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Letter to the Editor |
Departamento de Pediatría and EU Ciencias de la Salud, Facultad de Medicina, Universidad de Zaragoza, C/ Domingo Miral, s/n, 50009 Zaragoza, Spain, E-mail: gereva{at}comz.org
Dear Sir:
Ball et al (1) recently showed a negative association between percentage of body fat mass (%FM) and physical activity level (PAL) in boys but not girls aged 6–9 y. These results suggest that lower PALs may contribute to the rise in prevalence of childhood obesity, particularly in boys, and the authors conclude that physical activity is one factor contributing to body fatness in boys.
Physical activity and obesity are linked by the energy balance equation, according to which obesity is the result of accumulated excess energy and physical activity is a component of energy expenditure. Despite this simple relation, there is no consistent evidence for an influence of childhood activity on later fatness, with studies roughly divided between finding either no effect or a protective effect of physical activity (2, 3). In a 4-y longitudinal study of 75 white children, Goran et al (4) reported that the main predictors of change in %FM during preadolescent growth were sex, initial fatness, and parental fatness, but not reduced energy expenditure. There are some contradictory and inconclusive results on the role of physical activity in the development of obesity. Obesity is the result of a slow and gradual process in which there is a daily energy imbalance of 
2% of daily energy flux. Methods used to measure differences in energy expenditure between obese and nonobese groups are not very accurate, particularly in cross-sectional studies (2, 3).
Because the results obtained by Ball et al are categorical and surprising, we carefully examined their methods and conclusions. We argue that the negative association found between %FM and PAL in the boys may have been caused mainly by a direct association between body weight and predicted resting energy expenditure (REE) rather than, as the authors assume, by a hypothetical relation between inactivity and obesity. Total energy expenditure (TEE) was measured by using the doubly labeled water technique, and REE, which is the major component of TEE, was predicted with the use of the Schofield equations on the basis of body weight, age, and sex (5): REE (kJ) in boys = {[22.7 x weight (kg)] + 505} x 4.184, and REE (kJ) in girls = {[20.3 x weight (kg)] + 486} x 4.184. PAL and activity energy expenditure (AEE) were calculated with the ratios TEE:REE and [(TEE - REE):TEE] x 100, respectively. In these 2 ratios, REE is inversely related to PAL and to AEE, whereas in the Schofield equations, REE is directly related only to weight. Therefore, PAL and AEE could appear to decrease when body weight increases, and in a group of children with a small age range, body weight will increase mainly when %FM increases. Ball et al showed that TEE was not related to %FM in boys but that PAL and AEE (as percentage of TEE) were inversely related to %FM when REE was considered in these ratios.
We also evaluated REE data from a group of 40 prepubertal children consisting of 20 boys (8 obese and 12 nonobese) and 20 girls (8 obese and 12 nonobese) (6, 7). %FM was calculated with the use of skinfold-thickness measurements and the equations of Siri (8) and Brook (9). As we expected, REE data predicted by Schofield equations (5) showed a strong positive relation with %FM in the entire cohort (r = 0.65, P < 0.0001), in the boys (r = 0.70, P < 0.0001), and in the girls (r = 0.89, P < 0.0001). When REE was measured by indirect calorimetry, the results were different: the relations between REE and %FM were weaker in the entire cohort (r = 0.31, P = 0.05), in the boys (r = 0.30, NS), and in the girls (r = 0.63, P < 0.05). Indirect calorimetry is the reference method for measuring REE in research studies. As seen in our study, the use of REE predictive equations instead of calorimetry may considerably change the results and conclusions obtained in a study. Thus, we suggest that it would have been more accurate for Ball et al to calculate AEE and PAL with REE data measured by calorimetry and thus obtain reliable relations between energy expenditure and body composition.
REFERENCES
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