Читать книгу Health Psychology - Michael Murray - Страница 36

The importance of the molecule vitamin D has been recognized since its discovery by Edward Mellanby in 1920. The chemical structure of vitamin D was determined in 1932, and it was only then found to be a steroid hormone, more specifically, a secosteroid. Vitamin D normally arises in the skin from sunlight or it can come from food, such as oily fish, or from supplements. It is metabolized in the liver and kidney. Vitamin D metabolites appear to be involved in a host of cellular processes, including calcium homeostasis, immunology, cell differentiation and regulation of gene transcription (Bouillon et al., 1995). Vitamin D is the main hormone regulating calcium phosphate homeostasis and mineral bone metabolism. A variety of tissues can express vitamin D receptor (VDR) and vitamin D is implicated in the regulation of the IS, the cardiovascular system, oncogenesis and cognitive functions (Halfon et al., 2015).

Hormones can act as immunomodulators, altering the sensitivity of the IS. T cells have a symbiotic relationship with vitamin D, by binding to the steroid hormone version of vitamin D, calcitriol, but T cells express the gene CYP27B1, the gene responsible for converting the pre-hormone version of vitamin D, calcidiol, into the steroid hormone version, calcitriol. The decline in hormone levels with age is partially responsible for the weakened immune responses in older people. Conversely, some hormones are regulated by the IS, notably thyroid hormone activity. The age-related decline in immune function is also related to decreasing vitamin D levels in the elderly. As people age, two things happen that negatively affect their vitamin D levels. First, they stay indoors more due to decreased activity levels. This means that they get less sun and therefore produce less vitamin D via solar radiation. Second, as a person ages the skin produces less vitamin D.

Hypovitaminosis D is associated with decreased muscle function and performance and an increase in disability. On the other hand, vitamin D supplementation improves muscle strength and gait, especially in elderly patients. A reduced risk of falls has been attributed to vitamin D supplementation due to direct effects on muscle cells. Finally, a low vitamin D status is associated with a frail phenotype. Many authorities recommend vitamin D supplementation for frail patients.

Vitamin D deficiency is a factor in a variety of illnesses. Pereira-Santos et al. (2015) found that the prevalence of vitamin D deficiency was 35% higher in obese people and 24% higher in overweight people. Vitamin D deficiency was associated with obesity irrespective of age, latitude, cut-offs to define vitamin D deficiency and the Human Development Index of the study location. Altered vitamin D and calcium homeostasis are also associated with the development of Type 2 diabetes mellitus. Pittas et al. (2007) reviewed observational studies and clinical trials in adults with outcomes related to glucose homeostasis. Observational studies showed an association between low vitamin D status, calcium or dairy intake and prevalent Type 2 diabetes mellitus or metabolic syndrome. There are inverse associations with the incidence of Type 2 diabetes mellitus or metabolic syndrome. Trials with vitamin D and/or calcium supplementation suggest that combined vitamin D and calcium supplementation may have a preventive role for Type 2 diabetes mellitus only in populations at high risk (i.e., those with glucose intolerance).