Physical activity is associated with improved body composition, but the cause and effect of the association is not fully understood. Assumption often dictates that it must be the exercise that causes the improvements in body weight, but it is just as likely that weight gain (as a result of a poor diet) causes a gradual reduction in physical ability and motivation to perform exercise. In fact known mechanisms can explain the ability of a Western style diet to chip away at the energy regulatory and neurochemical motivational pathways and thus reduce the capacity to exercise. The association between physical activity and body weight is not straightforward, but a multi layered riddle set inside a paradox. Different types of exercise, the individual’s diet, the intensity of the exercise and other lifestyle factors all play a part to distort the picture and muddy the waters on this issue. However, one thing is clear. Aerobic exercise is not effective at causing successful long term fat loss in obese individuals when performed in isolation.
Another assumption often made about exercise is that it is effective at causing fat loss because it increases energy expenditure and thus ‘burns calories’. This is a very tenuous statement to make as the actual amount of energy required for even long periods of low intensity exercise is very small compared to the resting metabolic rate. In fact more calories are burnt during a typical night’s sleep than during hours of intense and gruelling aerobic exercise. However, is is not often considered that exercise might have other effects that induce weight loss. For example, exercise can lower blood sugar levels and thus has a beneficial glycaemic effect. As high levels of postprandial and fasting blood glucose are associated with weight gain, this may be a major health benefit of exercise. The health benefits of nutritional approaches to lowering blood glucose levels is only just being appreciated as a major determinant of obesity, and foods such as oats, legumes and fruit owe many of their health benefits to their beneficial glycaemic effects.
A recent study investigated the effects of physical activity on postprandial metabolic regulation1. Using a randomised cross-over study design, 70 subjects were assigned to a number of different interventions that included sitting for 9 hours, sitting for 15 minutes followed by walking for 30 minutes followed by sitting for 8 hours and 15 minutes, or bouts of walking for 1 minute 40 seconds over the course of 9 hours to a total time of 30 minutes. Food was provided as a liquid meal at 60, 240 and 420 minutes following the start of the study period. Walking intensity and duration was controlled by having the participants walk on a treadmill to a set speed and for a set time. The results showed that the different interventions had quite different effects on blood sugar. For example, the regular activity intervention lowered plasma insulin area under the curve by 866.7 IU per litre over 9 hours compared to prolonged sitting, and by 542 IU per litre over 9 hours compared to the continuous exercise group.
Therefore shorter and more frequent bouts of exercise appear to be more effective at lowering plasma insulin levels compared to the same amount of exercise performed in one bout. The reductions in plasma insulin levels were likely a result of lowered plasma glucose levels which were lowered by 18.9 mmol/L over 9 hours in the regular activity group when compared to prolonged sitting group, and by 17.4 mmol per litre compared to the continuous exercise group. The fact that regular shorter activity sessions produces beneficial glycaemic effects over continuous exercise suggests that those that lead less sedentary lifestyles through performing many smaller physical tasks such as gardening, housework and walking up stairs, may receive more beneficial health effects than those who lead sedentary office bound lifestyles but perform single activity sessions in gyms. The effects of exercise on body weight are highly complex and it is disingenuous to suggest that these effects occur through a basic ‘burning of calories’.
Dr Robert Barrington’s Nutritional Recommendation: Humans are designed to do things. Keeping active is an important part of human health and evidence now supports a role for such activity in maintaining proper blood glucose levels. While many consider exercise to involve hours of continuous and monotonous running on a hamster wheel, this need not be the case. Exercise should be fun and does not require a gym. However, no exercise will be effective if combined with a low quality Western style diet. Eating high quality foods is important in any exercise programme as a high quality diet is the cornerstone of health.
RdB
