A study in the June 2012 issue of Archives of Pediatrics and Adolescent Medicine (1) received substantial media coverage and is directly related to the ability of physical activity to help reduce disparities in childhood obesity. White and Jago analyzed data from the National Growth and Health Study that studied the development of obesity in African American and non-Hispanic White girls from age 9 to 19. The rationale for the study was that previous research showed African American girls expended less energy during physical activity and oxidized less fat during exercise than white girls. The present analyses only dealt with the 2 year period from age 14 to 16 when the girls (n=1148) had physical activity objectively measured by accelerometers.

The main finding was that the most active white girls had an 85% lower chance of becoming obese over 2 years, and the most active African American girls had only a 15% lower chance of becoming obese, which was not significant.

My concern is that the results will lead to a premature conclusion that physical activity is irrelevant for obesity control among African American adolescent girls. A reduced emphasis on increasing physical activity in this population would be a mistake at this time because:

1. The results can only address the effects of physical activity within the range studied, which is likely to be low. It is not possible to interpret the absolute amount of physical activity in the study (such as minutes or kilocalories expended), due to the nature of the early version of the accelerometer. However, a previous publication from the same study indicated self-reported physical activity was low and declining for both races during the years analyzed.  In fact, the median reported activity level in African American adolescent girls dropped to zero at age 16.(2)
2. Physical activity is very low among adolescent girls of all races and ethnicities. In analyses of the NHANES accelerometer data, the prevalence of meeting the 60 minute moderate-to-vigorous physical activity guideline was only 4% for African American 12-15 year old girls, though it was less than 3% for non-Hispanic and Mexican American girls.(3) It is possible that too few of the girls were active enough to experience any obesity prevention effects.
3. A study of physical activity trends based on accelerometer measures in the NHANES study suggest the emergence of race-ethnic disparities. From 2003 to 2006, there was a significant increase in total physical activity among non-Hispanic white 6-11 year-olds, but a significant decrease among African American and Mexican American children of the same ages. There were no trends among adolescents.(4) Now would be the wrong time to halt attempts to increase physical activity among African American youth.
4. It would be unfortunate if it was conclusively proven that physical activity does not contribute to the prevention of obesity among African American girls. But even that conclusion would not justify cessation of efforts to promote physical activity among this population. Physical activity has many physical and mental health benefits in youth beyond obesity control.(5) Thus, there is a public health imperative to increase physical activity among African American adolescent girls who are at high risk for several chronic diseases.

References

1. White J, Jago R. Prospective associations between physical activity and obesity among adolescent girls: Racial differences and implications for prevention. Arch Pediatr Adolesc Med 2012;166(6):522-7.

2. Kimm SYS, Glynn NW, Kriska AM, Barton BA, Kronsberg SS, Daniels SR, Crawford PB, Sabry ZI, Lui K. Decline in physical activity in black girls and white girls during adolescence. New Engl J Med 2002;347:709-15.

3. Whitt-Glover MC, Taylor WC, Floyd MF, Yore MM, Yancey AK, Matthews CE. Disparities in physical activity and sedentary behaviors among US children and adolescents: Prevalence, correlates, and intervention implications. J Publ Health Policy 2009;30:S309–S34.

4. Gortmaker SL, Lee R, Cradock AL, Sobol AM, Duncan DT, Wang, YC. Disparities in youth physical activity in the United States: 2003-2006. Med Sci Sports Exerc 2012;44(5):888-93.

5. Physical Activity Guidelines Advisory Committee. Physical Activity Guidelines Advisory Committee Report, 2008. Washington, DC: U.S. Department of Health and Human Services, 2008.