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Letter to the Editor |
National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, 4212 North 16th Street, Room 541, Phoenix, AZ 85016 E mail: arline_salbe{at}nih.gov
Dear Sir:
It was with great interest that we read the recent publication by Ekelund et al (1) on reduced physical activity in obese 18-y-old adolescents. In this carefully designed study, the authors measured physical activity by using the doubly labeled water method in conjunction with accelerometry. They discerned that obese adolescents are less physically active than are matched control subjects, despite no significant differences in the energy cost of physical activity between the groups. In addition, the physical activity level (PAL) was lower in the obese group than in the control group, and there was a negative relation between PAL and percentage body fat in the cohort. As the authors point out, many contradictory results in the literature show positive, negative, or no relations between physical activity and adiposity, including our own results, which showed no relation between activity and adiposity in 5-y-old Pima Indians (2). Ekelund et al also note that in cross-sectional studies such as their own, it is not possible to discern whether an inactive lifestyle causes obesity or whether obesity causes an inactive lifestyle.
We recently published a follow-up study of the same Pima Indian cohort, in whom metabolic measurements were made at 5 and 10 y of age, which may cast some light on this relation (3). At 5 y of age, there was little relation between PAL and adiposity (percentage body fat by dual-energy X-ray absorptiometry: r = -0.05, P = 0.55); however, at 10 y of age this relation was significant and negative (r = -0.28, P = 0.05). In keeping with this, 5-y-old children who were "overweight" [by National Center for Health Statistics criteria: body mass index (BMI) ≥ 95th percentile] or "at risk of overweight" (95th > BMI ≥ 85th percentile) showed a very modest rise in PAL over time, whereas in children at low risk (BMI < 85th percentile), PAL increased by 70% over baseline by age 10 y. PAL at age 5 y, which tracked only modestly to age 10 y (r = 0.34, P = 0.008), was not an independent predictor of weight gain by 10 y of age.
Our study illustrates many points. First, our results show that PAL, at least at age 5 y, is not an important correlate of adiposity or a predictor of future weight gain. This suggests that food intake may be the critical determinant of excess weight gain at these early ages. Second, our data show that even in the same cohort, the relation of adiposity with PAL varies with age. This may help to explain some of the discrepancies in the literature, eg, that PAL is generally negatively related to adiposity in older children and adults and positively related to adiposity in younger children.
Hypothetically, this may in turn reflect an increased role of physical activity in the etiology of obesity in older children. Although the negative relation between PAL and adiposity at 10 y of age in our study and in adolescents (r = -0.53, P = 0.0001) in the study by Ekelund et al may in part reflect developing social consequences of overweight at older ages (obesity causing lifestyle change), it is hard to escape the conclusion that the lack of increase in PAL in heavier children by age 10 y compared with their leaner peers is at least exacerbating their weight gain (lifestyle causing obesity). Further prospective assessment of these cohorts should help to determine whether PAL predicts weight gain in early adolescence and adult life, as suggested by Ekelund et al.
REFERENCES
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  U. Ekelund, L. B Sardinha, S. A Anderssen, M. Harro, P. W Franks, S. Brage, A. R Cooper, L. B. Andersen, C. Riddoch, and K. Froberg Associations between objectively assessed physical activity and indicators of body fatness in 9- to 10-y-old European children: a population-based study from 4 distinct regions in Europe (the European Youth Heart Study) Am. J. Clinical Nutrition, September 1, 2004; 80(3): 584 - 590. [Abstract] [Full Text] [PDF]
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