Specific vitamin D receptors are found in parts of the brain and spinal cord (Maxwell, 2001). Seasonal changes in 25-OHD and 1,25-OHD could have an effect on hormonal function, mood and behaviour. For example, seasonal affective disorders (SAD) appear to have a latitude gradient, with mood changes due to a reduction in daylight hours and altered circadian secretion of melatonin. Whether seasonal changes in UV light and vitamin D contribute is unknown.
Mortality rates from colon cancer are highest in those areas that receive the least amount of sunlight. A prospective study of 26000 volunteers investigated the association between 25-OHD and the risk of colon cancer. In those with 25-OHD concentrations of 50nmol/L (20ng/ml) or more, the risk of colon cancer was decreased threefold. However, confounding factors such as consumption of milk, meat or fat in the diet were not considered but these observations and previous epidemiological and laboratory studies suggest good vitamin D status in conjunction with calcium nutrition might lower the risk of colon cancer (Garland et al, 1989).
Experimental evidence from animals, both in vitro and in vivo, has shown an immunological role for 1,25 OHD3 in both lymphocytes and monocytes (Yang et al, 1993). Strict lactovegetarians, particularly in immigrant Asians, have an 8.5fold increased risk of tuberculosis compared with those who ate meat or fish daily. Since vitamin D deficiency is more common among vegetarian Asians and it is known to have effects on immunological function in animals, vitamin D deficiency may be responsible for reduced immunocompetence (Maxwell, 2001).
The mechanism for the immunological role of vitamin D is not known, but the hormone receptor for 1,25-OHD is now recognised as one of a superfamily, the so-called 'Steroid-Thyroid-Retinoid-Superfamily'. It is understood that if these nuclear receptors and their activating substances are to recognise response elements within responsive genes, they must act in pairs and a member of the retinoid family must serve as a partner if the dimer is to function. Thus vitamin A, usually in the form of retinoic acid, is a regulator for several hormone response systems including vitamin D (Kliewer et al, 1994) (see section 3.5.2). The anti-infection properties of vitamin A are widely recognised and interactions between vitamins A and D status may therefore be important in regulating immune function.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.