A powerful association exists between hypovitaminosis D and metabolic syndrome in obese patients that is independent from body fat mass and its clinical correlates, according to a study titled ‘Hypovitaminosis D is Independently Associated with Metabolic Syndrome in Obese Patients’, published in Plos One.
According to the study authors, the outcomes from the researcher indicates that the association between low 25(OH) D3 levels and metabolic syndrome is not merely induced by vitamin D deposition in fat tissue but reinforces the hypothesis that hypovitaminosis D represent a crucial independent determinant of MS.
They explain that vitamin D is a lipophilic hormone synthesised in the skin by ultraviolet-mediated isomerisation of 7-dehydrocholesterol and subsequently converted to active 1,25(OH)2D3 by two consecutive renal and hepatic hydroxylations.
However, as well as playing a role in calcium-phosphate regulation and bone metabolism, it has a potential role in the development of insulin resistance-related conditions, such as obesity, type 2 diabetes mellitus, systemic hypertension and metabolic syndrome. Previous studies have reported that vitamin D insufficiency is now considered to involve more than 75% of those with MS.
They also note that obese patients show a reduced response of serum 25(OH) vitamin D3 levels to ultraviolet-B irradiation and to oral vitamin D administration, compared with non-obese individuals.
Therefore, the investigators wanted to examine whether there an independent association between low 25(OH) vitamin D3 and metabolic syndrome in obese patients with and without metabolic syndrome.
“Two groups of subjects with the same degree of obesity, selected based on the presence or absence of MS, were compared to determine the contribution of body fat mass in the association between vitamin D insufficiency and the development of metabolic abnormalities,” they write. “This is to our knowledge the first study designed to establish the role of body fat mass in the link between hypovitaminosis D and MS.”
The investigators recruited 107 consecutive obese patients, 61 patients with a diagnosis of metabolic syndrome (30 patients with 3, 21 patients with 4 and 10 patients with 5 metabolic syndrome components) and 46 without MS, among subjects referring to the Endocrinology day-hospital of Sapienza University of Rome who underwent metabolic evaluation.
The two groups were comparable for sex, age, BMI, waist circumference and body fat percentage.
They report that serum 25(OH) vitamin D3 levels were significantly reduced in obese patients with metabolic syndrome, compared to obese subjects without metabolic syndrome, all comparable for sex, age, BMI, waist circumference and body fat mass percentage (13.5(3.3–32) ng/ml vs. 17.4(5.1–37.4) ng/ml, p<0.007, respectively). This difference persisted also excluding patients affected by type 2 diabetes (n = 26) from study population (14.4(5.7–32) ng/ml vs. 17.4(5.1–37.4) ng/ml, p = 0.01, respectively).
As expected, patients with metabolic syndrome had PAS, PAD, FBG, HbA1c and blood insulin significantly higher and HDL lower than subjects without metabolic syndrome. Reduction of insulin sensitivity as expressed by increased HOMA-IR and lower ISI was detected in metabolic syndrome patients, compared with non-MS.
Serum 25(OH) vitamin D3 concentrations inversely correlated with FBG (Pearson’s coefficient: -0.26, p<0.007) serum phosphate (Pearson’s coefficient: −0.21, p<0.03) and PTH levels (Pearson’s coefficient: −0.28, p<0.003) but were not associated with anthropometrical parameters, fat mass percentage, the diagnosis of type 2 diabetes and insulin resistance indexes in the univariate analysis.
Linear multivariate regression analysis demonstrated that low serum 25(OH) vitamin D3 levels are associated with the presence of metabolic syndrome independently from gender, age, BMI and serum PTH concentrations (Table 1).
Table 1: Multivariate linear regression analysis
“In this study, we demonstrated that serum 25(OH) vitamin D3 levels are significantly lower in obese patients affected by metabolic syndrome than in obese subjects without metabolic syndrome and comparable for sex, age, BMI, waist circumference and body fat mass,” the authors write. “The association between low serum 25(OH) vitamin D3 concentration and the diagnosis of MS was independent from PTH levels and the presence of type 2 diabetes.”
They also note that although the existence of an independent association between hypovitaminosis D and dysmetabolic conditions such as metabolic syndrome, type 2 diabetes, hypertension and liver steatosis has been demonstrated in several studies, the potential cause-effect relationship between the presence of low 25(OH) vitamin D3 levels and obesity/obesity-related conditions is still debatable.
“Reduced serum 25(OH) vitamin D3 concentrations represent a determinant of metabolic syndrome in obese patients,” they conclude. “The insulin-sensitising action of vitamin D rather than the distribution volume of this hormone is likely to be responsible for the thigh association between hypovitaminosis D and dismetabolic conditions.”