Fat soluble vitamin - Vitamin D
Dietary vitamin D exists as either ergocalciferol (vitamin D2) or cholecalciferol (vitamin D3). Ergocalciferol (D2) is derived from the UV irradiation of the plant sterol ergosterol, which is widely distributed in plants and fungi. (D3 ) is formed from the action of UV irradiation on 7-dehydrocholesterol in the skin of animals including humans.
Dietary sources are relatively insignificant, compared with the synthesis in the skin from exposure to sunlight or ultraviolet rays, because there are not many rich food sources of vitamin D.
Vitamin D is not classically a vitamin but a pro-hormone, acting as a precursor to one of the hormones involved in calcium homeostasis. Cholecalciferol is metabolised to the active steroid hormone 1,25-dihydroxyvitamin D3 in the liver and kidney. In this form it works as a hormone regulating the amount of calcium absorbed in the intestine. It is also essential for the absorption of phosphorus and for normal bone mineralisation. Vitamin D is also involved in the regulation of cell proliferation and differentiation. Vitamin D is also an activator of insulin-like growth factor (IGF-1) and, associated with this, poor vitamin D status is linked to sarcopenia (age related loss of skeletal muscle) which affects up to 25% of those over the age of 65 years and more than half of those over 85.
Deficiency of vitamin D results in poor calcification of the skeleton and hence skeletal deformity in children (rickets) and it leads to pain and bone fragility in adults (osteomalacia). Osteoporosis is not due to vitamin D deficiency but vitamin D may be beneficial in treatment. In the UK some groups of people such as Asian, black, older, institutionalised and housebound people and those who habitually cover the skin are vulnerable to vitamin D deficiency as a result of limited exposure to sunlight. Poor vitamin D status and rickets in children used to be commonplace in the UK but fortification and supplementation policies following the Second World War made rickets a thing of the past. However, in recent years, cases are again being reported, particularly in some ethnic minority groups from the Middle East and Indian subcontinent. Poor vitamin D status (a blood level of 25hydroxyitaminD below 25nmol/L – judged to be sufficient to prevent rickets) is also commonplace in the white population of the UK, emphasising the importance of balancing the need for sun exposure with the use of sun screen. For example, 36% of men and 38% of women aged 65-84 and living in institutions had low status in the NDNS for this age group. Amongst other age groups, the worst statistics were for young adults aged 19-24 years; 24% males and 28% of females. As sunlight is the major source, status tends to be lower in the winter/ spring than summer/autumn. Many young women enter pregnancy with poor stores of the vitamin.
It is recommended that pregnant and lactating women and people aged 65 years and over take vitamin D supplements (10µg per day). For other ‘at risk’ groups, for example ethnic groups that have limited sun exposure because of their style of dress, supplements may also be necessary. Infants are recommended to receive supplements containing 7.5µg of vitamin D; and these are available under the Healthy Start Scheme.
Excessive dietary vitamin D intake may lead to hypercalaemia (high calcium level in the blood), and some infants are especially sensitive to hypercalcaemia resulting from vitamin D toxicity. It is thought that skin synthesis is self-regulating.
Oily fish, eggs, fortified cereals and margarine are the main dietary sources of vitamin D. In the UK, the law states that margarine must be fortified with vitamin D (and vitamin A). Vitamin D is also often voluntarily added to reduced fat spreads, as is vitamin A. Human milk contains low levels of vitamin D, but infant formula is fortified with 0.001-0.0025 mg/100kcal.
Most vitamin D is obtained through the action of sunlight on our skin during the summer months. The latitude and strength of the sun in the UK means that the skin can only make vitamin D between 11am and 3pm, during the months of April to October.
© British Nutrition Foundation