Traditionally vitamin D has been viewed as an essential nutrient necessary only for correct calcium metabolism. This is reflected in the classic vitamin D deficiency diseases of the bones including osteomalacia and rickets in adults and children, respectively. However, more recently vitamin D has been shown to elicit far wider metabolic control than just its traditional role in calcium metabolism. More recent studies indicate that vitamin D deficiency diseases may include cancer, insulin resistance, cardiovascular disease and diabetes. New information regarding the function and molecular role for vitamin D has called into question the recommended intake of 200 to 400 IU of vitamin D as being too low for maintenance of human health. In the absence of strong sunlight intakes of 2000 IU per day for adults are now recommended as the minimum requirement to maintain health. Such a high intake would require the use of supplements as intakes of vitamin D at this level are not possible through diet alone.
However, in a twist to this story, some researchers have suggested that foods might contain the biologically active form of vitamin D, called 25-hydroxyvitamin D. Following the ingestion of vitamin D or its production in the skin through the action of ultraviolet light, it is transported to the liver and hydroxylated to form 25-hydroxyvitamin D. This hydroxylated form of vitamin D is the generally accepted biological marker for vitamin D status and one of the known activated forms. The realisation that food contains this activated form of 25-hydroxyvitamin D, and that it has superior biological activity to its non-hydroxylated substrate, has lead some to question the reported vitamin D content of some foods. The presence of 25-hydroxyvitamin D in certain foods may dramatically increase its total vitamin D activity even if the 25-hydroxyvitamin D is only in small amounts. Human milk for example contains a range of vitamin D metabolites including the activated 25-hydroxyvitamin D form.
Human milk is interesting because it is an ideal food to grow a human. With this in mind, the nutritional properties of milk are of relevant to human nutrition in terms of understanding which substances are essential for growth. The vitamin D content of human milk has been analysed by researchers and a number of metabolites and forms of the vitamin found1. Traditionally human milk was thought to contain around 10 IU/L of vitamin D, and was therefore a poor source of the vitamin, despite the fact that breastfed infants do not develop vitamin D deficiencies. However, human milk may contain around 40 to 50 IU/L of total vitamin D activity because of the presence of vitamin D metabolites. In total five main metabolites are present in human milk. These are vitamin D3, vitamin D2, 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D. The 25 hydroxyvitamin D content of the milk is around 163 ng/L, and this provides 33 IU/L of vitamin D activity alone.
Both vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol) have been detected in human milk at concentrations of 338 and 41 ng/L, respectively (equivalent to 14 to 16 IU of vitamin D activity). Cholecalciferol is the animal form of vitamin D, whereas ergocalciferol is the plant form, the latter not being produced in humans. This suggests that some of the vitamin D in human milk is derived from the diet of the mother. Roughly 75 % of the vitamin D activity of human milk is provided by the 25-hydroxyvitamin D metabolite, which explains the efficacy of milk against rickets despite its reported low vitamin D content. The ability of vitamin D to prevent rickets despite low concentrations of vitamin D lead some research to look for vitamin D metabolites and one water soluble metabolite was detected. This fraction was identified as vitamin D-3β-sulphate but was later dismissed as not relevant to human nutrition. However, the more recent identification of 25-hydroxyvitamin D in milk explains is antirachitic activity.
Dr Robert Barrington’s Nutritional Recommendation: The traditional view of vitamin D is outdated in that as a steroid hormone, vitamin D has a far wider metabolic role than was once considered. Food sources of vitamin D are limited, and in this respect dietary intakes of vitamin D have been dismissed as inadequate to provide the new higher intakes suggested by more modern research. However, the realisation that hydroxylated versions of the vitamin may provide most of the vitamin D activity in foods calls into questions traditional food tables that have calculated vitamin D activity from its non-hydroxylated forms. In human milk for example, 75 % of the total vitamin D activity comes from 25-hydroxyvitamin D. Until our understanding of the vitamin D activity of foods improves through careful analysis of food including all active metabolites, supplements of vitamin D are recommended in order to ensure adequate intake of this important steroid hormone.
RdB
