December 6 - 7, 2007 Advisory Committee Meeting Minutes
Energy Balance Subcommittee Report
Edward Howley, Ph.D., led the subcommittee report on energy balance. Dr. Howley introduced members of the subcommittee including, Dr. Pate, Dr. Yancey and the subcommittee' s consultants, Loretta DiPietro, Yale University, Shiriki Kumanyika, University of Pennsylvania, John Jackicic, University of Pittsburgh, Joe Donnelly, University of Kansas and CDC Liaison Dr. Kohl.
Dr. Howley prefaced the report with the thought that energy intake and expenditures is significantly important in this area making energy balance different then the other areas represented by the Committee. The group addressed two major questions:
How much physical activity is needed relative to weight stability, weight loss, weight regain following weight loss, and measures of central adiposity?
Do age, gender, race/ethnicity and socioeconomic status affect physical activity recommendations?
Dr. Howley asked Dr. Jackicic to address the first research question. In order to deal with the issue of energy balance it was important for the group to adopt a definition of weight maintenance. The group chose the recent definition by Stevens, et., al., who defined weight maintenance as plus or minus 3% for change in body weight. Additionally, the clinical definition of weight loss as also defined by Stevens, et., al., as greater than or equal to a five percent reduction in body weight.
The issues addressed by Dr. Jackicic focused on whether physical activity is sufficient to result in weight stability and is physical activity sufficient to result in weight loss. The group identified 24 studies from the CDC database of which 22 showed a favorable effect of physical activity on body weight. The most favorable effect appears to result from 30 to 60 minutes of physical activity per day. Of 18 randomized controlled studies 14 showed a significant effect of physical activity on body weight. The positive studies showed a weight reduction of .5 to 3 kilograms, which appeared no more than a 3% change in body weight. If adhering to the Stevens definition this reduction was not weight loss but weight stability. In summary, physical activity appears to result in less than a 3-kilogram change in body weight or less than 3% change in body weight. In order to achieve these results 30 to 60 minutes per day of physical activity is necessary. Resistance exercise from a weight perspective appears to be less effective, compared to endurance exercise, but that may be due to the confounder that resistance exercise may increase fat-free mass. Physical activity alone does not appear to result in significant reduction in body weight.
For maintenance of weight loss it is generally accepted that physical activity is critically important. Through review of the available studies it appears that more physical activity is better for maintenance of weight loss. Some shortcomings of the studies include few have been powered adequately resulting in sample size issues. A design issue also exists in which none of the studies have taken individuals who successfully lost weight and then randomized them to different doses of activity to truly examine the effect of activity on weight loss maintenance.
With reference to adiposity half the studies report no change. Studies that did show change only resulted in modest change. This would seem to indicate physical activity has a modest impact on adiposity.
Dr. Howley continued the report by addressing factors that might impact the relationship between physical activity and energy balance. Age was one factor that was identified and in general, decrease in physical activity was associated with an increase in body mass and fat mass in middle and older adults. Excess weight loss seems to be a main concern in individuals 65 and older leading to the loss of more lean mass than fat mass resulting in sarcopenic obesity. Observational studies point towards even modest physical activity as having a positive impact in this age group while randomized control trials show aerobic training as having only a minimal impact. Similarly, intense resistance training resulted in an increase in fat-free mass in muscle area with minimal changes in body weight in both men and women.
In terms of gender and physical activity, epidemiological studies found an inverse association between physical activity and weight gain in men and women in contrast to intervention studies were varied. Some showed weight and fat loss in men and not women. Some found no change in either and some found similar changes in both. Special attention should be given to pre-menopausal women, as there is some evidence that they may be more resistant to weight change or require greater volume of physical activity. The data however on gender is too sparse and varied to come to strong conclusions.
From a race perspective the literature seems to show dramatic differences in obesity in women, and not men, among different ethnicities. However, there is not enough data to extrapolate conclusions on energy balance with respect to race.
Overall, the Energy Balance section of the Committee Report should include some kind of statement regarding dietary restraint and the amount of physical activity required to achieve a goal will vary among people. Physical activity alone though is sufficient to achieve weight stability.
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