The medical establishment still recognises childhood rickets, or osteomalacia and osteoporosis in adults as the only real consequence of vitamin D deficiency. However, a growing body of research is demonstrating that the effects of low blood levels of vitamin D are far more wide ranging than this myopic mainstream viewpoint (figure 1). As well as being necessary for proper calcium homeostasis, vitamin D is an essential component of the immune system and deficiency is increasingly being linked with many degenerative diseases such as cancer, rheumatoid arthritis, multiple sclerosis, diabetes, and cardiovascular disease. Sun exposure is the most efficient way of maintaining proper vitamin D levels, but paranoia about the dangers of the sun (stirred up by corporate vested interest) are causing an epidemic of vitamin D deficiency due to inadequate sun exposure.
Figure 1. Some of the health implications attributed to vitamin D.
To make vitamin D3 (cholecalciferol) the body requires ultraviolet B sunlight photons in the ~290 to ~315 nm range to penetrate the skin. The energy in the UVB light converts 7-dehydrocholesterol in the cell membranes of dermal fibroblasts and epidermal keratinocytes into cholecalciferol. Cholecalciferol diffuses from the cell membranes and binds to the vitamin D binding protein in capillaries and enters circulation, where it passes to the liver to be converted to 25(OH)D3 (here). Research has shown that this process of vitamin D formation is not efficient in high latitudes in winter month. Therefore populations above around 40 latitude will not be able to synthesise enough vitamin D. Vitamin D toxicity is not possible from sun exposure because excess sun exposure causes excess cholecalciferol to be converted to products with little biological activity.
There is a gradual reduction in the efficiency of vitamin D3 production as we age. From age 20 to age 70 there is roughly a 25 % reduction in cholecalciferol production. In addition, application of sunscreen (SPF 8.) has been shown to reducecholecalciferol production by greater than 95 %. Populations with dark skin require up to ten times longer in the sun in order to achieve adequate levels of cholecalciferol in the circulation because of their increased skin pigmentation. Since the industrial revolution, the percentage of the population spending daylight hours inside has increased, causing sun exposure times to fall. cholecalciferol can be stored in fat cells which is subsequently released into circulation during periods of sun abstinence. However, obesity prevents this release of cholecalciferol because it is stored in deep tissue.
Estimates suggest that the body uses 3000 to 5000 IU of cholecalciferol a day. In the absence of sun exposure, the intake of cholecalciferol must be around 1000 IU in order to maintain 25(OH)D3 of at least 30 ng/mL. Estimates suggest that blood levels of over 40 ng/ mL are within the normal range. Blood levels of 25(OH)D3 that were previously thought to be adequate for good health (5-10 ng / mL) are increasingly being found to be insufficient. Exposure to the whole body to tanning lamps or the sun is able to produce 10,000 to 20,000 IU of cholecalciferol, which is up to 50 times the recommended levels of 400 IU for adults. Studies have shown that administration of up to 10,000 IU per day for 20 weeks in studies resulted in a complete absence of toxicity.
Our diets are generally a poor source of cholecalciferol, as it is present in high quantities only in oily fish. Fortified food such as milk and orange juice are also poor sources of the vitamin. Therefore, more than 90 % of the cholecalciferol requirement for most people comes from sun exposure. Most multivitamins contain around 400 IU of cholecalciferol, which would suggest that additional supplementation is required to maintain optimal levels. If the multivitamin contains the vitamin D2 form (ergocalciferol), then amount of additional supplementation will have to be increased because ergocalciferolis only about 20 to 40 % as effective as cholecalciferol in maintaining serum concentrations of 25(OH)D (here). Artificial tanning is associated with higher levels of vitamin D3 (here) and is an effective way to maintain levels in winter months if sun exposure is not available.
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