Energy expenditure study makes erroneous inferences and recommendation.

Abstract

Dear Editor-in-Chief: In the January issue of MSSE, Fulton et al. (1) report daily energy expenditure to be greater on the day women participated in 30 min of continuous walking compared with an equivalent volume of intermittent activity. The authors conclude “for the purposes of total energy expenditure, selection of a continuous mode of walking may offer additional benefit over an intermittent mode, given the same total prescribed duration.” Fulton et al. suggest these data may clarify differences between recent physical activity recommendations and Healthy People 2000(7) and may have “important consequences for the nation’s health.” Considering the potential importance of the inferences and recommendation of these highly respected authors, as evidenced by the appearance of these inferences in Reuters Medical News (6), we feel compelled to comment. Fulton et al. observed a mean difference of 59.9 kcal·min-1 in estimated energy expenditure between the two conditions, a difference of 2.7%. For this to be meaningful, it is critical that the measurement error of the testing device is less than 2.7%. Although test-retest reliability studies have, at times, yielded high intraclass correlation coefficients for accelerometry, clearly there is error associated with this technique. Nichols et al. (4) examined the construct validity of the triaxial accelerometer and in so doing report a standard error of the estimate of 0.014 kg·min-1·min-1. Applying this estimate to the body weight of Fulton’s subjects, they should have expected an error of approximately 0.9 kcal·min-1. If we generously assume the duration of a subject’s total daily activity to be as little as 2 h, a difference of over 100 kcal is necessary to have any confidence whatsoever that the two conditions resulted in different daily caloric expenditures. Additionally, Jakicic et al. (2) evaluated interdevice reliability and reported a difference in estimated energy expenditure of approximately 15% when two accelerometers were used simultaneously during walking. As a result of these and other similar studies, the consensus appears to be that accelerometry is useful for providing a reasonable estimate of energy expenditure (4) but not for determining whether similar activities are appreciably different in energy expenditure. Therefore, the authors should have inferred that the limitations of the accelerometer are such that a 60 kcal·d-1 difference is not sufficient to suggest that 30 min of continuous versus intermittent exercise results in appreciably different total daily energy expenditures. Fulton et al. also report a 2.6% lower intensity for the intermittent exercise condition, which is in close agreement with the 2.7% lower daily energy expenditure. Such agreement, in the absence of a true counterbalanced design, is grounds for concern. Moreover, the authors downplay the 2.6% difference in intensity as “marginal,” all the while suggesting that a 2.7% difference in total caloric expenditure poses potentially “important consequences for our nation’s health.” Finally, the more recent Healthy People 2010 initiative (8) suggests that we focus on increasing the proportion of adults who engage in regular, preferably daily, moderate physical activity, rather than whether the activity is continuous or intermittent. Until this problem is resolved, the issue of continuous versus intermittent exercise will only have “marginal” consequences for our nation’s health. (3,5) Robert H. Wood, Ph.D. Michael A. Welsch, Ph.D.