Energy and Macronutrient Intake in the Midwest Exercise Trial 2 (MET-2)

Abstract

Purpose 

This study aimed to examine the effect of exercise training over 10 months at two levels of energy expenditure on energy and macronutrient intake in a sample of previously sedentary, overweight/obese young adults.

Methods 

We conducted a 10-month trial in 141 young adults who were randomized to either supervised exercise 5 d·wk⁻¹ at 400 and 600 kcal per session or nonexercise control. Participants were instructed to maintain their usual ad libitum diet. Energy/macronutrient intake was assessed at baseline and 3.5, 7, and 10 months over 7-d periods of ad libitum eating in a university cafeteria using digital photography. Foods consumed outside the cafeteria were assessed using multiple-pass recalls.

Results 

There were no significant between-group differences in absolute energy intake at baseline or at any other time point in the total sample or in men. In women, absolute energy intake was significantly greater in the 600-kcal-per-session group versus controls at both 3.5 and 7 months. There were no significant between-group differences in relative energy intake (kcal·kg·d⁻¹) at any time point in the total sample, in men or women. There were no significant within- or between-group differences of change in absolute or relative energy intake in any of the three study groups in the total sample or in men or women. No clinically relevant changes in macronutrient intake were observed.

Conclusions 

Aerobic exercise training does not significantly alter energy or macronutrient intake in overweight and obese young adults. The possibility of a threshold level beyond which increased exercise energy expenditure fails to produce a more negative energy balance and potential sex differences in the energy intake response to increased levels of exercise are potentially important.

Compensatory increases in energy intake may be at least partially responsible for the limited magnitude of mean weight loss induced by aerobic exercise training without energy restriction (³³). For example, King et al. (²⁰) demonstrated significant increases in energy intake among participants who did not reduce weight or fat mass in response to aerobic exercise training (12 wk, 5 d·wk⁻¹, 500 kcal per session, 70% HR_max). However, empirical evidence for an effect of exercise on energy intake or dietary macronutrient composition is not compelling. Our recent systematic review, which included cross-sectional, acute/short-term, nonrandomized, and randomized trials (⁹), and a recent meta-analysis of acute trials by Schubert et al. (³⁰) both indicated that observed increases in postexercise energy intake only partially compensate for the energy expended during acute or short-term (2–14 d) exercise and dietary macronutrient composition was generally unchanged. Results from our systematic review on the response of energy and macronutrient intake to exercise training were in general agreement with those from acute and short-term trials (⁹). Only two of 36 (approximately 6%) nonrandomized and randomized trials identified, ranging in duration from 3 to 72 wk, reported an increase in energy intake, and 18 of 31 (58%) trials reported alterations in dietary macronutrient composition in response to exercise training. However, the literature on this topic should be interpreted cautiously because several design issues limit our full understanding of how exercise training influences energy intake. For example, available randomized trials have been conducted in small samples (i.e., ≤20 per group) (^12,27) over time frames of ≤12 wk (^21,24,33). In addition, with the exception of the study by Donnelly et al. (¹⁰), energy intake in most studies has been assessed using 2- to 7-d food records (^4,21,24,27). Food records have been shown to underestimate energy intake when compared with energy expenditure assessed by doubly labeled water (²⁸). The adequacy of self-report measures of energy intake to form the basis of scientific conclusions has been recently questioned (²⁹).

Data from the Midwest Exercise Trial 2 (MET-2) afforded a unique opportunity to examine the effect of exercise training at two levels of exercise energy expenditure (EEEx) on energy and macronutrient intake in a sample of previously sedentary, overweight/obese young adults. The primary aim of MET-2 was to evaluate the role of aerobic exercise training without energy restriction on weight and body composition; however, several secondary outcomes, including changes in energy and macronutrient intake, were included a priori in the original study design. A detailed description of the design and methods for MET-2 (¹⁴), results for the primary outcome (weight change) (¹¹), as well as changes in non-EEEx and physical activity (³⁸) have been published. A description of differences in changes in both energy intake and energy expenditure between participants who achieved or failed to achieve clinically significant weight loss (≥5%) in response to exercise will be presented in a forthcoming manuscript. Briefly, MET-2 randomized young adults age 18–30 yr with body mass index (BMI) 25–40 kg·m⁻² to a 10-month, 5 d·wk⁻¹ supervised exercise intervention at two levels of EEEx (400 or 600 kcal per session) or to nonexercise control.

METHODS

Participant eligibility

Potential participants were excluded for the following reasons: age outside the range 18–30 yr, BMI outside the range 25–40 kg·m⁻², history of chronic disease (i.e., diabetes, heart disease, etc.), elevated blood pressure (>140/90), lipids (cholesterol, >6.72 mmol·L⁻¹; triglycerides, >5.65 mmol·L⁻¹), or fasting glucose (>7.8 mmol·L⁻¹) , use of tobacco products, taking medications that would affect physical performance (i.e., beta-blockers), or metabolism (i.e., thyroid, steroids), inability to perform laboratory tests or participate in moderate- to vigorous-intensity exercise, and currently engaged in planned physical activity greater than 500 kcal·wk⁻¹ as assessed by recall (³²).

Recruitment

Potential participants were obtained from a variety of sources including advertisements in the local and campus newspapers and radio stations, flyers in campus buildings, and postings on our laboratory Web site. All forms of advertisements included our dedicated study phone number as well as our laboratory Web site and study e-mail address. Potential participants were to complete a Web-based initial eligibility screener regarding height and weight to determine BMI, use of medications, smoking and drinking habits, and current levels of physical activity. Those without Web access completed the initial screen by hard copies or telephone interview. Participants who seemed eligible on the basis of the initial eligibility questionnaire met with the project coordinator who described the study, answered questions, obtained a written informed consent, and assessed height and weight to determine final eligibility. Approval for this study was obtained from the Human Subjects Committee at the University of Kansas—Lawrence. A total of 2338 individuals completed an online initial eligibility questionnaire. One-hundred forty one individuals were determined to be eligible and were randomized to one of the three study groups. Participants were compensated for participation.

Randomization and blinding

Participants were stratified by sex and randomized within each sex by the study statistician (approximately 80% exercise and approximately 20% control). All participants were instructed to continue their typical patterns of dietary intake and nonexercise physical activity over the 10-month intervention. The blinding of participants to group assignment was not possible because of the nature of the intervention. However, both investigators and research staff were blinded at the level of outcome assessments, data entry, and data analysis.

Exercise training

Exercise, consisting primarily of walking/jogging on motor-driven treadmills, was supervised by trained research staff and conducted in a dedicated exercise facility in the Energy Balance Laboratory at the University of Kansas—Lawrence. To provide variety and decrease the potential for overuse injuries, alternate activities including stationary biking, walking/jogging outside, and use of elliptical trainers were permitted for 20% of the total exercise sessions (i.e., one session per week). The exercise protocol was designed to progress in intensity and amount from baseline to the end of month 4 both to provide time to adapt to exercise and prevent injuries.

The duration of exercise required to elicit either 400 or 600 kcal per session for each participant in the exercise groups was determined as follows: at the baseline assessment, treadmill speed/grade was set at 3 mph/0% grade and was adjusted by increments of 0.5 mph/1% grade until the participant reached 70% of HR_max ± 4 bpm. HR_max was the highest HR achieved during the assessment of maximal aerobic capacity using a modified Balke protocol (¹). EEEx was then assessed over a 15-min interval (1-min epochs) using a ParvoMedics TrueOne2400 indirect calorimetry system (ParvoMedics Inc., Sandy, UT). The average EEEx (kcal·min⁻¹) over the 15-min interval was calculated from measured oxygen consumption and carbon dioxide production using the Weir equation (³⁶). This value was used to provide the goal for the duration of exercise sessions for the first month of the intervention. For example, prescribed EEEx during month 1 = 150 kcal per session and EEEx = 9.2 kcal·min⁻¹, then exercise duration = 150 kcal per session divided by 9.2 kcal·min⁻¹ = 16 min per session. Similar procedures to determine exercise duration were conducted at the end of each month over the course of the 10-month intervention to adjust for potential effects of changes in both body weight and cardiovascular fitness on EEEx; thus, over time, the treadmill speed and/or grade were increased. The duration and intensity of all exercise sessions were verified by a downloadable HR monitor (RS 400; Polar Electro, Inc., Woodbury, NY) set to collect HR in 1-min epochs. All exercise sessions and assessments of EEEx were preceded by a brief warm-up on the treadmill (approximately 2 min, 3–4 mph, 0% grade). Treadmill speed and grade were subsequently increased to achieve the prescribed target HR. In addition, the level of perceived exertion (²), treadmill speed/grade, and HR were recorded by the research assistant at 10-min intervals during each exercise session. This procedure provided interaction between the study staff and participants and helped maintain compliance as well as a detailed description of each exercise session. Compliance to the exercise protocol, an essential element of an efficacy study, was defined as successfully completing >90% of the scheduled exercise sessions. Successful completion was defined as maintaining the target exercise HR ± 4 beats per minute for the prescribed duration of the exercise session. Participants who were noncompliant during any 3-month interval (months 0–3, 3–6, or 6–9) or during the final month (month 10) were dismissed from the study.

Control group

Participants assigned to the nonexercise control group were instructed to continue their typical patterns for physical activity and dietary intake over the 10-month study. With the exception of assessment of EEEx, the same outcome assessments were completed with both the exercise and control groups.

Energy/macronutrient intake

Energy and macronutrient intake was assessed at baseline and at 3.5, 7, and 10 months over 7-d periods of ad libitum eating in a University of Kansas cafeteria using digital photography. Two digital photographs were obtained before and after consumption of each meal, with the cafeteria trays placed in a docking station to standardize the camera angle (Fig. 1). One photograph was taken at a 90° angle above the tray, and one photograph was taken at a 45° angle to maximize depth perception and identification of food and beverage items. Notes were placed on the tray to identify types of beverages (e.g., diet vs regular soft drink, skim vs whole milk, etc.) and any other food items that would be difficult to identify from the photo. Foods consumed outside the cafeteria (i.e., snacks, noncafeteria meals) over the 7-d periods were assessed using multiple-pass recall procedures using food models and standardized, neutral probing questions. The type and amounts of food and beverages consumed at the cafeteria and results from recalls were entered into the Nutrition Data System for Research (NDS-R versions 2005 and 2006; University of Minnesota, Minneapolis, MN) for the quantification of energy and macronutrient intake. Before data collection, all nutrition research staff completed standardized training, conducted by a registered dietitian, covering the digital photograph methodology, multiple-pass dietary recalls, and NDS-R computer coding, with refresher sessions every 2 months. Before collecting data on study participants, nutrition research staff were required to satisfactorily evaluate digital photographs from 10 sample meals (before and after) and complete ten 24-h recalls from non-study participants. Dietary intake data were then entered into NDS-R. Energy and macronutrient intake estimated from digital photographs was compared with weigh-and-measure values. An error ≤5% was required before data collection on study participants. Recalls were evaluated according to a published dietary recall documentation checklist (³⁴). An error rate ≤5% on both the recall documentation checklist and computer coding was required before data collection. Interrater reliability coefficients for both digital photograph and 24-h recall assessments were ≥0.95. We have also demonstrated that 7 d of dietary data adequately characterize usual energy and macronutrient intake (¹⁶). Baseline data from the current study indicated that digital photography provided a significantly more accurate assessment of energy intake over 7 d (error, 6.8%) compared with energy intake assessed by 3-d food records during the same assessment period (error, 15.7 %) using total daily energy expenditure assessed by doubly labeled water as the standard. Thus, digital photography may provide better estimates of energy and macronutrient intake for studies on the effect of exercise training on dietary behavior where 24-h recalls or 3- to 7-d food records have been typically used (^24,27). Participants were required to eat a minimum of two meals per day on weekdays and one meal per day on weekends over the 7-d period in the cafeteria. Participants who were noncompliant with the energy intake assessment protocol were dismissed from the study.

FIGURE 1: Setup for the assessment of energy intake by digital photography. A, Photo at 45°. B, Photo at 90°.

Diet quality

Diet quality was estimated using the Healthy Eating Index 2010 (HEI-2010) developed by the United States Department of Agriculture (¹⁷) to assess conformance to the 2010 dietary guidelines for Americans (⁵). The HEI-2010 was calculated using NDS-R output obtained from digital photograph data following the method developed by Miller et al. (²³) modified for the 2010 guidelines. HEI-2010 scores range from 0 to 100. Scores >80 are considered “good,” scores between 51 and 80 are classified as “needs improvement,” and scores <51 are classified as “poor” diet quality.

Analysis

Baseline measures and demographic characteristics were summarized using means and SD for continuous variables and frequencies and percentages for categorical variables. ANOVA was performed to examine group differences in baseline characteristics. ANOVA and paired-sample t-tests were conducted to compare energy and macronutrient intake between and within treatment groups. General linear mixed modeling was also used to examine overall group differences (group effect), change over time (time effect), and group differences in this change (group–time interaction). Model parameters were estimated for each outcome, along with unconstrained correlations among repeated assessments, which provide better model fit than other error covariance structures according to Akaike Information Criterion and Bayesian Information Criterion (³). Models were adjusted for age and sex, thereby providing unbiased estimates of the treatment effects. When the group effect or group–time interaction was significant at 0.05 alpha level, adjusted means were compared pairwise using Bonferroni correction for type 1 error inflations. All analyses were conducted using SAS 9.3 (SAS Institute, 2002–2010).

RESULTS

Participants

Ninety-one of the 141 participants randomized at baseline (65%) complied with the study protocol and completed all outcome assessments. The completion rates were 75%, 70%, and 60% for the control and 400- and 600-kcal-per-session groups, respectively. Approximately 44% of those who did not complete the study were dismissed by the investigators for failure to comply with the study protocol. Additional reasons for dropout included lack of interest/time, schedule conflicts, and unwillingness to comply with the dietary assessment protocol. The baseline characteristics of the 91 participants who completed the study are presented in Table 1. The sample mean age was approximately 23 yr, BMI was approximately 31 kg·m⁻², and the sample was composed of approximately 50% women and 16% minorities. There were no differences in baseline characteristics (age, BMI, body composition, percentage of female, and energy intake) between participants who completed and did not complete the study protocol, with the exception of a small but significantly higher level of aerobic fitness (P < 0.05) in participants who completed (33.4 ± 5.9 mL·kg·min⁻¹) versus that in participants who did not complete the study protocol (31.4 ± 5.5 mL·kg·min⁻¹). No major adverse events were reported among participants in either the exercise or control groups.

TABLE 1: Baseline participant characteristics.

Exercise compliance/EEEx

Attendance at exercise sessions (≥91%) did not differ by sex or exercise group. The mean EEEx from month 4 to 10 for the 400- and 600-kcal-per-session groups was 402 ± 6 and 604 ± 7 kcal per session, respectively. There were no differences in exercise intensity or between men and women assigned to exercise at 400 or 600 kcal per session. Because of higher body weight, men required less time to complete the 400 (men, 31 ± 6 min; women, 48 ± 7 min) or 600 kcal per session (men, 42 ± 8 min; women, 63 ± 9 min) protocols compared with women.

Weight/body composition

Results for change in weight and body composition, assessed by dual energy x-ray absorptiometry, are presented in detail elsewhere (¹¹). Briefly, weight change over the 10-month intervention in both the 400-kcal-per-session (−3.9 ± 4.9 kg; 4.3%) and 600-kcal-per-session (−5.2 ± 5.6 kg; 5.7%) groups was significantly different from that in controls (+0.5 ± 3.5 kg; 0.5%); however, weight change did not differ significantly between the exercise groups. There were no significant differences in weight change between men and women in either the 400-kcal-per-session (men: −3.8 ± 5.8 kg, −3.7%; women: −4.1 ± 4.2 kg, 4.9%) or 600-kcal-per-session (men: −5.9 ± 6.7 kg, 5.9%; women: −4.4 ± 2.1 kg, 5.4%) groups. Fat mass decreased significantly from baseline to 10 months in both the 400-kcal-per-session (−3.5 ± 4.8 kg) and 600-kcal-per-session (−5.2 ± 5.2 kg) groups but not in controls (0.2 ± 3.2 kg). There were no significant differences for change in fat mass between men and women in either the 400-kcal-per-session (men, −3.6 ± 5.3 kg; women, −3.4 ± kg) or 600-kcal-per-session groups (men, −5.9 ± 6.0 kg; women, −4.4 ± 4.3 kg). There were no significant changes in fat-free mass in any study group; thus, the reductions in body weight observed in the exercise groups were a result of decreased fat mass.

Energy intake

Absolute energy intake (kcal·d⁻¹) over the 10-month intervention in the total sample and in men and women is presented in Figure 2A and Table 2. There were no significant group differences in absolute energy intake at baseline and at 3.5, 7, or 10 months in the total sample or in men. However, in women, absolute energy intake was significantly greater in the 600-kcal-per-session group compared with that in controls at both 3.5 and 7 months. Mixed modeling revealed that after controlling for age and sex, there were no significant between- or within-group differences (group or time effect) or group–time interaction in absolute energy intake in the total sample or in men or women. There was a consistent pattern for change in absolute energy intake from baseline to 10 months in the total sample and in men and women. That is, absolute energy intake increased in the 600-kcal-per-session group and decreased in both the 400-kcal-per-session and control groups. Increased energy intake was observed in the 600-kcal-per-session group in spite of a mean weight loss of 5.7%.

TABLE 2: Absolute and relative energy intake (mean ± SD) across 10 months by intervention group (400 and 600 kcal per session and control) in the total sample and by sex.

FIGURE 2: A. Absolute energy intake (kcal·d⁻¹) over 10 months in the total sample, men and women. B. Relative energy intake (kcal·kg⁻¹ body weight·d⁻¹) over 10 months in the total sample, men and women. *The 600-kcal-per-session group was significantly greater than the control (P <0.05). **Significant group–time interaction based on mixed modeling. Total sample (P < 0.01), men (P = 0.03) and women (P = 0.01).

Although we observed no significant change in mean energy intake in response to a 10-month aerobic exercise program; interindividual variability was considerable (Fig. 3). In the total sample, energy intake at 10 months was increased compared with that at baseline in approximately 33% of controls (mean increase, 438 ± 358 kcal·d⁻¹), approximately 42% of the 400 kcal-per-session group (293 ± 159 kcal·d⁻¹), and approximately 62% of the 600-kcal-per-session group (444 ± 243 kcal·d⁻¹). Interestingly, despite the high percentage of participants in the exercise groups that increased energy intake over the 10-month intervention, 71% of participants in the 400 kcal-per-session group and 81% of the participants in the 600-kcal per-session group lost weight.

FIGURE 3: Interindividual variability in change in absolute energy intake (kcal·d⁻¹), 10 month minus baseline) by intervention group.

Energy intake relative to body weight (kcal·kg·d⁻¹) over the 10-month intervention is presented in Figure 2B and Table 2. There were no significant group differences in relative energy intake at baseline and at 3.5, 7, or 10 months in the total sample or in men or women. However, mixed modeling showed that after controlling for age and sex, the group–time interaction was significant in the total sample (P < 0.01) and in both men (P = 0.03) and women (P = 0.01). Relative energy intake increased from baseline to 10 months in the 600-kcal-per-session group and was essentially unchanged or slightly decreased in the 400-kcal-per-session and control groups.

Macronutrient intake

Macronutrient intake at baseline and at 3.5, 7, and 10 months in the total sample and by sex is presented in Table 3. There were no significant group differences in intake of CHO, fat, or protein expressed as either grams per day or as a percentage of energy intake at baseline and at 3.5, 7, or 10 months in the total sample. In men, fat intake as a percentage of energy intake was significantly higher in the 400-kcal-per-session group compared with that in the 600-kcal-per-session group at baseline and 7 months. There were no significant group differences in fat intake as a percentage of energy intake at any time point in women. However, in women, fat intake (g·d⁻¹) was significantly higher in the 600-kcal-per-session group compared with that in controls at 3.5 and 7 months, reflecting the decreased absolute energy intake in control women. In addition, protein intake (g·d⁻¹) in women was significantly higher in the 400-kcal-per-session group compared with that in the 600-kcal-per-session group at 3.5 months and significantly greater in the 600-kcal-per-session group compared with that in controls at 7 months; however, no significant group differences in protein intake as a percentage of energy intake were observed at any time point. Mixed modeling revealed no significant treatment effects on the intake of CHO, fat, or protein expressed as either grams per day or as a percentage of energy intake in the total sample or in men or women.

TABLE 3: Macronutrient intake (mean ± SD) across 10 months by intervention group (400 and 600 kcal per session and control) in the total sample and by sex.

HEI-2010

There were no significant between- or within-group differences in HEI in the total sample or in men or women. The HEI-2010 scores averaged across all periods were 37.6 ± 8.9, 35.6 ± 8.4, and 36.7 ± 8.5 for the 400-kcal-per-session, 600-kcal-per-session, and control groups, respectively. HEI-2010 scores were significantly higher in women (38.8 ± 9.0) compared with those in men (34.6 ± 7.7, P < 0.01).

DISCUSSION

We found no significant change in energy intake assessed by digital photography in response to a 10-month supervised aerobic exercise program at two measured levels of EEEx in a sample of previously sedentary, overweight, and obese young adults. This finding is consistent with results from previous randomized and nonrandomized trials conducted by our group (^12,13,31) and others (^{6,7,19,20,24,27}). However, randomized and nonrandomized trials have also reported both significantly increased (²²) and decreased energy intake in response to exercise training (^8,21). The observation that exercise training does not induce compensatory increases in mean energy intake is relatively consistent; however, these trials were not specifically designed to address this issue. In addition, these trials have generally assessed energy intake by self-report food records (^4,21,24,27), which may not provide data of sufficient quality to adequately address this question (²⁹). However, trials that used more precise estimates of energy intake, such as weigh-and-measure test meals (^6,7,20,27), or observed weigh-and-measure ad libitum eating (¹³) have also reported no change in mean energy intake in response to exercise training. In addition, most previous trials have not prescribed exercise by level of energy expenditure or assessed the actual level of EEEx achieved (⁹). Without precise measures of both energy intake and EEEx, it is not possible to quantify the association between these two variables.

The change in energy intake in response to exercise training displayed considerable individual variability, which is at least partially responsible for the high levels of individual variability in weight change observed in this trial (¹¹) and other exercise and weight loss trials (^8,10,20). Interestingly, approximately 52% of participants in the exercise groups increased energy intake over the 10-month intervention (mean increase, approximately 369 kcal·d⁻¹), with 38% of exercise participants increasing energy intake ≥200 kcal·d⁻¹. However, 77% of exercise group participants lost weight. As described in detail in a companion article (³⁸), total daily energy expenditure assessed by doubly labeled water increased approximately 250 kcal·d⁻¹ in the exercise groups from baseline to 10 months. Therefore, in most participants, the increased energy intake did not fully compensate for the imposed EEEx and resulting increased total daily energy expenditure, allowing most participants to achieve a negative energy balance. The observation that both absolute and relative energy intake tended to increase in the 600-kcal-per-session group but not in the 400-kcal-per-session group suggests there may be a level of EEEx that induces compensatory increases in energy intake. However, the increased energy intake even in the 600-kcal-per-session group was insufficient to fully compensate for that level of EEEx and an associated mean increase in total daily energy expenditure of approximately 289 kcal·d⁻¹ (³⁸), as 81% of participants in the 600 group lost weight (mean, −5.7%). Thus, exercise without energy restriction represents a positive behavioral approach, which may be an attractive first-line weight loss recommendation for overweight or obese young adults who are unwilling or unable to comply with energy restriction or intensive behavioral counseling. Identification of characteristics of participants who increase energy intake in response to aerobic exercise, and the levels of EEEx at which compensatory increases occur, will be important for tailoring exercise interventions for both the prevention and treatment of obesity. However, in this trial and in previous trials (¹³), we have been unable to identify any baseline characteristics that differentiate participants who do or do not increase energy intake in response to aerobic exercise.

Our finding of no significant sex difference for change in energy intake in response to exercise training is in agreement with previous results from our group (¹³) and others (^7,37). The current trial and the trial by Caudwell et al. (⁷) both showed nonsignificant differences for weight loss between men and women in response to exercise of equal energy expenditure, which argues against the notion that women are unable to achieve clinically significant weight loss with exercise without energy restriction. In the current trial, mean weight loss in men in the 600-kcal-per-session group (−5.9%) was greater than that in the 400-kcal-per-session group (−3.7%); however, in women, mean weight loss values in the 400-kcal-per-session (−4.9%) and 600-kcal-per-session groups (−5.4%) were nearly identical. Sex differences in energy intake between the 400- and 600-kcal-per-session groups offer a potential explanation for the observation of greater weight loss with increased EEEx in men but not in women. In men, energy intake during the exercise intervention (i.e., mean of months 3.5, 7, and 10) was similar in both the 400-kcal-per-session (3205 kcal·d⁻¹, 33.8 kcal·kg·d⁻¹) and 600-kcal-per-session groups (3208 kcal·d⁻¹, 32.8 kcal·kg·d⁻¹). However, in women, energy intake during the intervention was greater in the 600-kcal-per-session group (2619 kcal·d⁻¹, 34.2 kcal·kg·d⁻¹) compared with that in the 400-kcal-per-session group (2459 kcal·d⁻¹, 31.3 kcal·kg·d⁻¹). However, on the basis of a relatively small sample, potential sex differences in the energy intake response to different levels of EEEx may warrant additional investigation.

We found no clinically relevant changes in macronutrient intake in response to aerobic training, a result in agreement with previous reports from our group (^12,13,31) and others (^{8,19,20,22,24,27}). However, changes in macronutrient intake in response to exercise training have been reported. For example, Brandon et al. (⁴) reported a significant increase in absolute CHO intake in White but not African American women, whereas Kirkwood et al. (²¹) reported a significant increase in fat intake as a percentage of total energy intake, with no change in the percentage of energy intake from CHO or protein. Studies have also reported both significant increases (^39,40) and decreases in CHO intake (⁴) and significant increases (¹²) and decreases in fat intake (³⁹) in response to exercise training.

Diet quality, as assessed by the HEI-2010, was poor and did not change in response to exercise training. Thus, simply engaging in aerobic exercise training does not result in changes in diet quality that may be associated with improved health. The average HEI-2010 score observed in our sample of young adults (36.6) was lower than that reported for the average American (53.5) (³⁵). Diets were very low in fruits, vegetables, seafood, and plant proteins and very high in sodium. Our observation of poor diet quality among college-age individuals and higher diet quality in women compared with that in men has been reported previously (^15,18). We are unaware of other trials that have evaluated the effect of exercise on HEI-2010.

The strengths of the current investigation include the following: 1) the use of a randomized efficacy design, 2) a relatively long intervention (10 months), 3) inclusion of both men and women, 4) the use of supervised exercise at two verified levels of energy expenditure, and 5) multiple assessments of energy and macronutrient intake using digital photography. Potential limitations may include the following: 1) the study was not specifically designed to detect differences in change in energy and macronutrient intake either within or between intervention groups; 2) although energy intake was assessed using digital photography, our protocol provided direct observation of a minimum of 57% of meals consumed at baseline and at 3.5, 7, and 10 months, and thus, our inability to detect significant changes in energy and macronutrient intake may be a function of underreporting of foods not consumed under direct observation; and 3) participant attrition (35%). We emphasize that MET-2 was an efficacy trail designed to answer questions relative to the effect of exercise training on body weight, energy intake, etc., when the exercise was competed as intended. Thus, higher rates of attrition, when compared with effectiveness trials, are expected. However, our observed attrition rate was similar to the attrition rate we used in our power calculations for our primary aim (33%) and nearly identical to attrition rates reported in other longer-term randomized trials (>6 months), which have evaluated the effect of exercise training on energy intake (approximately 36%) (^25,26). In addition, MET-2 was an efficacy trial conducted in overweight and obese young adults. Thus, generalizability of results to other groups, such as middle-age or older adults, or comparisons with trials using and intent-to-treat analysis are unwarranted.

In summary, we found no significant change in energy or macronutrient intake in response to a 10-month supervised exercise program in overweight and obese young adults. The possibility of a threshold beyond which further increases in EEEx do not produce a more negative energy balance and potential sex differences in the energy intake response to increased levels of EEEx observed in this study are potentially important and worthy of investigation in an adequately powered trial. Randomized trials designed and powered to evaluate the effect of additional exercise parameters, e.g., mode, frequency, intermittent vs continuous, time of the day, and participant characteristics including age, body weight/composition, race/ethnicity, and aerobic capacity on energy and macronutrient intake, are warranted. This information will be important for both the design and targeting of weight management intervention using exercise alone or exercise in combination with energy restriction.

This study was supported by the National Institutes of Health grant R01-DK049181.

The clinical trial registry for this article is NCT011865.

The authors report no conflict of interest.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.