Vitamin D

Food

Few foods naturally contain vitamin D. The flesh of fatty fish (such as trout, salmon, tuna, and mackerel) and fish liver oils are among the best sources [17,1]. An animal’s diet affects the amount of vitamin D in its tissues. Beef liver, egg yolks, and cheese have small amounts of vitamin D, primarily in the form of vitamin D3 and its metabolite 25(OH)D3.

Animal-based foods typically provide some vitamin D in the form of 25(OH)D in addition to vitamin D3. The impact of this form on vitamin D status is an emerging area of research. Studies show that 25(OH)D appears to be approximately five times more potent than the parent vitamin for raising serum 25(OH)D concentrations [17,20,21]. One study found that when the 25(OH)D content of beef, pork, chicken, turkey, and eggs is taken into account, the total amount of vitamin D in the food is 2 to 18 times higher than the amount in the parent vitamin alone, depending on the food [20].

In foods and dietary supplements, vitamin D has two main forms, D2 (ergocalciferol) and D3 (cholecalciferol), that differ chemically only in their side-chain structures. Both forms are well absorbed in the small intestine. Absorption occurs by simple passive diffusion and by a mechanism that involves intestinal membrane carrier proteins [4]. The concurrent presence of fat in the gut enhances vitamin D absorption, but some vitamin D is absorbed even without dietary fat. Neither aging nor obesity alters vitamin D absorption from the gut [4].

Moreover, 25(OH)D absorbed from the diet has been found to be more potent in increasing serum concentrations of 25(OH)D than is the equivalent amount of nonmetabolized dietary vitamin D (4). When the 25(OH)D content of animal-derived foods is not measured and included in tables of food composition, the ability to obtain a reasonable estimate of total vitamin D intake, and in turn its impact on vitamin D status, is not only uncertain but very likely to lead to underestimates of true intake. There have been reports concerning the discrepancy between recommended intakes for vitamin D and the reported vitamin D intakes based on national surveys that are coupled to available tables of food composition (5). This has caused public health concern and led to discussions about the need to increase vitamin D fortification of the food supply. However, tables of food composition in the United States currently report quantitative amounts in foods for only the nonmetabolized forms of vitamin D. This may explain, in part, the discrepancy between the reported population-based intakes of vitamin D and the measures of serum 25(OH)D from national surveys, the latter being much higher than would be expected given the estimates of intake (6). We, in turn, agree with the conclusion that some, but not all, of the difference between intake and serum measures can be attributed to the contribution of sunlight (7), and the failure to take into account the contributions of 25(OH)D from food is likely a significant factor in this regard.