In this 12-week trial, exercise with its inherent mild weight loss had a significant beneficial impact on body composition, the risk factors associated with metabolic syndrome, and cardiovascular and metabolic disease in an older obese population. The addition of a diet-based weight loss program to the exercise intervention did not lead to greater improvements in this study. Although the exercise intervention was performed at a relatively high intensity based on each individual's aerobic capacity and was sustained for a relatively prolonged duration for each exercise session, this was nevertheless a walking program. What this highlights, in part, is the relatively low aerobic fitness of these individuals and more importantly that a walking program can bring about significant weight loss and an improvement in clinical outcomes in 12 weeks. The improvement in aerobic fitness was associated with the improvement in insulin sensitivity based on clamp measures. More than 90% of glucose disposal during a clamp is taken up by skeletal muscle. These data suggest that the central and peripheral adaptations that are reflective of increased VO2max
may contribute to the improvement in glucose disposal in these participants. This may occur via improved blood flow to the muscle, thus increasing insulin delivery, or enhanced muscle function leading to improved insulin signaling and glucose uptake (21
). Collectively, these observations provide strong support for prescribing brisk, sustained walking exercise for older obese adults.
The addition of caloric restriction (500 kcal/d) to the exercise intervention was successful in generating greater weight loss, but surprisingly, this did not translate to greater improvements in the clinical measures that underpin the metabolic syndrome and presumably chronic disease risk. Although several previous studies on exercise and caloric restriction (23
) have been published, we believe that it is important to distinguish studies of older versus younger adults and quantify the exercise sessions and the amount of caloric restriction. In some studies that have compared caloric restriction with exercise, participants often exercised three sessions per week, each session lasting around 30 minutes, with minimal change in body weight (26
). Although exercise in these studies improved the cardiovascular profile, greater weight loss produced through dieting or a combined intervention had a superior outcome in younger (29
), older (27
), or mixed–age group populations (25
). However, mechanisms of energy balance are dysregulated in old age, and the response to a negative energy balance is attenuated (31
). Consequently, older adults experience greater loss of muscle mass with intentional caloric restriction compared with younger adults (14
). Weiss and colleagues have shown in older men and women that muscle mass and absolute work capacity decrease in response to 12 months of caloric restriction but not in response to a similar amount of weight loss induced by exercise (33
). The loss of muscle with advancing aging is clinically known as sarcopenia and is strongly related to impaired mobility, increased mortality and morbidity, and lower quality of life (34
). Although both groups of participants in our study preserved muscle mass, we did observe a trend for a greater loss of muscle in the caloric restriction group (~4%, p
= .11). These observations suggest that there is a need for caution and a careful assessment of the risk or benefit when prescribing diet-induced weight loss in the elderly.
It is well established that aging is associated with an increased accumulation of truncal fat, and studies have previously shown that there is a preferential loss of fat from the central regions of the body with exercise training in older adults (7
). In the present study, the reduction in visceral fat was associated with improvements in insulin resistance and was also correlated with the change in fasting glucose. Recent reports and reviews conclude that independent of total weight loss, or even reductions in waist circumference, reduced visceral fat contributes to reduced cardiovascular risk (6
). We observed that the improvement in metabolic syndrome and insulin resistance was comparable for both interventions. The reduction in visceral fat was also similar in both groups, whereas the reduction in total body weight and subcutaneous fat was greater during the exercise or caloric restriction trial. Our correlation data underscore the importance of fitness and weight loss in improving insulin resistance, possibly through exercise-induced weight loss–associated mobilization of visceral fat stores and improved skeletal muscle insulin sensitivity. Data from this study suggest that there may be a threshold for improving metabolic syndrome, and this can be reached by exercise alone or by exercise plus caloric restriction. Fitness had a very strong correlation with insulin sensitivity (), and this observation supports recently published data that fitness is a strong and independent predictor of all-cause mortality in adults 60 years or older (13
). The combined effects of reduced visceral fat and improved fitness appear to be powerful contributors to reduced cardiometabolic health risk.
The interventions produced marked improvements in resting blood pressure. These data are consistent with the observations of Dengel and colleagues who reported improved blood pressures after exercise training with and without weight loss in older men (28
). The change in systolic blood pressure correlated with the reduction in visceral fat. The mechanistic link between systolic blood pressure and visceral fat may reside with inflammatory cytokines such as tumor necrosis factor α and interleukin-6, which are known to increase with advancing age (35
). These cytokines together with adhesion molecules such as vascular adhesion molecule-1 and intercellular adhesion molecule-1 may contribute to increased endothelial activation and hypertension (36
). It is possible that the decrease in visceral fat contributed to a reduction in proinflammatory cytokines, thereby removing the stimulus for endothelial dysfunction and hypertension.
One limitation of this study is the relatively small number of participants studied. It is possible that with additional participants, some of the group differences may have reached statistical significance. However, the between-group p values shown in suggest that these numbers would have to be increased substantially to attempt to demonstrate such an effect. Furthermore, it is likely that these cardiometabolic benefits would require even greater caloric restriction with a subsequent greater weight loss and an increased likelihood of losing muscle mass. In addition, participants in this study were evaluated when they were fully trained, and so the effects of exercise were maximized. It is worth considering that because the effects of exercise are short lived, the EX + CR group may have had a more long-lived benefit because the known effects of caloric restriction on insulin resistance tend to persist longer than exercise alone. If measures had been obtained 7–10 days after the intervention, the effect of the greater weight loss with caloric restriction may have been more evident. On the other hand, delaying the insulin sensitivity measure too long after the last exercise may result in a “de-training” effect, which would in turn mask the exercise benefit.
In conclusion, data from this study suggest that frequent habitual exercise with its inherent mild weight loss had a significant beneficial impact on body composition, the risk factors associated with metabolic syndrome, and cardiovascular and metabolic disease in an older obese population. The addition of a diet-based weight loss program to the exercise intervention did not lead to greater improvements in these older obese adults. Although it would seem that exercise alone may be adequate to achieve significant improvements in health risk, one must also consider that the effects of exercise are short lived, whereas the benefits that accrue from caloric restriction may be sustainable for a longer period of time.