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Letter to the Editor |
Connecticut Department of Public Health, 410 Capitol Avenue, Hartford, CT 06134-0308, E-mail: anthony.polednak{at}po.state.ct.us
Dear Sir:
Kimm et al (1) reported a "tantalizing suggestion" of a "genetic modulator" of the higher susceptibility of African American women than of white women to obesity. The CC genotype of exon 5 of the uncoupling protein (UCP) gene UCP3 is actually less common among blacks than among whites (1), so that genetic susceptibility would depend on associations between the genotypes and fat mass in blacks but not in whites. "Corroboration from a larger study" (1) is needed because only 6 black patients had the CC genotype, and chance findings are possible even if statistically significant (as for resting energy expenditure). A nonsignificant trend was reported in mean fat mass across the UCP3 exon 3 genotypes for black women, but the mean for CT was not intermediate between the means for CC and TT. The association for white women also was not significant, but data on mean fat mass by genotype were not given.
Human genetic variation is much greater within a race than between races (ie, 
85% compared with 15% of variation in allele frequencies; 2, 3). However, the importance of the limited genetic variation between races for disease or risk factors is largely unknown, so that genetic explanations for racial-ethnic differences cannot be ruled out (3).
An autosomal genetic hypothesis for black-white differences in obesity would need to explain the absence (in the third National Health and Nutrition Examination Survey, 1988–1994) of black-white differences in the age-adjusted prevalence of overweight or obesity among men (4). An alternative hypothesis would involve sociocultural factors in women. In the 1985 National Health Interview Survey, black women were less likely than white women to perceive themselves as overweight when categorized by actual weight relative to ideal weight (5). In a survey in East Baltimore, the ideal body image size selected did not differ between black and white men, but black women selected a significantly larger ideal image than did white women (6). In the Baton Rouge Children’s Study, the black-white difference in total daily energy expenditure was explained by lower energy expended in physical activity in black girls (7). Neighborhood factors such as the availability of safe areas for exercise and the availability and cost of healthier foods may be important in explaining racial differences in obesity among women (8).
Whereas the search for "thrifty" genotypes, and for population differences in their frequencies, will undoubtedly continue, so should work on black-white differences in leisure-time physical activity among women and studies of variation in living circumstances that might explain racial differences in obesity prevalence and success in obesity treatment programs (8).
REFERENCES
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