The role of vitamin D in human health

Vitamin D is a steroid hormone precursor produced photochemically when ultraviolet light rays interact with cholesterol in the skin. It is then activated in the kidneys and liver to become active hormone calcitriol (1,25 dihydroxy-vitamin D). Sources of vitamin D are: diet (certain fish and fish oils), sunlight exposure and supplements. Latitude, skin pigmentation, age, and use of sunscreens dramatically affect skin production of vitamin D. There are also seasonal variations in vitamin D biosynthesis so that one cannot make vitamin D during winter months.

1,25 dihydroxy-vitamin D (calcitriol) is a classical calcitropic hormone that is important to many aspects of good health. It has an important role in the regulation of cell growth, bone formation, immune function, muscle strength, hair growth, fighting infection, reducing the risk of autoimmune disease and possible protection against certain cancers. In addition to its well known calcitropic action it has numerous non-calcitropic actions which are important in cell proliferation and differentiation (particularly in the skin, muscle and immune system).

When assessing vitamin D status 25(OH) vitamin D should be measured. Levels below 25 nmol/L are regarded as severe deficiency. Levels between 25 and 50 nmol/L are regarded as mild deficiency. Suboptimal concentrations of vitamin D are between 50 and 75 nmol/L. Levels above 75 nmol/L are regarded as normal; in patients with osteopenia/osteoporosis one should aim for vitamin D levels between 100 and 160 nmol.L.

Vitamin D deficiency is a worldwide problem as a result of reduced sun exposure (including the use of sunscreens) and inadequate consumption from diet and supplements. Vitamin D deficiency is associated with stunting of growth and rickets in children and with increased risk of osteoporosis and osteomalacia in adults. Vitamin D deficiency is also associated with an increased risk of many chronic diseases such as some common cancers (breast cancer, prostate cancer, bowel cancer), autoimmune disease(type 1 diabetes and multiple sclerosis) and cardiovascular diseases (increased risk of sudden cardiac death).

Vitamin D supplementation has an important immunomodulatory role and is associated with improved outcomes in multiple sclerosis and tuberculosis. It is currently believed that vitamin supplementation has significant anti-oncogenic activities (linear dose response in relative risk reduction has been documented for colon cancer).

In the absence of sun exposure at least 1000IU of vitamin D per day is required to maintain healthy levels of vitamin D in circulation (between 75 and 150 nmol/L). In those with severe vitamin D deficiency (levels below 12 nmol/L) 50,000IU per week for 12 weeks followed by 50,000 IU per month for another three months is needed to normalize vitamin D level. For those with concentrations between 12 and 37 nmol/L 50,000 IU per week for four weeks followed by 50,000 IU per months for five months is needed to normalise vitamin D levels. In mild deficiency (levels between 40 and 75 nmol/L) one should supplement 50,000 IU per month for six months. The best supplementation of vitamin D is achieved via oral route. Both vitamin D2 (ergocalciferol) which is of plant origin and vitamin D3 (cholecalciferol) which is of animal origin are equally potent in correcting vitamin D deficiency.