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Department of Physical Education and Health, Orebro University, Orebro S-701 82, Sweden
Dear Sir:
We appreciate the comments of Salbe et al on our recently published paper (1). In one of very few prospective studies on the relation between energy expenditure and weight gain in young persons, Salbe et al (2) showed that there is a positive relation between physical activity level (PAL = total energy expenditure/resting metabolic rate) and body weight and no significant association between PAL and percentage body fat at 5 y of age. However, the relation between PAL and body weight and between PAL and percentage body fat was inverse and significant at 10 y of age. Furthermore, it was observed that PAL at age 5 y did not predict percentage body fat at age 10 y. Another observation was the more pronounced increase in PAL over the 5-y period in lean children than in those who were at high risk of overweight at 5 y of age (2).
The relation between estimates of energy expenditure that are suggested to reflect physical activity, such as PAL and activity expenditure (AEE), and body weight is highly complex in growing children. In general, PAL and AEE increase with age in children aged 3–16 y, whereas AEE expressed per kilogram of body weight seems to be independent of age. This may indicate that an increase in PAL and AEE with age is attributable to the increase in body weight (3). In contrast with PAL and AEE, cross-sectional and longitudinal studies using objective and self-reported measures of physical activity showed decreases in physical activity with age, which are especially remarkable during the adolescent period (4–6). It could therefore be hypothesized that the generally lower PAL values observed in younger children than in older children are due to the lower body weights (or body size) of the younger children and that PAL may not be the best indicator of physical activity.
Furthermore, if an increase in PAL with age is mainly due to an increase in body weight (3), we speculate that PAL and body movement assessed by accelerometry are different entities and that an increase in PAL in children during growth, as in the study by Salbe et al (2), may not reflect an increase in physical activity, expressed as body movement, per se. Salbe et al also suggest that food intake may be the most important determinant of excess weight gain in young children, whereas physical activity may have a greater effect in older children and adolescents. This is an interesting hypothesis that requires additional attention.
We agree with Salbe et al that further assessments of their and our own cohorts and other well- designed longitudinal studies are needed to shed light on the issue of whether physical activity is a predictor of weight gain or whether obesity causes a sedentary lifestyle in children, adolescents, and young adults. In fact, it was recently suggested that physical inactivity did not predict later obesity, whereas, on the other hand, the degree of obesity predicted later inactivity in adults (7). However, to determine the effect of various aspects of physical activity, including sedentary behavior, on future weight gain, sophisticated energy expenditure and body-movement assessment methods are needed, especially when examining this relation in young people.
From a public health perspective, more research is required to better understand the minimal amount of physical activity needed to prevent weight gain as well as the complex interaction between genetic and environmental factors in the development of obesity during various stages of life. An enhanced understanding of these issues can only be achieved by combining accurate methods for assessing physical activity and energy expenditure. We therefore suggest that future research combines measures of energy expenditure and objective body-movement assessment techniques to unravel which dimensions of physical activity may predict obesity and its related disorders.
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