Obesity and Vitamin D Deficiency: Correlation or Causation?
One consistent finding in vitamin D research is between obesity and deficiency. People with higher measures of body fat tend to have lower vitamin D levels. The most recent issue of the journal Nutrition, Metabolism and Cardiovascular Disease today has three articles that look at the relationship between obesity and vitamin D levels. The research to date has been unable to determine whether lower vitamin D levels are caused by lifestyle factors related to obesity, by dilution due to higher body mass, or vitamin D sequestering from increases in fat mass, or another reason. The recent articles may shed some more light on this question that has been puzzling researchers for decades (see early report from Compston and colleagues).
The first article (Jablonski et al.) looks at how vitamin D levels affect the obesity-related condition non-alcoholic fatty liver disease (NAFLD). The researchers conducted a case-control study in 607 NAFLD patients and matched them with controls for age, sex, race, and season of circulating serum 25(OH)D measurement. Around 16% of cases and 11% of controls were vitamin D deficient (defined as less than 15 ng/ml in this group). There were some significant differences found between the two groups, including a higher body mass index, higher incidence of diabetes, lower vitamin D levels and higher incidence of vitamin D deficiency in the case group. After correcting for BMI, there was still a higher odds of vitamin D deficiency and insufficiency in NAFLD patients. This study identified found that vitamin D deficiency was more common in NAFLD patients, and this was independent of BMI.
The article by Lee and co-workers investigated a pediatric population in Korea. The researchers used school-based health examinations in 1660 Korean children to also measure circulating vitamin D levels. Obesity in children was defined as being greater than the 95th percentile for age and gender, and waist circumference above the 90th percentile for age and gender. Around 14% of the children had a BMI that was considered obese, and the mean serum 25(OH)D level was around 19 ng/ml, the level of sufficiency. This means that roughly half the children had circulating levels of vitamin D that were too low. Children who were obese or had a large waist circumference had a significantly lower vitamin D level. There were significant trends for decreasing levels of vitamin D when obesity and waist circumference increased.
The third study (Sakamoto and associates) was observational and conducted in a Seventh Day Adventist population, who are predominantly non-smoking, non-drinking and vegetarian. Two racial groups were investigated: Blacks and Whites. Rates of vitamin D deficiency (defined as less than 20 ng/ml) were high, at 15% in Whites and 55% in Blacks. This study investigated the relationship between vitamin D levels and blood pressure.
These three studies showed that vitamin D deficiency is prevalent in diverse populations in the US and Korea. Two studies showed a relationship between adiposity and vitamin D deficiency, and one indicated that vitamin D deficiency and BMI are independent of each other. Further research is needed to determine whether obesity is a risk factor or cause of vitamin D deficiency, and the effects of improving vitamin D status on obesity (see article by Vinh Qu and Nguy on the underlying mechanisms of how vitamin D and obesity are related).-jb-
K.L. Jablonski, A. Jovanovich, J. Holmen, G. Targher, K. McFann, J. Kendrick, M. Chonchol. Low 25-hydroxyvitamin D level is independently associated with non-alcoholic fatty liver disease. Nutrition, Metabolism and Cardiovascular Diseases, Volume 23, Issue 8, August 2013, Pages 792–798.http://dx.doi.org/10.1016/j.numecd.2012.12.006
S.H. Lee, S.M. Kim, H.S. Park, K.M. Choi, G.J. Cho, B.J. Ko, J.H. Kim. Serum 25-hydroxyvitamin D levels, obesity and the metabolic syndrome among Korean children. Nutrition, Metabolism and Cardiovascular Diseases, Volume 23, Issue 8, August 2013, Pages 785–791.http://dx.doi.org/10.1016/j.numecd.2012.04.013
R. Sakamoto, K. Jaceldo-Siegl, E. Haddad, K. Oda, G.E. Fraser, S. Tonstad. Relationship of vitamin D levels to blood pressure in a biethnic population. Nutrition, Metabolism and Cardiovascular Diseases, Volume 23, Issue 8, August 2013, Pages 776–784. http://dx.doi.org/10.1016/j.numecd.2012.04.014