Changes in the concentrations of vitamin D and its metabolites in the plasma of healthy subjects orally given physiological doses of vitamin D2 by multivitamin or vitamin D preparations

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.

Plasma 25-hydroxyvitamin D responses of younger and older men to three weeks of supplementation with 1800 IU/day of vitamin D

OBJECTIVE

The objective of this study was to compare changes in plasma 25-hydroxyvitamin D (25(OH)D) levels of younger and older men after three weeks of oral vitamin D supplementation.

METHODS

Nine younger men (22 to 28 years) and nine older men (65 to 73 years) with self-reported vitamin D intakes below 200 IU/d were enrolled in February and randomized to 1800 IU/d of ergocalciferol (vitamin D2, n=11) or to a control group (n=7) and followed for three weeks. Blood was collected at baseline, and after one, two and three weeks for measurement of plasma concentrations of total 25(OH)D, 25(OH)D2 and 25(OH)D3.

RESULTS

In both the younger and older supplemented men, 25(OH)D2 and total 25(OH)D concentrations increased significantly during the study, whereas values of these metabolites did not change in younger or older control subjects. No group showed significant changes in 25-hydroxyvitamin D3. There was a significant interaction between age group and supplement group, suggesting that the effect of vitamin D2 supplementation on changes in 25(OH)D2 changes with age. The mean increase in 25(OH)D2 was greater in the younger supplemented men than in the older supplemented men (37+/-9 nmol/L vs. 19.5 nmol/L, p=0.027), and this accounted for their significantly greater increase in total 25(OH)D.

CONCLUSION

These data are consistent with an age-related decline in the absorption, transport or liver hydroxylation of orally-consumed vitamin D.

Vitamin D deficiency in older people

Because of changes that occur with aging, older people with any other risk factors for vitamin D deficiency are likely to have inadequate stores of this vitamin. The consequences of vitamin D deficiency are likely to be losses in bone, strength, and function and the development of pain. Many questions remain regarding screening, prevention, and treatment of vitamin D deficiency. Supplementation may be unnecessary in most healthy, ambulatory seniors. Excessive supplementation in this group may lead to vitamin D toxicity. There does seem to be a role for supplementation in homebound older people who will not get adequate vitamin D from sunlight exposure. This population is at particular risk of developing vitamin D deficiency. Issues such as inadequate diet, physiologic changes with aging, polypharmacy, and diseases that interfere with vitamin D metabolism contribute to this risk. In such circumstances, a recommendation of 800 IU per day is reasonable. An alternative to daily dosing is a single oral dose of 100,000 IU of vitamin D (ergocalciferol or cholecalciferol) every 3 to 6 months. A simple maneuver is for geriatricians, who see many chronically ill patients with low vitamin D stores (who are likely to be seen in the office every 3 to 6 months), to administer vitamin D during the office visits. These dosing schedules have not been associated with toxicity and can be considered safe in homebound (sunlight-deprived) older adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevation of blood vitamin D2 levels does not impede the release of vitamin D3 from the skin

The mechanism for the transfer of fat-soluble vitamin D3 from the avascular basal cellular layers of the epidermis to dermal capillaries and peripheral circulation is unknown, although vitamin D-binding protein (DBP) is thought to mediate this process. To evaluate the effect of increased occupancy of vitamin D carrier(s) on vitamin D3 removal from the skin, serial serum vitamin D2 and D3 concentrations were determined in three groups of six healthy volunteers given combinations of an oral dose of vitamin D2 (50,000 IU) and a fixed dose of UVB radiation (27 mJ/cm2). Serum vitamin D3 levels increased significantly following UVB (time effect, P < .01 by ANOVA), but the response remained unchanged after pretreatment with vitamin D2, increasing from 3 +/- 1 to 14 +/- 5 ng/mL (mean +/- SEM), versus UVB alone, 5 +/- 1 to 16 +/- 5 ng/mL. Elevation of serum vitamin D2 levels was also similar in the groups given vitamin D2 alone (< 1 to 64 +/- 8 ng/mL) and vitamin D2 + UVB (< 1 to 45 +/- 8 ng/mL). There was no time or treatment effect for changes in serum levels of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, or (DBP) levels (P > .1). We conclude that vitamin D3 egress from the skin is not affected by elevated circulating vitamin D concentrations; thus, the cutaneous release of vitamin D is probably mediated by a protein such as DBP with a high carrying capacity for the vitamin.