Relationships of low serum vitamin D₃with anthropometry and markers of the metabolic syndrome and diabetes in overweight and obesity

Low serum 25 hydroxyvitamin D₃ (vitamin D₃) is known to perturb cellular function in many tissues, including the endocrine pancreas, which are involved in obesity and type II diabetes mellitus (TIIDM). Vitamin D₃ insufficiency has been linked to obesity, whether obesity is assessed by body mass index (BMI) or waist circumference (waist). Central obesity, using waist as the surrogate, is associated with the metabolic syndrome (MetSyn), insulin resistance, TIIDM and atherosclerotic cardiovascular disease (CVD). We tested how vitamin D₃ was related to measures of fat mass, MetSyn markers, haemoglobin A_1c (HbA_1c) and MetSyn in a cross-sectional sample of 250 overweight and obese adults of different ethnicities. There were modest inverse associations of vitamin D₃ with body weight (weight) (r = -0.21, p = 0.0009), BMI (r = -0.18, p = 0.005), waist (r = -0.14, p = 0.03), [but not body fat % (r = -0.08, p = 0.24)], and HbA_1c (r = -0.16, p = 0.01). Multivariable regression carried out separately for BMI and waist showed a decrease of 0.74 nmol/L (p = 0.002) in vitamin D₃ per 1 kg/m² increase in BMI and a decrease of 0.29 nmol/L (p = 0.01) per 1 cm increase in waist, with each explaining approximately 3% of the variation in vitamin D₃ over and above gender, age, ethnicity and season.

The similar relationships of BMI and waist with vitamin D₃ may have been due to associations between BMI and waist, or coincidental, where different mechanisms relating hypovitaminosis D₃ to obesity occur concurrently. Previously reviewed mechanisms include that 1) low vitamin D₃, may impair insulin action, glucose metabolism and various other metabolic processes in adipose and lean tissue 2) fat soluble-vitamin D₃ is sequestered in the large adipose compartment, and low in serum, 3) obese people may be sensitive about their body shape, minimising their skin exposure to view and sunlight (not tested). We showed evidence for the first theory but no evidence to support the second.

In the current study, serum vitamin D₃ was inversely related to weight, BMI and markers of TIIDM (large waist, raised HbA_1c) but not to adipose mass nor to MetSyn per se.