Relationship between aerobic fitness level and daily energy expenditure in weight-stable humans TERESA A. SHARP, GEORGE W. REED, MING SUN, NAJI N. ABUMRAD, AND JAMES 0. HILL Departments of Pediatrics, Preventive Medicine, and Surgery, Vanderbilt University, Nashville, Tennessee 37232 Sharp, Teresa A., George W. Reed, Ming Sun, Naji N. Abumrad, and James 0. Hill. Relationship between aerobic fitness level and daily energy expenditure in weight-stable humans. Am. J. Physiol. 263 (Endocrinol. Metab. 26): E121- E128, 1992.-The relationship betweenexercise and energy ex- penditure is unclear, with some suggestions that exercise leads to increased energy expenditure over and beyond the increase due to the exercise itself. In this cross-sectional study, we ex- aminedthe relationshipsamong aerobic fitness level, body com- position, and total daily energyexpenditure in 78 subjects. Daily energy expenditure (determined in a whole room calorimeter) wassignificantly correlated with both fat-free mass (FFM) and aerobic fitness (estimated from maximum aerobic capacity or VO 2 max ). However, multiple-regressionanalysis demonstrated that, after accounting for FFM, VO, max did not explain a sig- nificant amount of the remaining variation in energy expenditure. In addition, the relationship betweenresting met- abolic rate and both FFM and VO, max wasevaluated usingdata from 214 weight-stable subjectsanalyzed retrospectively. The results were identical with the results obtained from the 78 subjects in that VO, max did not have effects independent of FFM on energy expenditure. We conclude that aerobic fitness does not have a direct effect on energy expenditure. However, it may have effects that are mediated through body composition, since in both populations studied here, ir0, max was positively correlated with FFM and negatively correlated with adiposity. exercise; resting metabolic rate; sleeping metabolicrate; thermic effect of food; energyexpenditure due to physical activity; whole room indirect calorimeter THE ROLE OF INCREASED physical activity on net energy balance is unclear (13, 14, 22). For example, there is no agreement on whether the increase in energy expendi- ture caused by exercise extends beyond the acute bout of exercise itself. If there is such an increase, this could occur as a sustained elevation of resting energy expen- diture after exercise (2, 8)) an increase in the thermic effect of food (4), or an increase in some other compo- nent of energy expenditure. Although most investigators find an elevation in energy expenditure after a bout of exercise, the magnitude and duration of this postexer- cise energy expenditure is controversial. There are some reports that energy expenditure is elevated only for a few minutes after bouts of exercise (2), but others report that such elevations can last for at least 24 h and may depend on the intensity and duration of the exercise (8). Similarly, the data are mixed regarding whether the thermic effect of food is increased by exercise or exercise training (4, 10, 18, 23). Before assessing whether energy expenditure is greater in exercisers than in nonexercisers, it is neces- sary to recognize that energy expenditure is highly cor- related with fat-free mass (FFM; see Ref. 27). The ques- tion then becomes whether effects of exercise on energy expenditure are independent of FFM. The current liter- ature contains many studies that have attempted to an- swer this question. In general, studies can be grouped into cross-sectional and longitudinal studies. Cross-sec- tional studies have examined the relationship between energy expenditure [usually resting metabolic rate (RMR)] and a measure of habitual exercise level (usually maximum oxygen consumption or VO, max) in weight- stable subjects. Longitudinal studies have added (or in- creased the level of) exercise and examined the sequen- tial changes in energy expenditure. One limitation to most previous studies has been the short-term nature of the measures of energy expenditure, necessitating esti- mates rather than direct measures of total daily energy expenditure. The purpose of this cross-sectional study was to use a whole room indirect calorimeter to assess the contribu- tion of fitness level to determining total daily energy expenditure and the individual components of energy expenditure. A second purpose was to assess the contri- bution of fitness level to daily oxidation rates of protein, carbohydrate, and fat. METHODS We examinedthe relationshipsamong ire, max, body compo- sition, and total daily energy expenditure (using a whole room indirect calorimeter) in 78 adult subjects. In addition, we present retrospective data showing the relationship among RMR, body composition, and \jo2 maxin 214 adult subjects. This latter data set was accumulated over a period of 5 yr and consists of data obtained from subjects participating in several other researchprojects. These data are presented to provide additional support for the conclusions reachedwith the 78 sub- jects studied in the whole room calorimeter. Subjects. Thirty-nine men and thirty-nine women partici- pated in this study, which was approved by the Vanderbilt University Committee for the Protection of Human Subjects. Attempts were made to recruit subjects who varied widely in body composition and usualactivity levels. The retrospective data were accumulated from 214 weight- stable subjects.This was a highly heterogeneous group of sub- jects, chosen from all weight-stable subjects studied (in a variety of different protocols) during the past 5 yr on which measures of body composition, energy expenditure, and ir0, max were available. Characteristics of all subjectsare shown in Table 1. Procedure. All subjects had the following measures taken: 1) body composition by underwater weighing; 2) measurement of VO 2max by treadmill test; 3) measurement of total daily energy expenditure and estimation of each component of energy ex- penditure usinga whole room indirect calorimeter; and 4) mea- surement of RMR using a ventilated hood system.There wasno set order of testing. Measurements of VO, max and energy ex- penditure were madeat least 36 h after the last bout of planned exercise.All subjects were weight stablefor at least 6 mo before the study and remained weight stable throughout the study. Data for individuals who did not complete all measurement 0193-1849/92 $2.00 Copyright 0 1992 the American Physiological Society El21 by 10.220.32.247 on June 22, 2017 http://ajpendo.physiology.org/ Downloaded from