Can Diet and Exercise Really Change Metabolism?

, University of Alberta

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Can We Increase Our Metabolic Rate?

There is evidence to suggest that increased food intake can increase RMR. This has been demonstrated in particular in patients with anorexia nervosa participating in nutritional repletion programs. Increasing food intake alone can result in an increase in RMR greater than that which would be expected in association with the body mass gain.

It is also known that exercise increases metabolic rate during a bout of exercise and immediately after the exercise. However, whether there is a longer-term effect remains unresolved. Many of the studies in this area have investigated exercise effects during an energy restriction, but the results are mixed. Ballor[24] suggests that the more severe the dietary restriction, the smaller the effect of exercise training on metabolic rate and conservation of FFM. In animal studies, an interaction between the severity of dietary restriction and exercise training has been observed: Exercise has been found to elevate RMR during moderate dieting, but not during severe dietary restriction.[24] That is, a certain level of dietary intake seems to be needed for exercise to stimulate metabolic processes effectively.

In a study to determine the effect of exercise regimen on metabolic rate and body composition, women who had a history of chronic dieting were randomized into 2 exercise groups (endurance training and resistance training).[25] They were compared to a control group of women who did not exercise. The prescribed exercise program was of moderate intensity, designed to minimize potential injury and maximize compliance. One group participated in an outdoor walking program of progressively increasing distances. The other group participated in a weight-training fitness class of progressively increasing resistance exercises. The exercise was done 3-4 times per week for 3 months. At this level of intensity, there were no exercise-induced changes in FFM or REE, regardless of the type of exercise. In addition, there was no weight loss in either of the exercise groups, but the control group gained an average of 2.5kg over the same 3-month period. Positive effects resulting from the exercise included an increase in fitness level and maintenance of initial body weight for the duration of the study. This study suggests that exercise at a level beyond moderate intensity may be required to induce changes in FFM and REE.[25] However, the tradeoff is this: A sedentary overweight population may be less likely to adhere to a more vigorous exercise program.

One study that showed a significant effect of exercise on RMR was done in an elderly population.[26] This study involved an exercise program for 18 elderly people (66 years old [+/-1.4 year]; 10 males, 8 females) who participated in cycling exercise for an 8-week period (3x/wk). For the first week they cycled at 60% of maximal oxygen consumption (VO2 max) until 150kcal was expended. By the eighth week, they cycled at 80% VO2 max until 300kcal was expended. There was no change in body weight, FM, or FFM during the study. VO2 max significantly increased from 1.9 (+/-0.1) L/min to 2.1 (+/-0.1) L/min (P <0.01). When measured on an outpatient basis, exercise training significantly increased RMR by 11% (P <0.01) (pre 1.17 [+/-0.031] kcal/min versus post 1.29 [+/-0.019] kcal/min). It was suggested that this increase is not just related to the exercise but to an increased energy flux. Increased food must have been consumed to compensate for the energy expenditure, as there was no weight change over the 8-week period. Both nutritional and sympathetic factors can mediate changes in metabolism in response to endurance exercise.[26]

Adequate food intake appears to be a key signal to the body to maintain energy-producing processes. In essence, it takes food to stimulate and sustain metabolism. For a depleted patient with anorexia nervosa, increased food intake can stimulate RMR. For an individual trying to lose weight, an adequate food intake can complement exercise, whereas a restricted food intake can antagonize an exercise program. The idea of maintaining a certain level of energy flux appears to be an important one. Increasing muscle mass and therefore FFM through weight training indirectly should increase RMR. The level of training required to do this has not been clearly shown, however, and it appears to be a very high intensity level.

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