Vitamin D status is measured by using the plasma concentration of 25-hydroxyvitamin D 25(OH)D which is formed following hydroxylation of vitamin D in the liver. Because vitamin D3 is formed in the skin from the action of ultra violet B radiation, sun exposure increase vitamin D and 25(OH)D plasma concentrations. It is known that avoiding sun exposure and the use of sun screens decreases cutaneous vitamin D production, but there is also a decline is general vitamin D status with age. Other factors may therefore be acting to reduce the production of vitamin D in the skin. One factor that has been identified in the literature is the thickness of the skin. During the ageing process skin thickness declines and this has been shown to be associated with a decline in the circulating levels of 25(OH)D.
For example, researchers1 have measured the skin fold thickness on the back of the hand, serum 25(OH)D, sun exposure and body weight in 433 postmenopausal women to investigate any associations. As had been reported elsewhere, serum 25(OH)D was associated with the length of exposure to the sun. Interestingly, this exposure had a 2 month lag time, which suggests that raised 25(OH)D levels in the summer months may provide adequate vitamin D status throughout the autumn. Serum 25(OH)D was also associated with skin thickness and inversely associated with body mass index. The seasonal variation in vitamin D status was greater in lean subjects compared to overweight subjects suggesting that body size alters the metabolism of vitamin D. Because skin fold thickness declines with age, this data suggests that the poorer vitamin D status of older individuals may be due to deteriorate is skin condition.
Additional factors are known to cause a decline is vitamin D status with age, including lifestyle factors that may limit the exposure to the sun. For example, elderly individuals in care homes have very poor vitamin D status compared to age matched controls living independent lives. However, this study confirms that age related declines in vitamin D status may also have physiological causes. The autumn fall and spring rise of 25(OH)D in the lean subjects was more rapid than those of the overweight individuals. Because vitamin D is fat soluble, it can be stored in adipose tissue. The larger size of adipose tissue in overweight individuals may act as a pool of vitamin D that is released to plasma as levels decline in the autumn, and which stores newly formed vitamin D created in the spring.
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