Vitamin D, obesity, the metabolic syndrome and its sequelae

The increased risk of vitamin D deficiency in obesity is well established. There is substantial variation in the estimated strength of the association between different population groups, likely reflecting methodological issues in measuring adiposity and 25(OH)D concentrations. Genetic studies have shown that higher body mass index (BMI) is a causal risk factor for low 25(OH)D concentrations. Clinical trial evidence for benefits of vitamin D supplementation in terms of affecting weight gain or adiposity remains limited. Some trials, typically with concomitant calcium supplementation and low calcium intakes, have suggested potential reductions in central fat deposits. Vitamin D supplementation is less effective in increasing 25(OH)D concentrations in obese compared with normal-weight participants. Evidence of harm by vitamin D deficiency is becoming well established. At the physiological and biochemical levels, active hormonal vitamin D has many mechanistic associations that may help counteract ill effects of obesity.