Different metabolism of vitamin D2 and vitamin D3 in epileptic patients on carbamazepine

Vitamin D: sources, physiological role, biokinetics, deficiency, therapeutic use, toxicity, and overview of analytical methods for detection of vitamin D and its metabolites

Vitamin D has a well-known role in the calcium homeostasis associated with the maintenance of healthy bones. It increases the efficiency of the intestinal absorption of dietary calcium, reduces calcium losses in urine, and mobilizes calcium stored in the skeleton. However, vitamin D receptors are present ubiquitously in the human body and indeed, vitamin D has a plethora of non-calcemic functions. In contrast to most vitamins, sufficient vitamin D can be synthesized in human skin. However, its production can be markedly decreased due to factors such as clothing, sunscreens, intentional avoidance of the direct sunlight, or the high latitude of the residence. Indeed, more than one billion people worldwide are vitamin D deficient, and the deficiency is frequently undiagnosed. The chronic deficiency is not only associated with rickets/osteomalacia/osteoporosis but it is also linked to a higher risk of hypertension, type 1 diabetes, multiple sclerosis, or cancer. Supplementation of vitamin D may be hence beneficial, but the intake of vitamin D should be under the supervision of health professionals because overdosing leads to intoxication with severe health consequences. For monitoring vitamin D, several analytical methods are employed, and their advantages and disadvantages are discussed in detail in this review.

Vitamin D bioavailability: state of the art

There has been renewed interest in vitamin D since numerous recent studies have suggested that besides its well-established roles in bone metabolism and immunity, vitamin D status is inversely associated with the incidence of several diseases, e.g., cancers, cardio-vascular diseases, and neurodegenerative diseases. Surprisingly, there is very little data on factors that affect absorption of this fat-soluble vitamin, although it is acknowledged that dietary vitamin D could help to fight against the subdeficient vitamin D status that is common in several populations. This review describes the state of the art concerning the fate of vitamin D in the human upper gastrointestinal tract and on the factors assumed to affect its absorption efficiency. The main conclusions are: (i) ergocalciferol (vitamin D2), the form mostly used in supplements and fortified foods, is apparently absorbed with similar efficiency to cholecalciferol (vitamin D3, the main dietary form), (ii) 25-hydroxyvitamin D (25OHD), the metabolite produced in the liver, and which can be found in foods, is better absorbed than the nonhydroxy vitamin D forms cholecalciferol and ergocalciferol, (iii) the amount of fat with which vitamin D is ingested does not seem to significantly modify the bioavailability of vitamin D3, (iv) the food matrix has apparently little effect on vitamin D bioavailability, (v) sucrose polyesters (Olestra) and tetrahydrolipstatin (orlistat) probably diminish vitamin D absorption, and (vi) there is apparently no effect of aging on vitamin D absorption efficiency. We also find that there is insufficient, or even no data on the following factors suspected of affecting vitamin D bioavailability: (i) effect of type and amount of dietary fiber, (ii) effect of vitamin D status, and (iii) effect of genetic variation in proteins involved in its intestinal absorption. In conclusion, further studies are needed to improve our knowledge of factors affecting vitamin D absorption efficiency. Clinical studies with labeled vitamin D, e.g., deuterated or (13)C, are needed to accurately and definitively assess the effect of various factors on its bioavailability.

Serum 25-hydroxyvitamin D levels in vitamin D-insufficient hip fracture patients after supplementation with ergocalciferol and cholecalciferol

Vitamin D insufficiency is commonly associated with hip fracture. However, the equipotency of ergocalciferol and cholecalciferol supplementation in this patient group has not been studied in a randomized trial using high-performance liquid chromatography (HPLC) measurement of serum 25-hydroxyvitamin D (25OHD). The objective of this study was to determine if ergocalciferol and cholecalciferol are equipotent therapies in vitamin D-insufficient hip fracture patients. Ninety five hip fracture inpatients with vitamin D insufficiency (25OHD<50 nmol/L) were randomized, double-blind, to treatment with ergocalciferol 1000 IU/day (n=48) or cholecalciferol 1000 IU/day (n=47) for three months. All participants were also given a placebo matching the alternative treatment to maintain blinding of treatment allocation. The primary endpoint was total serum 25OHD measured by HPLC. Secondary endpoints included 25OHD measured by radioimmunoassay (RIA), intact parathyroid hormone (iPTH), and bioactive (1-84) whole PTH (wPTH). Seventy patients (74%) completed the study with paired samples for analysis. Cholecalciferol supplementation resulted in a 31% greater increase in total HPLC-measured 25OHD (p=0.010) and 52% greater rise in RIA-measured 25OHD (p<0.001) than supplementation with an equivalent dose of ergocalciferol. Changes in iPTH and wPTH were not significantly different between calciferol treatments (p>0.05). In vitamin D-insufficient hip fracture patients, supplementation with cholecalciferol 1000 IU/day for three months was more effective in increasing serum 25OHD than an equivalent dose of ergocalciferol. However, the lack of difference in PTH lowering between calciferol treatments raises questions about the biological importance of this observation.

Hypovitaminosis D in acutely injured pediatric burn patients

PROBLEM

The prevalence of vitamin D insufficiency, its etiology, and associated sequelae among acutely injured burn patients is unknown.

OBJECTIVE

This study assessed vitamin D and endocrine status, as well as the effect of anabolic agents, in pediatric patients who had sustained burns in excess of 25% total body surface area (TBSA).

SUBJECTS

Sixty-nine patients with a mean TBSA burn of 50.6+/-2.2% (range 27% to 94%) and full thickness injury of 41.3+/-3.0% (range 0% to 94%) were studied. Subjects ranged in age from 0.6 to 18 years (mean, 5.8+/-0.6 years). Main outcome measures Blood samples were obtained for serum 25-hydroxyvitamin D (D25), 1,25-dihydroxyvitamin D (D1,25), albumin, cortisol, triiodothyronine (T3), tetraiodothyronine (T(4)), thyroid stimulating hormone (TSH), and parathormone (PTH).

RESULTS

Two hundred eighty morning blood samples of D25 and D1,25 demonstrated that 45% and 26.2% were low and 8.9% and 11% were very low, respectively. At least one low D25 or D1,25 level occurred in 62.3% of all subjects. Very low levels were noted in 23.2% of all patients. There was an increased incidence of hyperparathyroidism in patients with very low serum D25. Vitamin D25 and D1,25 levels were lower in subjects with larger burns or inhalation injury, as well as those treated with thyroxine or oxandrolone. Serum albumin, cortisol, T(4), and TSH were not correlated with concentration of vitamin D.

CONCLUSIONS

Demonstration of a high incidence of low serum vitamin D indicates vitamin D status may be significantly compromised in burned children. It is unclear why vitamin D deficiency exists in this population. The most effective way to improve vitamin D status remains elusive at this time.

Effect of Vitamin D supplement use on serum concentrations of total 25OHD levels in elderly women

Vitamin D(2) and D(3) are generally considered equipotent in humans. As Vitamin D(2) supplements are commonly used by elderly in United States, we determined the contribution of 25OHD(2) to the total serum 25OHD levels by HPLC in elderly women who reported taking Vitamin D(2) supplements (n = 56) and also in a group of randomly selected unsupplemented women (n = 60). In addition, we compared the total serum 25OHD measured by HPLC with competitive protein-binding assay (CPBA), a method routinely employed to measure Vitamin D status. A correlation of 0.91 (P < 0.001) was observed between the two methods for the serum total 25OHD measurement. The mean serum 25OHD level in Vitamin D(2) supplemented group was significantly higher than in unsupplemented group measured by HPLC (32 versus 28 ng/ml) and marginally higher measured by CPBA (33 vs. 31 ng/ml). Seventy eight percent of women taking Vitamin D(2) supplements had appreciable amounts of circulating 25OHD(2,) which constituted about 25 percent of their total serum 25OHD. It is also interesting to note that Vitamin D deficiency was less prevalent in elderly women taking Vitamin D(2) supplements (1.8%) compared to women not taking any supplements (12%).

Effects of increasing doses of 1 alpha-hydroxyvitamin D2 on calcium homeostasis in postmenopausal osteopenic women

This study is the first reported administration of 1 alpha-hydroxyvitamin D2 (1 alpha-OHD2) to human subjects. A total of 15 postmenopausal osteopenic women were given increasing oral doses of 1 alpha-OHD2, beginning with a low dose of 0.5 microgram/day. In 15 subjects, the doses were raised at weekly intervals to 1.0, 2.0, 4.0, and 5.0 micrograms/day, and in 5 of these subjects, the dose was further increased to 8.0 or 10.0 micrograms/day. Mean urine calcium +/- SEM showed a dose-related increase from 134 +/- 17 mg/24 h on 0.5 microgram/day to 198 +/- 21 mg/24 h on 4.0 micrograms/day (p < 0.05) and to 241 +/- 35 mg/24 h on 5.0 micrograms/day (p < 0.05). No subjects had hypercalciuria (> 350 mg/24 h, the upper limit of the laboratory normal range) at doses less than 5.0 micrograms/day; 5 subjects had hypercalciuria at or above 5.0 micrograms/day (3 at 5.0 micrograms/day, 1 at 8.0 micrograms/day, and 1 at 10.0 micrograms/day). Mean serum calcium increased slightly on the 4.0 micrograms dose only (p < 0.05) but remained well within the normal range. Mean creatinine clearance and BUN, used as measures of renal function, showed no significant changes. Routine blood and urine assays also showed no significant changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D2 and 25,26-dihydroxyvitamin D2 in a single plasma sample by mass fragmentography

A specific and sensitive assay for the measurement of the concentration of 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D2 and 25,26-dihydroxyvitamin D2 in a single plasma sample is described, using stable isotope dilution mass fragmentography. After addition of appropriate deuterium-labelled internal standards, plasma samples were treated with acetonitrile to precipitate protein, and vitamin D metabolites were extracted on prepacked microparticulate reverse-phase cartridges. Further purification was achieved using straight-phase cartridges and high-performance liquid chromatography. Gas chromatography-mass spectrometry was carried out after appropriate derivatisation of samples and standards. The method has been evaluated in terms of specificity, recovery of added standards, and reproducibility.

Comparisons between two receptor assays for 1,25-dihydroxyvitamin D

We present a competitive protein binding assay (CPBA) for 1,25(OH)2D employing 1,25(OH)2D receptor from calf thymus, which was compared with a CPBA-employing receptor from rachitic chick intestine. The thymus receptor assay was more sensitive, specific and precise than the intestinal receptor assay. The thymus receptor assay measured both 1,25(OH)2D2 and 1,25(OH)2D3 with equal affinity, whereas 1,25(OH)2D2 was 1.1 times less potent than 1,25(OH)2D3 in the displacement from the chick intestinal receptor. Mean serum values of 1,25(OH)2D in normal subjects, post-menopausal women, pregnant women, and patients with chronic renal failure measured by the two assay systems did not differ. Furthermore, both assays showed that 1,25(OH)2D was unchanged in post-menopausal women after treatment with vitamin D2 or vitamin D3, 4000 IU/day for 8 weeks. We conclude that the high sensitivity of the thymus receptor and the equal affinity for the D2 and D3 analogue make the thymus receptor assay a reliable alternative to the chick intestinal receptor assay.

Different metabolism of vitamin D2/D3 in epileptic patients treated with phenobarbitone/phenytoin

Serum concentrations of vitamin D metabolites were measured before and during treatment with either vitamin D2 or vitamin D3, 4000 IU per day for 24 weeks, in 22 epileptic outpatients receiving phenobarbitone/phenytoin. The serum concentration of total 1,25(OH)2D did not change during the treatment period in any of the treatment groups. On the other hand, in the vitamin D2 group, serum 25(OH)D2, total 25(OH)D, and 24,25(OH)2D increased significantly during the trial, whereas serum concentrations of the vitamin D3 metabolites were unchanged. In the vitamin D3 group, serum concentrations of the vitamin D3 metabolites increased significantly, whereas the vitamin D3 metabolite levels remained unchanged. However, vitamin D3 treatment resulted in a 2-4-fold greater increase in serum concentrations compared to vitamin D2 treatment. Treatment with vitamin D2 and vitamin D3 in the same dose in IU results in considerably different serum concentrations of the vitamin D metabolites.

Discrepancy between serum 1,25-dihydroxycholecalciferol measured by radioimmunoassay and cytosol radioreceptor assay

A radioimmunoassay for determination of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) in serum, was compared with the radioreceptor assay using cytosol receptor from rachitic chick intestine in order to clarify whether differences in specificity can explain discrepancies in the clinical application of the two techniques. In the literature, seasonal fluctuations in serum 1,25(OH)2D3 are only observed when using radioimmunoassay. Treatment of anticonvulsant osteomalacia with vitamin D3 results in a marked increase in radioimmunologically measured 1,25(OH)2D3, but no effect on the serum 1,25(OH)2D level, measured by the radioreceptor assay, could be observed. The present study demonstrates that another unknown compound is coeluted on high pressure liquid chromatography with 1,25(OH)2D3, and recognised only by the antiserum. It can be concluded that the present radioimmunoassay cannot replace the cytosol receptor assay or either supplement by measuring the 1,25(OH)2D3 metabolite only.