On the regulatory importance of 1,25-dihydroxyvitamin D3 and dietary calcium on serum levels of 25-hydroxyvitamin D3 in rats

The Impact of Thyme and Rosemary on Prevention of Osteoporosis in Rats

Osteoporosis poses an important public health problem which affects millions of people worldwide. There is a direct link between calcium deficiency in diet and induction of osteoporosis and bone loss. The current study was conducted to evaluate the protective effect of thyme (Thymus vulgaris L.) and rosemary (Rosmarinus officinalis L.) against osteoporosis in rats with low calcium intake. Essential oils of rosemary and thyme were analyzed. The experiment was carried out on growing male Sprague–Dawley rats; the experimental animals were divided into 5 groups: 1, control negative was fed standard balanced diet; 2, control positive was fed balanced diet with low calcium level (L Ca) (Ca 0.1% w/w); 3, (L Ca) + thyme powder (5% w/w); 4, (L Ca) + rosemary powder (5% w/w); 5, (L Ca) + orally administration with CaCO₃ (27 mg/kg body weight). Blood samples were collected for different biochemical analyses in plasma (calcium (Ca), phosphorus (P), magnesium (Mg), tumor necrosis factor-alpha (TNF-α), C-reactive protein (CRP), malondialdehyde (MDA), parathyroid hormone (PTH), C-terminal telopeptide (CTX), and 1,25-(OH)2-vitamin D3). Femur mass, length, and bone mineral density (BMD) were recorded, and histopathological studies for femurs were examined. Low-calcium diet induced osteoporotic changes in positive control rats (decrease in Ca, vitamin D3, and BMD and increase in CTX, PTH, TNF-α, CRP, and MDA). Supplementation with thyme and rosemary inhibited significantly the development of bone loss, increased Ca and vitamin D3 in plasma, improved BMD, and also prevented the inflammation and oxidative stress (improved TNF-α, CRP and MDA) compared to the positive control. The histopathological examination of treated groups showed an improvement in bone histology and protection against bone loss. However, thyme powder showed more effective impact than rosemary. Our study demonstrates that thyme and rosemary effectively mitigated calcium deficiency-induced bone loss and maybe considered as promising candidates for preventing bone resorption and osteoporosis.

Sunlight exposure is just one of the factors which influence vitamin D status

Studies on the determinants of vitamin D status have tended to concentrate on input - exposure to ultraviolet B radiation and the limited sources in food. Yet, vitamin D status, determined by circulating concentrations of 25-hydroxyvitamin D (25(OH)D), can vary quite markedly in groups of people with apparently similar inputs of vitamin D. There are small effects of polymorphisms in the genes for key proteins involved in vitamin D production and metabolism, including 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol, the precursor of vitamin D, to cholesterol, CYP2R1, the main 25-hydroxylase of vitamin D, GC, coding for the vitamin D binding protein which transports 25(OH)D and other metabolites in blood and CYP24A1, which 24-hydroxylates both 25(OH)D and the hormone, 1,25-dihydroxyvitamin D. 25(OH)D has a highly variable half-life in blood. There is evidence that the half-life of 25(OH)D is affected by calcium intake and some therapeutic agents. Fat tissue seems to serve as a sink for the parent vitamin D, which is released mainly when there are reductions in adiposity. Some evidence is presented to support the proposal that skeletal muscle provides a substantial site of sequestration of 25(OH)D, protecting this metabolite from degradation by the liver, which may help to explain why exercise, not just outdoors, is usually associated with better vitamin D status.

Significance of Leydig Cell Neoplasia in Rats Fed Lactitol or Lactose

Lactitol is a disaccharide sugar alcohol (polyol) which is derived from lactose by catalytic hydrogenation and which may be used as a noncariogenic, reduced calorie sugar substitute in different foods. In the context of the safety evaluation of lactitol, a chronic carcinogenicity/toxicity study was conducted in a Wistar-derived strain of rats. In addition to effects that occur commonly in rats fed high doses of polyols, an increased incidence of Leydig cell tumors was observed in rats fed a diet with 10% lactitol for their lifetime. A comparison group receiving a diet with 20% lactose exhibited the same effect. At the 5% dose level of lactitol, no testicular changes were seen. Although lactitol is not genotoxic in standard in vitro tests and was also not associated with tumor formation in female rats and mice of either sex, it was necessary to assess the relevance of the testicular neoplastic growth for human safety. A comparative evaluation of the spontaneous and chemically induced formation of Leydig cell tumors in rats and humans demonstrates that the spontaneous occurrence is extremely low in humans but rather high in rats. Chemical agents or experimental conditions that in rats are associated with interstitial cell hyperplasia or neoplasia have not been associated with similar effects in humans. This is also true for lactose which, in Western countries, is consumed regularly and in substantial amounts with dairy products. Since lactitol is essentially not hydrolyzed in the small intestine, it gains access to the metabolism only after fermentation by the intestinal flora. It is therefore reasonable to assume that the testicular effects of lactitol and lactose were mediated by changes in the digestive tract such as by the known increase of calcium absorption which occurs in lactitol- and lactose-fed rats but not in humans or by effects of these compounds on the enterohepatic cycling of steroid hormones. Although these mechanisms are not yet elucidated, the available data on Leydig cell tumors indicate that the effects seen in male rats are not relevant to humans. The major lines of evidence supporting the human safety of lactitol and lactose are: (1) the lack of genotoxicity of lactitol, (2) the rat specificity of the testicular effects of lactose and lactitol, (3) the long history of safe consumption of lactose in humans, (4) the insensitivity of the human Leydig cells to agents and conditions that are known to cause neoplastic growth of Leydig cells in rats, (5) the generally very low spontaneous incidence of Leydig cell tumors in the human population, and (6) the absence of any epidemiological evidence establishing a link between nutritional factors and the occurrence of Leydig cell tumors in humans.

Lack of effect of calcium intake on the 25-hydroxyvitamin d response to oral vitamin D3

This study was conducted to examine the effect of calcium intake on the rise in serum 25-hydroxyvitamin D [25(OH)D] levels in response to supplemental vitamin D(3). Fifty-two healthy older men and women were randomly assigned to take calcium (500 mg twice daily with meals) or placebo tablets for 90 d between October 1 and the end of March. All participants were placed on 800 IU/d (20 microg/d) vitamin D(3). Serum 25(OH)D measurements were made at baseline and on d 30, 60, and 90. The mean baseline 25(OH)D values were 19.2 +/- 6.4 ng/ml (47.9 +/- 15.9 nmol/liter) in the calcium group and 19.6 +/- 6.7 ng/ml (49.1 +/- 16.7 nmol/liter) in the control group (P = 0.808). The difference in pattern of change in 25(OH)D was not statistically significant (group by time interaction, P = 0.651); the calcium group increased 6.5 +/- 5.9 ng/ml (16.2 +/- 14.8 nmol/liter; P < 0.001), and the control group increased 6.6 +/- 7.0 ng/ml (16.6 +/- 17.4 nmol/liter; P < 0.001). The 95% confidence interval for difference in mean increase, calcium vs. control, was -3.8 +/- 3.5 ng/ml (-9.6, 8.7) nmol/liter. In older men and women, the level of calcium intake, within the range of 500-1500 mg/d, does not have an important effect on the rise in serum 25(OH)D that occurs in response to 800 IU (20 microg)/d vitamin D(3).

Low-calcium diets increase both production and clearance of 1,25-dihydroxyvitamin D3 in rats

Administration of large doses of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] to animals induces 1,25(OH)2D3 side-chain oxidative pathways. This study determined if the elevated plasma 1,25(OH)2D3 seen in rats fed low-Ca diets is associated not only with an increased production rate (PR) but also with an increased metabolic clearance rate (MCR) of the hormone. In vitamin D-replete rats fed a Ca-deficient diet for 3-4 wk, the PR increased 21-fold, plasma levels 15-fold, and the MCR by 37%. The increased MCR in Ca-deficient rats was associated with a 48% increase in hepatic microsomal UDP glucuronyl transferase enzyme activity, whereas 1,25(OH)2D3 catabolism by homogenates of liver and small intestinal mucosa was unchanged. In contrast to the effects of low-Ca diets, acute (7 h) pharmacological elevation of plasma 1,25(OH)2D3 to 1.5 ng/ml in normal rats did not influence the MCR. Thus chronically elevated 1,25(OH)2D3 levels are necessary to stimulate clearance. In conclusion, 1,25(OH)2D3 clearance in rats can be stimulated not only by chronic pharmacological doses of 1,25(OH)2D3 but also by the physiological stimulus of a low-Ca diet. Hence, plasma 1,25(OH)2D3 levels can be regulated by changes in both PR and MCR.

Oral verapamil and calcium and vitamin D metabolism in rats: effect of dietary calcium

Prior studies showed that chronic oral verapamil administration increased plasma immunoreactive parathyroid hormone (irPTH) but decreased 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels in rats fed a high (1.2%)-Ca diet. These and other findings suggested that verapamil may induce target-organ PTH resistance. This study determined the effects of verapamil (4, 20, or 100 mumol.kg-1.day-1 for 2 wk) in rats fed high (1.2%)-, low-normal (0.47%)-, and low (0.02%)-Ca diets (higher irPTH levels). With 1.2 and 0.47% Ca diets, verapamil administration was associated with increases in irPTH (92 and 44%, respectively) and decreases in 1,25(OH)2D3 levels (22 and 21%, respectively), increases in duodenal Ca transport (13 and 8%, respectively), and increases in tibia mineral content (1.3 and 2.8%, respectively). The decrease in 1,25(OH)2D3 levels was caused by decreased production, not by increased clearance. In contrast, verapamil was without effect in rats fed the 0.02% Ca diet. Thus severe dietary Ca deficiency abolished the stimulatory effects of verapamil on irPTH levels, Ca absorption, and tibia mineral content. Importantly, these results indicate that verapamil, in contrast to nifedipine, appears not to have adverse effects on Ca homeostasis in rats, irrespective of dietary Ca intake.

The calcium ionophore A23187 is a potent stimulator of the vitamin D3-25 hydroxylase in hepatocytes isolated from normocalcaemic vitamin D-depleted rats

The role played by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and/or by calcium on the C-25 hydroxylation of vitamin D3 (D3) was studied in hepatocytes isolated from D-depleted rats which were divided into four treatment groups: Group 1 served as controls, Group 2 received calcium gluconate, Groups 3 and 4 were infused with 1,25(OH)2D3 at 7 and 65 pmol/24 h x 7 days respectively. The treatments normalized serum calcium in all but the controls which remained hypocalcaemic, while serum 1,25(OH)2D3 remained low in Groups 1 and 2 but increased to physiologic and supraphysiologic levels in Groups 3 and 4. The data show that basal D3-25 hydroxylase activities were not significantly affected by any of the treatments. Addition of CaCl2, EGTA, or Quin-2 in vitro revealed that relative to basal values, EGTA strongly inhibited the enzyme activity in all groups (P less than 0.0001), except in G 1; Quin-2 and CaCl2 had no significant effect on the activity of the enzyme in any of the groups. Addition of 1,25(OH)2D3 or A23187 in vitro in the presence of CaCl2 revealed that 1,25(OH)2D3 did not significantly affect enzyme activity, while A23187 was found to stimulate its activity in vitamin D-depleted animals, but most specifically in Group 2 (P less than 0.001); low serum calcium (Group 1) dampened (P less than 0.01), and 1,25(OH)2D3 treatment in vivo totally blunted (P less than 0.001) the response to A23187. The data suggest that 1,25(OH)2D3 supplementation in vivo has per se little or no effect on the basal D3-25 hydroxylase activity. The data show, however, that the magnitude of the response to various challenges in vitro is greatly influenced by the conditioning in vivo of the animals. They also show that A23187 can be a potent stimulator of the enzyme activity, which allowed us to demonstrate a significant reserve for the C-25 hydroxylation of D3 which is well expressed in hepatocytes obtained from D-depleted calcium-supplemented rats.

Effect of calcium intake on serum levels of 25-hydroxyvitamin D3

The effects of high calcium intake on vitamin D metabolism were investigated. To the normal diet of 14 healthy men, 2 g calcium were added daily for 6-7 weeks. The mean serum concentration of 25-hydroxyvitamin D3 increased from 73 +/- 7 to 94 +/- 6 nmol l-1 (P less than 0.05, Student's unpaired t-test; P less than 0.01, paired t-test) in the subjects receiving calcium, whereas there was only a minimal increase, from 67 +/- 5 to 71 +/- 4 nmol l-1 in a control group on a normal diet. At the end of the study the difference between the test group and the controls was highly significant (P less than 0.005). The calcium loading caused a statistically significant depression of the serum levels of 1,25-dihydroxyvitamin D. The results obtained are in agreement with previous studies in rats and indicate that calcium intake is of some importance for the serum level of 25-hydroxyvitamin D3. The findings are discussed in relation to our previous finding that there is a relationship between high 25-hydroxyvitamin D3 levels and hypercalciuria in renal-stone formers.