点击显示 收起
MRC Epidemiology Unit
Elsie Widdowson Laboratory
Fulbourn Road
Cambridge CB1 9NL
United Kingdom
E-mail: ulf.ekelund{at}mrc-epid.cam.ac.uk
Faculty of Human Movement
Technical University of Lisbon
Portugal
Department of Sports Medicine
The Norwegian University of Sport and Physical Education
Norway
Department of Public Health
University of Tartu
Estonia
Diabetes and Arthritis Epidemiology Section
NIDDK, National Institute of Health
Phoenix, AZ
Department of Exercise and Health Sciences
University of Bristol
Bristol
United Kingdom
London Sport Institute
Middlesex University
United Kingdom
Institute of Sport Science & Clinical Biomechanics
University of Southern Denmark
Odense
Denmark
Dear Sir:
We thank Maffeis for his comments on our recently published article (1). The main findings of our cross-sectional study are that total activity is not significantly associated with body fat, and that the time devoted to moderate-to-vigorous physical activity (MVPA) explains <1% of the variation in body fat in the children studied. Our results also suggest that the amount of time spent in sedentary activities and in light-intensity physical activities does not contribute to the explained variance in body fat in these children. These results imply that the intensity of activity may be more relevant than the total amount of PA, but our results should be interpreted while bearing in mind the methodologic limitations we discussed in our article. These limitations include the limited number of days of measurement and the uncertainty in defining cutoffs for different intensity levels with the use of an accelerometer (1).
The main concern that Maffeis has with our findings seems to relate to the fact that our results reduce "...the relevance of the intensity of PA in the maintenance of childhood obesity and seems to frustrate the reasonable expectancy for the role potentially played by exercise and PA in both the prevention and treatment of overweight."
We agree with Maffeis that it is disappointing that our results for PA do not explain a larger proportion of the variance in body fatness. However, the association between PA or sedentary behavior and obesity in young people has not been consistently shown, and we believe that our interpretation of the results from the present study is accurate. Nonetheless, we completely agree that sedentary behavior should be prevented for many reasons, which is reflected by the following quotes from our article: "The relations that we observed between PA and body fatness were small. Nonetheless, these findings may have important public health implications because sedentary behavior is common in most industrialized societies." and "Thus, effective preventive strategies need to address the underlying social, cultural, physical, and economical determinants of childhood obesity and are likely to include interventions designed to decrease sedentary behavior in children." (1).
Maffeis also put forward the hypothesis that time spent sedentary and in light-intensity PA may be more closely related to body fat in children. We are well aware that energy expenditure during weight-bearing PA is higher in obese than in normal-weight children when expressed in absolute values (2). We also recently showed that time spent in MVPA, measured by accelerometry, is significantly lower in obese children than in a matched normal-weight control group, whereas time spent in sedentary and in light-intensity PA and absolute PA energy expenditure (simultaneously measured by the doubly labeled water method did not differ significantly between groups (2). Maffeis argues that the higher energy cost of weight-bearing PA "promotes, by implication, the spontaneous reduction of time devoted to moderate-to-vigorous physical activity (MVPA) by the obese." However, this is questionable and needs to be demonstrated. Indeed, it may actually be the high body weight per se in obese children that contributes to their reduced activity levels.
Maffeis also suggests that obese children spend more energy (which is an effect of higher body mass) and oxidize much more fat during light-intensity exercise (3). Walking at 4 km/h, the lowest activity intensity examined in the study by Maffeis et al (3), represents an intensity similar to the lower cutoff we used for MVPA (1). We defined moderate-intensity PA as >2000 counts/min, which is broadly equal to a walking speed of 3–4 km/h in 9–10-y-old children (4, 5). Thus, MVPA—as defined in our article—includes intensity levels that Maffeis argues are favorable for increased fat oxidation.
Finally, Maffeis cites a study by Montgomery et al (6) that he suggests shows a clear relation between time spent in sedentary and light-intensity PA and overweight in young children and, thus, supports his hypothesis. However, we disagree with Maffeis's interpretation of these data. The only associations reported by Montgomery et al (6) are those between the percentage of daytime spent sedentary, in light PA, and in MVPA (measured by accelerometry) and PAL (ie, the ratio of total energy expenditure and resting metabolic rate). These authors observed that time spent sedentary explained most of the variance in PAL. However, we do not believe PAL can be used as a surrogate for overweight.
It is possible that time spent in sedentary and in light-intensity PA is more important than is the accumulated time spent in MVPA, or the total amount of PA, in preventing overweight and obesity in children. However, in our large population-based cross-sectional study, which included almost 1300 children from 4 different countries in Europe (1), and in our matched case-control study (2), we observed associations between MVPA and body fatness but not between body fat and the time spent being sedentary. Although one needs to be cautious when inferring causality from cross-sectional data, these findings support the view that MVPA and body fatness are related, whereas our objective measure of low intensities of PA and body fatness do not.
ACKNOWLEDGMENTS
None of the authors had a conflict of interest.
REFERENCES