analogs inhibit colon cancer cell proliferation. Few studies have evaluated the effect of vitamin D deficiency on the development and growth of colon cancer. To assess the antiproliferative effects of 25-hydroxyvitamin D [25(OH)D] and 1,25(OH)2D3 in vitro, we cultured MC-26 (a colon cancer cell line) in the presence of 25(OH)D3 and 1,25(OH)2D3 and performed [3H]thymidine incorporation. The proliferation of MC-26 was significantly inhibited by both 25(OH)D3 and 1,25(OH)2D3. To determine the effect of vitamin D deficiency on colon cancer proliferation, Balb/c mice were rendered vitamin D deficient by feeding them a vitamin D-deficient diet for 3 mo. A group of vitamin D-sufficient mice was given the same diet with supplemental vitamin D. The mice were injected with MC-26 colon cancer cells and the tumors were measured daily for 20 d. Vitamin D-sufficient mice had 40% smaller tumors than vitamin D-deficient mice. The tumors were evaluated for mRNA expression of the vitamin D receptor (VDR) and 25-hydroxvitamin D-1alpha-hydroxylase (1alpha-OHase) by quantitative RT-PCR. The expression of the mRNA for the VDR and the 1alpha-OHase was 37- and 6-fold higher, respectively, in the vitamin D-sufficient mice compared with the vitamin D-deficient mice. We conclude that vitamin D deficiency enhances the growth of colon cancer in mice. The tumor expression of VDR and 1alpha-OHase indicates possible autocrine/paracrine cell growth regulation by vitamin D.     

Effect of age on calcium absorption in postmenopausal women

BACKGROUND: It is assumed that calcium absorption decreases with age, but this is not well documented. We report a study that addresses this issue. OBJECTIVE: The aim was to establish the extent and timing of any age-related change in calcium absorption in postmenopausal women. DESIGN: We measured radiocalcium absorption (alpha) in 262 healthy postmenopausal women aged 40-87 y. We also measured the serum vitamin D metabolites, parathyroid hormone (PTH), and other biochemical variables. RESULTS: Radiocalcium absorption decreased with age (P = 0.018); it was 28% lower in the 25 women aged >75 y than in the rest (P < 0.001). It was significantly related to serum 1,25-dihydroxyvitamin D [1,25(OH)(2)D] in the whole set and in both the younger and older subsets, but it was not related to either 25-dihydroxyvitamin D [25(OH)D] or PTH or to any other measured variable. No decrease in 1,25(OH)(2)D was seen with age to account for the decrease in calcium absorption, so radiocalcium absorption corrected for serum 1,25(OH)(2)D decreased significantly after age 75 y. On multivariate analysis, the serum 1,25(OH)(2)D concentration was a positive function of 25(OH)D (P < 0.001), albumin (P = 0.010), and PTH (P = 0.012) and a negative function of serum creatinine (P = 0.003). PTH was a negative function of calculated ionized calcium (P = 0.004) and 25(OH)D (P = 0.009) and a positive function of weight (P = 0.011) and age (P = 0.028). CONCLUSIONS: A late age-related decrease in calcium absorption is seen in postmenopausal women in addition to the decline that occurs at menopause. This decrease could be due to a decline in either the active calcium transport or diffusion component of the calcium absorption system.     

Calcium and cholecalciferol: effects of small supplements in normal men

The effect of small calcium and vitamin D supplements on mineral metabolism in normal persons is unclear. To investigate the biochemical response to these medications, we administered 1000 mg Ca and 25 micrograms cholecalciferol per day or a placebo to 92 normal men for 1 y. The Ca and cholecalciferol were tolerated well. 25-Hydroxycholecalciferol [25-(OH)D] and 24,25-dihydroxycholecalciferol [24,25-(OH)2D] levels rose in treated subjects; there was no definite change in 1,25-(OH)2D concentrations. The average difference in 25-(OH)D levels between treated and untreated subjects was 30 nmol/L at 1 y. Fasting serum Ca, alkaline phosphatase, creatinine, and parathyroid hormone levels and the fasting urinary excretion of Ca, phosphorus and cAMP, were not affected. However, 24-h urinary Ca excretion was higher in the supplemented group (3.5 +/- 1.9 vs 4.7 +/- 1.7 mmol/d, p = 0.006). Serum P concentrations were slightly higher in the supplemented group at 1 y. In normal men small calcium and cholecalciferol supplements are safe, provide adequate vitamin D nutrition and apparently increase net gastrointestinal Ca absorption.     

Effects of 3 diets with various calcium contents on 24-h energy expenditure, fat oxidation, and adipose tissue message RNA expression of lipid metabolism-related proteins

BACKGROUND: Evidence from molecular and animal research and epidemiologic investigations indicates that calcium intake may be inversely related to body weight, possibly through alterations in 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] metabolism. OBJECTIVE: We tested whether energy and substrate metabolism and adipose tissue enzyme messenger RNA (mRNA) expression can be altered by dietary calcium intake in healthy, nonobese, human volunteers consuming an isocaloric diet. DESIGN: Twelve healthy men [age: 28 +/- 2 y; body mass index (BMI; in kg/m(2)): 25.2 +/- 06] received 3 isocaloric diets [high calcium (1259 +/- 9 mg/d), high dairy (high/high); high calcium (1259 +/- 9 mg/d), low dairy (high/low); and low calcium (349 +/- 8 mg/d), low dairy (low/low)] in a randomized crossover design. At the end of the 7-d dietary periods, 24-h energy expenditure and substrate metabolism were measured, and fat biopsy specimens were obtained to determine mRNA expression in genes involved in the lipolytic and lipogenic pathways. RESULTS: The 24-h energy expenditure was 11.8 +/- 0.3, 11.6 +/- 0.3, and 11.7 +/- 0.3 MJ/24 h in the high/high, high/low, and low/low conditions, respectively. Fat oxidation in these conditions was 108 +/- 7, 105 +/- 9, and 100 +/- 6 g/24 h. These differences were not statistically significant. mRNA concentrations of UCP2, FAS, GPDH2, HSL, and PPARG did not differ significantly. Serum 1,25(OH)(2)D(3) concentrations changed from 175 +/- 16 to 138 +/- 15, 181 +/- 23 to 159 +/- 19, and 164 +/- 13 to 198 +/- 19 pmol/L in the high/high, high/low, and low/low conditions, respectively, and was significantly different between the high/high and low/low conditions (P < 0.05). CONCLUSION: Altering the dietary calcium content for 7 d does not influence substrate metabolism, energy metabolism, or gene expression in proteins related to fat metabolism, despite significant changes in 1,25(OH)(2)D(3) concentrations.     

The effect of a high-protein, high-sodium diet on calcium and bone metabolism in postmenopausal women and its interaction with vitamin D receptor genotype

The influence of a high-Na, high-protein (calciuric) diet on Ca and bone metabolism was investigated in postmenopausal women (aged 50-67 years) who were stratified by vitamin D receptor (VDR) genotype. In a crossover trial, twenty-four women were randomly assigned to a diet high in protein (90 g/d) and Na (180 mmol/d) or a diet adequate in protein (70 g/d) and low in Na (65 mmol/d) for 4 weeks, followed by crossover to the alternative dietary regimen for a further 4 weeks. Dietary Ca intake was maintained at usual intakes (about 20 mmol (800 mg)/d). Urinary Na, K, Ca, N and type I collagen cross-linked N-telopeptide (NTx; a marker of bone resorption), plasma parathyroid hormone (PTH), serum 25-hydroxycholecalciferol (25(OH)D3), 1,25-dihydroxycholecalciferol (1,25(OH)2D3), osteocalcin and bone-specific alkaline phosphatase (B-Alkphase) were measured in 24 h urine samples and fasting blood samples collected at the end of each dietary period. The calciuric diet significantly (P<0.05) increased mean urinary Na, N, K, Ca and NTx (by 19 %) compared with the basal diet, but had no effect on circulating 25(OH)D3, 1,25(OH)2D3, PTH, osteocalcin or B-Alkphase in the total group (n 24). There were no differences in serum markers or urinary minerals between the basal and calciuric diet in either VDR genotype groups. While the calciuric diet significantly increased urinary NTx (by 25.6 %, P<0.01) in the f+ VDR group (n 10; carrying one or more (f) Fok I alleles), it had no effect in the f- VDR group (n 14; not carrying any Fok I alleles). It is concluded that the Na- and protein-induced urinary Ca loss is compensated for by increased bone resorption and that this response may be influenced by VDR genotype.     

Efficacy of daily and monthly high-dose calciferol in vitamin D-deficient nulliparous and lactating women

BACKGROUND: We previously found a high prevalence of vitamin D deficiency and low medication regimen compliance in Arab and East Indian women residing in the United Arab Emirates (UAE). The appropriate dosing regimen for improving vitamin D status in this population is not known. OBJECTIVE: We aimed to determine the efficacy of daily and monthly supplementation with vitamin D2, the only high-dose calciferol available in the UAE, in lactating and nulliparous women. DESIGN: Healthy lactating (n = 90) and nulliparous (n = 88) women were randomly assigned to consume 2000 IU vitamin D2/d or 60,000 IU vitamin D2/mo for 3 mo. Serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured by radioimmunoassay at baseline and every month. RESULTS: Most women had vitamin D deficiency [ie, 25(OH)D < 50 nmol/L] at study entry. Mean +/- SD 25(OH)D concentrations at 3 mo were significantly higher than baseline in both lactating (39.8 +/- 12.4 and 25.2 +/- 10.7 nmol/L, respectively) and nulliparous (40.4 +/- 23.4 and 19.3 +/- 12.2 nmol/L, respectively) women (P < 0.001 for both). In total, vitamin D supplementation was effective in achieving serum 25(OH)D concentrations of >or=50 nmol/L in 21 (30%) of 71 women at endpoint. CONCLUSIONS: Oral vitamin D2 supplementation with 2000 IU/d or 60,000 IU/mo for 3 mo was safe, and it increased serum 25(OH)D concentrations significantly; however, only a small proportion of the women studied achieved concentrations of >or=50 nmol/L. This suggests that, when sunlight exposure is limited, doses of vitamin D2 higher than those currently studied may be needed. Monthly dosing appears to be a safe and effective alternative to daily dosing.     

Relationship of calcium absorption with 25(OH)D and calcium intake in children with rickets

Nutritional rickets has long been considered a disease caused by vitamin D deficiency, but recent data indicate that inadequate dietary calcium intake is an important cause of rickets, particularly in tropical countries. Children with rickets due to calcium deficiency do not have very low 25(OH)D concentrations, and serum 1,25(OH)(2) D values are markedly elevated. Studies of Nigerian children with rickets demonstrated they have high fractional calcium absorption. A high-phytate diet was demonstrated to increase calcium absorption compared with the fasting state, and enzymatic dephytinization did not significantly improve calcium absorption. When given vitamin D, children with rickets have a marked increase in 1,25(OH)(2) D concentrations without any change in fractional calcium absorption. No positive relationship was found between fractional calcium absorption and serum 25(OH)D concentrations in children on low-calcium diets. More research is needed to understand the interaction between calcium and vitamin D and the role of vitamin D in calcium absorption.     

25-Hydroxyvitamin D and 1,25-dihydroxyvitamin D of D2 and D3 origin in maternal and umbilical cord serum after vitamin D2 supplementation in human pregnancy

Serum concentrations of 25-hydroxyvitamin D (25-OHD) and 1,25-dihydroxyvitamin D [1,25-(OH)2D] of vitamin D2 and D3 origin were determined separately in 10 women before vitamin intake in early pregnancy, and repeated in maternal and cord serum obtained at delivery after 20 to 30 wk of vitamin D2 supplementation in a dose of 400 IU/day. Before supplementation 25-OHD2 and 1,25-(OH)D2D2 were present in just traceable or nondetectable concentrations, but the levels increased in all to a mean +/- 1 SD of 7.3 +/- 3.7 ng/ml and 37.2 +/- 18.1 pg/ml, respectively (p less than 0.0025), by the time of delivery. At delivery the total 25-OHD and 1,25-(OH)2D levels were always lower in the cord than in the maternal serum (30.7 +/- 14.2 versus 20.1 +/- 9.1 ng/ml, and 90.1 +/- 31.2 versus 37.3 +/- 11.6 pg/ml, p less than 0.0025). The paired concentrations of 25-OHD were closely related (r = 0.89, p less than 0.0005), while the association for 1,25-(OH)2D was not statistically significant (r = 0.53, p less than .01). The 25-OHD of D2 and D3 origin accounted for a similar proportion of the total 25-OHD in the maternal and cord serum (ratio of 25-OHD2 to 25-OHD3: 0.40 +/- 0.28 versus 0.45 +/- 0.29, p = NS), as did the respective 1,25-(OH)2D metabolites [ratio of 1,25-(OH)2D2 to 1,25-(OH)2D3: 0.73 +/- 0.35 versus 0.90 +/- 0.50, p = NS].(ABSTRACT TRUNCATED AT 250 WORDS)     

Moderate cholecalciferol supplementation depresses intestinal calcium absorption in growing dogs

Hormonal regulation of calcium (Ca) absorption was investigated in a cholecalciferol (vitamin D(3))-supplemented group (hVitD) vs. a control group (cVitD) of growing Great Danes (100 vs. 12.5 micro g vitamin D(3)/kg diet). Although Ca intakes did not differ, fractional Ca absorption was significantly lower in the hVitD group than in the cVitD group. There were no differences in plasma concentrations of Ca, inorganic phosphate, parathyroid hormone, growth hormone or insulin-like growth factor I between groups. Plasma 25-hydroxycholecalciferol [25(OH)D(3)] concentrations were maintained in the hVitD dogs at the same levels as in the cVitD dogs due to increased turnover of 25(OH)D(3) into 24,25-dihydroxycholecalciferol [24,25(OH)(2)D(3)] and 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)]. In hVitD dogs, the greater plasma 24,25(OH)(2)D(3) concentration and the enhanced metabolic clearance rate (MCR) of 1,25(OH)(2)D(3) indicated upregulated 24-hydroxylase activity. The increased MCR of 1,25(OH)(2)D(3) decreased plasma 1,25(OH)(2)D(3) concentrations. In hVitD dogs, the greater production rate of 1,25(OH)(2)D(3) was consistent with the 12.9-fold greater renal 1alpha-hydroxylase gene expression compared with cVitD dogs and compensated to a certain extent for the accelerated MCR of 1,25(OH)(2)D(3). The moderately decreased plasma 1,25(OH)(2)D(3) concentration can only partially explain the decreased Ca absorption in the hVitD dogs. Intestinal vitamin D receptor concentrations did not differ between groups and did not account for the decreased Ca absorption. We suggest that 24,25(OH)(2)D(3) may downregulate Ca absorption.     

Phosphate transport by plasma membranes of enterocytes during development: role of 1,25-dihydroxycholecalciferol.

The present studies were designed to investigate phosphate transport across the brush border and basolateral membranes of enterocytes and to determine the effect of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] on these processes in suckling and adolescent rats. Vitamin D deficiency was induced in suckling rats by feeding pregnant dams a vitamin D-deficient diet 48 h after insemination; they were then kept in the dark. Vitamin D deficiency in the adolescent rats was induced by feeding the vitamin D-deficient diet to weanling rats for 4 wk. V max values for Na(+)-dependent phosphate uptake in the brush border membranes of vitamin D-deficient and 1,25(OH)2D3-injected suckling rats was 0.7 +/- 0.1 and 1.5 +/- 0.2 nmol.mg protein-1.10 s-1 (P less than 0.01), respectively; V max values in adolescent rats were 0.2 +/- 0.05 and 0.36 +/- 0.04 nmol.mg protein-1.10 s-1 (P less than 0.05), respectively. Vmax values for Na(+)-dependent phosphate uptake in basolateral membranes of vitamin D-deficient and 1,25(OH)2D3-treated suckling rats were 0.006 +/- 0.001 and 0.047 +/- 0.006 nmol.mg protein-1.10 s-1 (P less than 0.01).     

Calcium and vitamin D3 supplements in calcium and vitamin D3 sufficient early postmenopausal healthy women

OBJECTIVE: To study the calcium homeostasis in healthy, calcium and vitamin D replete early postmenopausal women during oral supplementation with calcium and vitamin D3. DESIGN: A prospective, placebo-controlled, randomised, double-single-blind, 3-week study. SETTING: Outpatient clinic at Copenhagen University Hospital, Denmark. SUBJECTS: In all, 17 started, one was excluded. Totally, 16 healthy women, 45-61 y of age (mean 57.3 y) who were at least 4 y after menopause (mean 6.7 y) completed. INTERVENTIONS: All underwent three consecutive 7-day study periods. Each began with 4 days of normal diet followed by 3 days treatment of either C: one tablet of 1.250 mg calcium carbonate (ie 500 mg Ca2+ per tablet) twice daily (breakfast and dinner), or CD3: as in C but plus 400 IU vitamin D3 b.i.d., or P (only) placebo tablets b.i.d. RESULTS: At baseline plasma 25-hydroxycholecalciferol was normal (66+/-22 nmol/l) and the calcium intake without supplements 850 mg/day. In group C, the 24-h urinary calcium increased by 35% (6.9+/-2.0 mmol), vs the placebo group P (5.1+/-1.6 mmol) (P < 0.05). Addition of 800 IU vitamin D3 daily (CD3) did not increase calcium excretion further (6.6+/-2.1 mmol) but decreased plasma 1,25-(OH)2-vitamin D3 by 21% (P < 0.05). CONCLUSIONS: In this carefully controlled study calcium plus vitamin D3 supplements only had minor influences of uncertain significance on the calcium balance in healthy, calcium and vitamin D sufficient early postmenopausal women.     

Prevalence and functional significance of 25-hydroxyvitamin D deficiency and vitamin D receptor gene polymorphisms in Asian Indians

BACKGROUND: Recent studies show a wide prevalence of hypovitaminosis D in Asian Indians. OBJECTIVE: The objective was to assess the functional significance of 25-hydroxyvitamin D [25(OH)D] deficiency, vitamin D receptor (VDR) gene, and parathyroid hormone (PTH) gene polymorphisms in relation to bone mineral density (BMD) in urban Asian Indians. DESIGN: Serum total calcium, inorganic phosphorus, alkaline phosphatase, 25(OH)D, intact PTH, and BMD at lumbar spine, proximal femur, and forearm were measured in 105 adult subjects. The genotyping related to VDR (BsmI, FokI, and TaqI) and PTH (BstBI and DraII) gene single-nucleotide polymorphisms was carried out by polymerase chain reaction-restriction fragment length polymorphism analysis. RESULTS: The mean serum 25(OH)D concentration in the whole cohort was 9.8 +/- 6.0 ng/mL, which was inversely related with serum intact PTH values (P = 0.042). Ninety-nine (94.3%) of the 105 subjects had vitamin D deficiency with 25(OH)D concentrations < 20 ng/mL. The age- and body mass index (BMI)-adjusted BMD value at the hip was higher in subjects with serum 25(OH)D values > 9.0 ng/mL than in those with values < or = 9.0 ng/mL (0.893 +/- 0.114 compared with 0.839 +/- 0.112 g/cm2, respectively; P = 0.001). The mean forearm and spine BMD values in subjects with TT (VDR, TaqI) or bb (PTH, BstBI) genotypes were significantly higher than the values in subjects with Tt genotype and BB or Bb genotype, respectively. CONCLUSION: Functionally significant 25(OH)D deficiency affecting BMD at the hip region is prevalent in urban Asian Indians. However, variation in BMD at the spine and forearm is related to VDR and PTH gene polymorphisms rather than to vitamin D status, at least in this hypovitaminotic D population.     

Daily supplementation with 25 μg cholecalciferol does not increase calcium absorption or skeletal retention in adolescent girls with low serum 25-hydroxyvitamin D

In healthy adolescents, cross-sectional studies show either no or negative relationships between serum 25-hydroxyvitamin D [25(OH)D] and calcium (Ca) absorption. Using a 2-period metabolic balance study, the effect of vitamin D supplementation on Ca absorption and retention in adolescent girls was investigated. Eleven girls aged 12-14 y with a mean entry serum 25(OH)D of 35.1 nmol/L consumed a controlled intake (providing 5 μg vitamin D and 1117 mg Ca/d) for two 3-wk metabolic balance periods separated by a 1-wk washout period. Sunlight exposure was minimized by sunscreen with a sun protection factor ≥ 15. After the first metabolic balance period, participants received 25 μg/d cholecalciferol supplementation for 4 wk. Fractional Ca absorption was measured in each metabolic balance period using a stable Ca isotope method. All urine and fecal samples were collected and analyzed to measure net Ca absorption and Ca retention. Paired t tests and correlations were used to analyze the data. Daily supplementation with 25 μg vitamin D resulted in a mean increase in serum 25(OH)D of 13.3 nmol/L (P < 0.01) but a decrease in fractional Ca absorption of 8.3% (P < 0.05) and no significant change in fasting serum 1,25-dihydroxyvitamin D, parathyroid hormone, net Ca absorption, or Ca skeletal retention. In pubertal girls with vitamin D status considered insufficient in adults, vitamin D supplementation of 25 μg/d for 4 wk did not improve fractional Ca absorption, net Ca absorption, or Ca retention.