In humans vitamin is activated to 25-hydroxyvitamin D in the liver. Plasma levels of 25-hydroxyvitamin D are the accepted marker for vitamin D status. Levels of 25-hydroxyvitamin D are increasingly being reported as being insufficient for human health. Vitamin D can be produced by the actions of ultraviolet B radiation on the skin, but increasing trends towards avoiding sun exposure thorough lifestyle habits is reducing endogenously synthesised vitamin D. As a result people are becoming more reliant of dietary sources. However, dietary vitamin D is present in a limited number of foods, and reliance of dietary source of vitamin D can therefore result in severe vitamin D insufficiencies that manifest in diseases including type 2 diabetes, cancer and cardiovascular disease. One group of individuals that is at risk are strict vegetarians, who excludes intakes of vitamin D rich foods such fish and milk. Research suggest that vegetarians have lower levels of 25-hydroxyvitamin D than omnivores.
Researchers1 have investigated the vitamin D status of a number of different groups of individuals in order to assess 25-hydroxyvitamin D status. Strict vegetarians, lactovegetarians, lactoovovegetarians that ate some fish, and vegetarians taking vitamin D supplements or having regular sun exposure, were compared to control subjects who ate normal mixed diets and had no health problems. The results showed that dietary vitamin D intake was significantly lower in the vegetarian groups than the control group. However worryingly the vitamin D intake was below the 5 µg/d recommended allowance in all groups. The 25-hydroxyvitamin D levels of the strict vegetarians (27 nmol/L), the lactovegetarians (27 nmol/L) and lactoovovegetarians that ate some fish (39 nmol/L) were significantly lower than the control group (47 nmols/L). A number of individuals in all these groups had 25-hydroxyvitamin D levels below the lower reference limit suggesting the presence of a severe insufficiency and a significant increased risk of disease.
Interestingly those vegetarians taking supplements or being exposed to regular sun exposure had the highest levels of 25-hydroxyvitamin D (63 nmols/L). This highlights the importance of the sun and vitamin D supplements in providing adequate vitamin D status and supports data from other studies. The poor vitamin D status of the strict vegetarians was reflected in higher parathyroid hormone levels that indicated secondary hyperparathyroidism. In addition, calcium intake was lower in the strict vegetarians than the control group. Dietary fibre was also measured because it may inhibit calcium absorption, and dietary fibre was found to be higher in all the vegetarian groups than the control group. These results suggest that the vitamin D and calcium status of strict vegetarians may be such that their long term health may be compromised. Current recommendations for adults is the consumption of 2000 IU of vitamin D (in the D3 cholecalciferol form) every day in the absence of exposure to strong summer sunlight.
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