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Caloric Restriction with or without Exercise: The Fitness versus Fatness Debate

Posted: Sunday, February 14, 2010

There is a debate over the independent effects of aerobic fitness and body fatness on mortality and disease risks.

In this randomized clinical trial, researchers tested the effect of body weight and body fat loss with or without improvement in physical fitness on cardiometabolic risk factors in overweight men and women who underwent a 6-month CR regimen with or without regular aerobic exercise. Although both intervention groups experienced similar reductions in total body mass, fat mass, and visceral abdominal mass, the caloric restricted plus exercise group experienced greater improvement in insulin sensitivity, LDL cholesterol, and diastolic blood pressure than that in the CR by diet alone group.
The results strongly suggest that inclusion of regular aerobic exercise (or training) in a weight loss program yields cardiometabolic health benefits beyond those of weight loss alone. Results further support the argument that both fitness and fatness are important for reducing cardiometabolic risks, particularly during CR and may shed some light on how fitness or fatness may contribute to overall mortality.

The main difference between the two caloric-restricted groups was that one group was 25% caloric restricted by diet alone (i.e., consuming 75% of requirement), whereas the other group was in 25% energy deficit, 12.5% by decreasing food intake, and 12.5% by increasing energy expended through regular aerobic exercise. Not surprisingly, the caloric restricted plus exercise group experienced significant improvements in aerobic fitness with an average 10% improvement in absolute V·O2peak (L·min−1 and an average 22% improvement in V·O2peak relative to body weight. Of interest, however, is the slight albeit not statistically significant decrease in absolute aerobic fitness that occurred over the 6-month period in both the nonexercise groups. In comparison, Ross et al. found that weight loss induced by a 700-kcal daily energy deficit by diet resulted in a 5% decrease in aerobic fitness, whereas weight loss induced by a 700-kcal increase in exercise resulted in a 13% improvement in V·O2peak. In the current study, however, CR either by diet or diet plus exercise did not result in any significant changes in leg muscle peak power or endurance. Importantly, the data demonstrate that CR does not lead to reduced overall strength or functionality in humans when fed a nutritionally sound diet.

A unique aspect and strength of the study is that weight loss was achieved through provision of isoenergetic deficits (regardless of exercise) which resulted in almost identical reductions in total body weight and total and visceral adiposity. Such design allowed researchers to tease out the additional influence of exercise which according to the results is a necessary part of the prescription to gain full cardiometabolic improvements including improved insulin sensitivity and lowered LDL cholesterol and diastolic blood pressure. The statistically significant improvement in insulin sensitivity only in the CR + EX, but not CR group, is not surprising given the well-documented insulin-sensitizing effect of aerobic exercise driven predominantly by increased glucose transporter 4 (GLUT4) expression and trafficking in exercised skeletal muscle. Increased fatness, on the other hand, is associated with increased ectopic fat deposition in skeletal muscle and liver which may influence the insulin signaling cascade and impact circulating lipids. Somewhat surprising, however, is that inclusion of aerobic exercise did not result in additional improvements in HDL cholesterol and to a lesser extent systolic blood pressure. Such lack of effect may be related to the selection of healthy overweight rather than obese volunteers (who had relatively normal blood pressure values) and administration of a tightly controlled AHA-Step 1 diet which provided 30% of energy from fat.

The results of the current study along with those of a previous analysis reporting larger reduction in 10-yr CVD risk (38% in CR + EX vs 29% in CR and no change in controls) when exercise was included suggest that improvements in both fitness and fatness are needed for optimally reducing overall morbidity and mortality. Specifically, aerobic fitness (assessed by V·O2peak) had stronger associations than fatness with the Framingham Risk Score (model used to identify healthy individuals at risk for CVD), the ankle/brachial index (average ankle systolic blood pressure/arm systolic blood pressure; a physiological marker of cardiovascular risk), and hemoglobin A1c concentration. The results further suggest that regular aerobic exercise which improves V·O2peak also improves peripheral resistance during diastole above that noted with a reduction in fatness.  Although it is clearly established that elevated systolic blood pressure is a more powerful predictor of cardiovascular events than diastolic pressure, increased diastolic blood pressure causes an increased risk for end organ failure, cardiovascular death, and could indeed be an important reason why fitness is associated with reduced overall mortality.

Another important health benefit exerted by regular exercise beyond those of weight loss includes its impact on aerobic capacity, particularly in relation to daily functionality and overall mortality. Aerobic capacity has been shown to be a more powerful predictor of mortality among both healthy men and those with CVD than other established risk factors with each 1-MET increase in aerobic capacity conferring a 12% increase in survival. Based on these reported statistics, the CR + EX group, who improved their aerobic capacity by an average 1.6 METs would have an estimated 19.2% increase in survival. By contrast, the improvement in the aerobic capacity of the CR + EX group was parallel with an alarming tendency of absolute aerobic capacity to decline over the 6-month period in both the CR and control-treated groups.

Results of the current study suggest that beyond changes in fatness, combining CR with exercise is important for increasing aerobic fitness and optimizing improvements in risk factors for diabetes and CVD, including improved insulin sensitivity, LDL cholesterol, and diastolic blood pressure that are beyond those of body weight/body fat reduction alone. Improvements in other cardiometabolic risk factors, however, such as systolic blood pressure and HDL cholesterol might only be associated with changes in fatness and/or consumption of a healthy diet.

Source: Diabetes In Control: Medicine and Science in Sports and Exercise (R). 2010;42(1):152-159.

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