Metabolic syndrome is a disorder characterised by insulin resistance and abdominal obesity. This causes metabolic disturbances that include detrimental changes to blood lipids and deposition of fat in the viscera. The result is an increase in the risk of cardiovascular disease and type 2 diabetes. Metabolic syndrome is a Western lifestyle disease, and diet is thought to be the main causative factor in its development. In particular fructose consumption and high intakes of refined carbohydrates are thought to significantly increase the risk of developing the insulin resistance that characterises the disorder. However, other factors may play a role in the aetiology of insulin resistance and a number of factors have been indentified in the literature. For example, low intakes of vitamin D are though to contribute to insulin resistance and epidemiological studies have shown an inverse association between vitamin D intakes and cardiovascular disease.
An inverse association between vitamin D intakes and metabolic syndrome has recently been reported in the literature. Researchers used data from a cohort study of 4727 black and white subjects to investigate the relationship between dietary and supplemental vitamin D and metabolic syndrome over 20 years. High intakes of vitamin D, both supplemental and dietary, were inversely associated with the prevalence of abdominal obesity, a clinically relevant parameter for the diagnosis of metabolic syndrome. In addition, the authors reported that vitamin D intake was inversely associated with high plasma glucose levels and positively associated with high density lipoprotein levels. The hazard ratios for developing metabolic syndrome in the second to fifth quintile, compared to the lowest quintile for vitamin D intake, were 0.82, 0.84, 0.70 and 0.82, respectively. Therefore higher intakes of vitamin D may be protective of metabolic syndrome, which may also explain the protective effect against cardiovascular disease and type 2 diabetes.
The reason that vitamin D may be protective of metabolic disease is not fully understood. However, it may relate to the calcium paradox. Theory suggests that low plasma levels of 25-hydroxyvitamin D, an active metabolite of vitamin D, in combination with low dietary intakes of calcium, result in low plasma levels of calcium. This causes an increase in cellular levels of calcium ions, and this in turn interferes with cellular function. In particular, high cellular levels of calcium may interfere with the insulin signal cascade, which may be pivotal in inducing insulin resistance. Lifestyle changes have decreased sunlight exposure in Western living and as a result many individuals are deficient in vitamin D. In addition, research suggests that the current recommended plasma levels of vitamin are too low, and that they should be revised upwards. Dietary sources of vitamin D are limited and supplements are therefore recommended.
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