Based on objectively measured levels of PA, non-Hispanic White youth were the least active race/ethnic group and non-Hispanic Black youth were the most active race/ethnic group. Previous findings using self-report measures of physical activity in U.S. samples showed that non-Hispanic Black youth were the least physically active race/ethnic group (1
). However, using accelerometers to measure physical activity Owen et al (2008) found that Blacks recorded four more minutes per day in MVPA than Whites (35
). In our sample, non-Hispanic Black youth spent about 8 more minutes per day in MVPA than non-Hispanic White youth.
Troiano et al (2008) previously reported an age-related decline in physical activity in U.S. youth based on results from NHANES 2003–4 (42
). For 2003–4 and 2005–6 survey combined samples, we also found an age-related decline in physical activity. Pate et al (2009) reported a 4% annual decline in MVPA in adolescent girls (35
). Broderson et al (2007) found that physical activity dropped off between ages 11 and 12 (6
). In our sample, 6 to 11 year olds participated in twice as much MVPA than the older age groups, consistent with the observation that the most dramatic age-related decline in physical activity may occur at the start of puberty.
To better understand the age-related decline in MVPA observed in this sample, the 3-way interaction between age, BMI category, and race/ethnic group was examined for each gender. Non-Hispanic Whites and Blacks in higher BMI categories spent less time in MVPA than normal weight non-Hispanic White and non-Hispanic Black youth, whereas obese 12 to 15 year old Mexican American youth recorded the same amount of time in MVPA per day as normal weight youth. A previous study in adolescent Hispanic females found a trend for normal weight Mexican American females spending fewer minutes in MVPA than overweight females (8
). This suggests that BMI may interact with physical activity levels differently in Mexican American youth than in other race/ethnic groups. In the non-Hispanic White race/ethnic group, the largest difference in MVPA is between the normal and overweight groups, with the overweight and obese groups participating in approximately the same amount of MVPA. However, in the non-Hispanic Black race/ethnic group, the largest difference in MVPA is between the overweight and obese youth. Thus, the findings indicate that different race/ethnicities have different thresholds of BMI percentile past which MVPA declines.
The large difference in physical activity between males and females is particularly striking. Normal weight females of all race/ethnic groups achieved less physical activity than obese males of all race/ethnic groups. A recent study conducted in adolescents using accelerometers found that while the rates of decline of physical activity were the same for both genders, females participated in significantly fewer minutes of MVPA than males and MVPA dropped below 60 min/day one year earlier in females than in males (32
). Non-Hispanic Black youth had the largest gender differences in counts per minute and minutes spent in MVPA: females recorded about 140 counts per minute and 27 minutes per day less than males. Using previous prediction equations based on overweight youth (18
), this deficit is broadly similar to 600 kcal per day. The difference in physical activity levels between males and females in this sample may contribute to the fact that the non-Hispanic Black females had the highest prevalence of obesity.
Overall, the inverse association between physical activity levels and BMI percentile in this sample is consistent with previous findings (38
). Contrary to our expectations, higher levels of physical activity were not associated with lower prevalence of obesity across the race/ethnic groups. Non-Hispanic White youth had lower mean counts per minute and spent fewer minutes per day in MVPA than non-Hispanic Black and Mexican American youth, yet had a lower prevalence of obesity than the other race/ethnic groups. This paradox may be accounted for by the fact that non-Hispanic White youth may spend more time in activities not captured well by accelerometry such as swimming or bicycling. These differences could also be attributed to the higher socioeconomic status found in the non-Hispanic White youth since socioeconomic status has been inversely related to obesity and positively related to physical activity (24
). However, socioeconomic status was controlled for in all analyses; other factors may contribute to the pattern of obesity and physical activity in non-Hispanic White youth.
Genetic predisposition to obesity, socioeconomic status, and cultural differences in behavior may play a role in the race/ethnic differences found in this sample and elsewhere (46
). Non-Hispanic Black and Mexican American adults have the highest prevalence of overweight and obesity in the U.S. population (33
). Children of overweight and obese parents have been shown to have higher rates of obesity than children of normal weight parents (4
). Furthermore, non-Hispanic Black and Mexican American females have been shown to have lower basal metabolic rates and expend less activity energy than non-Hispanic White females which may put them at higher risk for overweight and obesity (17
). Dietary intake may also account for the differences in the obesity prevalence between non-Hispanic White youth and the minority race/ethnic groups, particularly given that there are race/ethnic differences in the consumption of unhealthy foods. Arcan et al (2009) found that non-Hispanic Black high school students were more likely to consume sugar-sweetened beverages and high-fat foods than other race/ethnic groups (2
). The higher rate of obesity in non-Hispanic Blacks may be explained by a higher intake of unhealthy foods, particularly in non-Hispanic Black females who have been found to have the lowest levels of physical activity and highest intakes of unhealthy foods (27
In this study adjustment for reported energy intake did not moderate the associations between total physical activity and age, race/ethnicity, or weight status and physical activity levels. Non-Hispanic White youth reported significantly higher energy intake than the other race/ethnic groups, and normal weight youth reported significantly higher energy intake than overweight or obese youth. Differences across weight status may be due to differential underreporting of energy intake (29
), while differences across race/ethnicity may be due to a combination of differential misreporting and actual differences in true intake. Although the observed differences suggest that energy intake was a potential confounder, analysis determined that it was not confounding the relationship between weight status and race/ethnicity and physical activity levels. Further analysis of the association between patterns of food intake and physical activity could be of interest because of the high quality data available in this survey.
No other past study that we are aware of has described race/ethnic differences in objectively measured physical activity in large representative sample of U.S. youth, however several limitations to the present study merit discussion. First, this is a cross-sectional analysis, thus we cannot determine any causal associations. However, the large sample size allows robust estimates of associations between variables of interest and may help inform future longitudinal studies. Second, accelerometers do not capture all types of physical activity (40
). However, accelerometers are considered to be an excellent objective measurement of physical activity in youth because they minimize self-report bias and eliminate human error in recalling previous physical activity (14
). Third, the accelerometers do not record the type of physical activity as do self-report measures, which prevents us from exploring the frequency of specific behaviors (i.e.: TV viewing) that could explain the observed differences in physical activity levels among race/ethnic groups. Fourth, the NHANES survey is designed to sample the three largest race/ethnic groups in the U.S. and therefore does not provide data sufficient for a national estimate for other minority groups such as Asians or other non-Mexican Hispanic populations that comprise a significant and growing proportion of the U.S. population. Finally, BMI percentile category is used here as a proxy measure of adiposity. While some findings indicate that it is not an accurate measure of body fat for all race/ethnic groups, Flegal et al (2009) recently demonstrated that it corresponds well with percent body fat in an adult sample (19
) and BMI has been shown to be significantly correlated with percent body fat in youth (36
). Furthermore, BMI percentile is a cost efficient and feasible measure in a large population-based study (15
As measured by accelerometry, non-Hispanic White youth engaged in less physical activity than both non-Hispanic Black and Mexican American youth yet had the lowest prevalence of obesity in this sample. Also, non-Hispanic Black females are the least physically active and have the highest prevalence of obesity in this sample. Mexican American 12 to 15 year old obese and normal weight youth had the same amount of MVPA. Explanations for differences in obesity rates between youth of different race/ethnic groups must be influenced by other factors than variations in physical activity levels.