February 28 - 29, 2008 Advisory Committee Meeting Minutes
Subcommittee Report: Energy Balance
Edward Howley, Ph.D., presented the energy balance subcommittee report. Dr. Howley started with strong note that energy balance is equally dependent on intake and expenditure and both sides must be addressed. The group addressed the following research questions:
How much physical activity is needed relative to: Weight stability and weight loss; Weight regain following weight loss; Measures of central adiposity.
Do the following factors affect physical activity recommendations for energy balance: Age and gender; Race/ethnicity and social economic status (SES).
In reviewing the data the subcommittee defined weight maintenance as less than 3% change in body weight and weight loss as more than a 5% reduction in body weight.
The cross-sectional evidence on physical activity and body weight included 24 studies from the CDC database of which 22 showed favorable effects of activity on weight. There was also a dose-response relationship with most favorable changes occurring with 30 – 60 minutes per day of activity. Prospective evidence included 9 studies all showing favorable effects with 30 – 45 minutes per day to reduce weight gain over time. RCT evidence included 19 studies of which 7 reported a significant effect.
Reviewing the effects of resistance exercise, 9 studies showed modest change in body weight. The potential reasons for these modest effects include weight loss may be accompanied by a concurrent increase in fat-free mass and dose of resistance exercise may not be sufficient.
Overall conclusions for weight loss and maintenance suggest that endurance activities show a clear dose-response across studies with a range of 180 – 360 minutes per week over 12 weeks to 18 months results in weight loss of 3kg or less. Resistance exercises show a magnitude of 1kg or less of weight loss. It is important to note that dietary intervention is also necessary.
To analyze physical activity and weight regain following weight loss the group identified 8 RCT studies with only 3 studies randomizing subjects to physical activity levels subsequent to weight loss and only 2 used a control group. Three observational studies were identified. From this data one can conclude that “more is better.” The estimated gross energy expenditure needed for weight maintenance after weight loss is the equivalent to 60 minutes at approximately 4mph or 80 minutes at approximately 3mph.
Researching the relationship between physical activity and body composition related to metabolic disorders the subcommittee reviewed RCT studies. Both aerobic and resistance training favorably affected body composition. Abdominal fat loss is proportional to overall fat loss, and is greater in overweight individuals. 45 – 60 minutes per day of moderate-to-vigorous activity is needed for clinically relevant changes in total and abdominal adiposity.
The subcommittees report on the relationship between physical activity, energy balance and gender and age included cross-sectional and prospective studies, RCTs, epidemiological studies as well as observational studies. There is some evidence that the amount of physical activity needed to maintain weight increases with age and may be higher for women; however, the evidence is not sufficient to recommend differential prescriptions based on gender or age alone.
Addressing the effect of race/ethnicity and SES, Dr. Howley noted there are a few studies on Latinos and American Indians; however, heterogeneity both between and within racial/ethnic croups limits extrapolation of results. The limited amount of data makes it unwise to draw conclusions as to whether physical activity recommendations should differ by race, ethnicity or SES.
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