I read with great interest the article by Guerrero et al.,1 who used a large population-based study and several spectroscopic and imaging methodologies to assess the contribution of body fat distribution to the differing rates of hepatic steatosis in the three major US ethnic groups (African American, Hispanic, and Caucasian). They suggested that the differing rates of hepatic steatosis among the three ethnic groups are associated with similar differences in visceral adiposity. Interestingly, in comparison with either Hispanics or Caucasians, African Americans appear to be more resistant to the hypertriglyceridemia associated with insulin resistance despite their lower levels of intraperitoneal and liver fat.1 Here I propose hypovitaminosis D as a potential underlying mechanism for the high prevalence of insulin resistance in African Americans on the basis of the following findings.
First, numerous studies have demonstrated that vitamin D insufficiency and hypovitaminosis D are more prevalent among African Americans than other Americans.2-6 This is primarily due to the fact that pigmentation reduces vitamin D production in the skin.2 A cross-sectional analysis of serum 25-hydroxyvitamin D levels showed that hypovitaminosis D was present in a substantial proportion of the studied African American population, even in the South and among those meeting recommended dietary guidelines.5 Moreover, no significant difference was found in the proportion of vitamin D insufficiency between obese and nonobese preadolescent African American children.6
Second, previous studies have established that vitamin D insufficiency and hypovitaminosis D are associated with insulin resistance.7-9 For instance, by using a hyperglycemic clamp technique in a group of healthy, glucose-tolerant subjects, Chiu et al.9 showed a positive correlation between the 25-hydroxyvitamin D concentration and insulin sensitivity, and subjects with hypovitaminosis D were found to be at higher risk for insulin resistance and metabolic syndrome.
These findings make it rational for us to infer that the prevalence of vitamin D insufficiency and hypovitaminosis D among African Americans can increase their risk of insulin resistance. This may account in part for the fact that despite the lower levels of intraperitoneal and liver fat in African Americans, their prevalence of insulin resistance is similar to that of Hispanics, who possess the highest intraperitoneal and liver fat levels.1 Therefore, vitamin D insufficiency and hypovitaminosis D in African Americans may provide useful clues for understanding their high prevalence of insulin resistance.