Abstract and Introduction
Abstract
Experimental studies suggest that vitamin D modulates the activity of adipocytes. The authors examined baseline serum 25-hydroxyvitamin D (25(OH)D) level in relation to prevalent and cumulative incident obesity in Norway. A cohort of 25,616 adults aged 19–55 years participated in both the second and third surveys of the Nord-Trøndelag Health Study (HUNT 2 (1995–1997) and HUNT 3 (2006–2008)). Serum 25(OH)D levels measured at baseline and anthropometric measurements taken at both baseline and follow-up were available for a random sample of 2,460 subjects. Overall, 40% of the 2,460 subjects had a serum 25(OH)D level less than 50.0 nmol/L, and 37% had a level of 50.0–74.9 nmol/L. The prevalence and cumulative incidence of obesity, defined as body mass index (weight (kg)/height (m)) ≥30, were 12% and 15%, respectively. Lower serum 25(OH)D level was associated with a higher prevalence of obesity. In the 2,165 subjects with baseline BMI less than 30, a serum 25(OH)D level less than 50.0 nmol/L was associated with a significantly increased odds ratio for incident obesity during follow-up (adjusted odds ratio = 1.73, 95% confidence interval: 1.24, 2.41). When prevalent and incident obesity were classified according to waist circumference (≥88 cm for women, ≥102 cm for men), similar results were obtained. In addition to prevalent obesity, a serum 25(OH)D level less than 50.0 nmol/L was significantly associated with new-onset obesity in adults.
Introduction
Both obesity and vitamin D insufficiency are highly prevalent worldwide. The two conditions are associated with each other, but the nature of the association requires clarification. Obesity has been proposed to lead to low body vitamin D status, although this directional association has been investigated in very few prospective epidemiologic studies. Because of its hydrophobic characteristics, vitamin D may move out of the circulation into the large amount of adipose tissue in obese persons, resulting in low serum levels of 25-hydroxyvitamin D (25(OH)D) even if the total body storage of vitamin D might be adequate. There is also a possibility that low vitamin D status increases the risk of obesity, since vitamin D appears to modulate the catabolic and anabolic activity of adipocytes in experimental studies. A recent study of Colombian children demonstrated that low serum 25(OH)D level was associated with a significant increase in adiposity, measured by changes in body mass index (BMI; weight (kg)/height (m)), skinfold thickness, and waist circumference (WC), during a follow-up period of approximately 3 years. By contrast, another prospective epidemiologic study from the United States did not observe any significant associations of 25(OH)D with changes in BMI and adipose tissue values in Hispanic and African-American adults after a 5-year follow-up. In the present study, we used data from the Nord-Trøndelag Health Study (HUNT) to investigate the cross-sectional and prospective associations of serum 25(OH)D level and obesity in adults, who were followed up for 11 years, on average.