Working out won't cut it for the joules

UNDER THE MICROSCOPE:IT IS WIDELY believed that regular aerobic exercise is a good way to lose weight. However, research has shown that exercise is not an effective way to lose weight, although regular exercise is undoubtedly good for your general health. The subject of exercise and weight is topical at the moment and features as the cover story of Timemagazine, August 17th, written by John Cloud.

The overall mechanism of how we gain or lose weight is easy to understand. Think of the body as a machine. Like any machine it must be supplied with fuel (energy) to operate. We take in energy as food and we expend that energy in maintaining our body and its processes and in carrying out work on the environment. If we take in more energy that we expend, the excess energy is converted to fat and we gain weight. If we expend more energy than we take in, we must burn up our internal body resources to make up the shortfall and we lose weight.

The energy content of food is expressed in terms of joules or, the old unit of energy, calories. The calorie is the more commonly used term colloquially.

So, if you want to lose weight you can do it in either or both of two ways – you can restrict your food intake and/or you can increase the amount of exercise you take. Reducing your food intake is far more efficient than exercise simply because you must do a lot of exercise to burn off even a small number of calories. It is a much better strategy to avoid taking in the calories in the first place.

The relationship between exercise and weight loss struck me forcibly several years ago when running on a treadmill in a gym. I ran for 30 minutes and stopped out of boredom and the nuisance of excessive perspiration. The treadmill had a calorie counter and I noted the number of calories my 30 minutes of sweaty exercise had burned off. I had eaten my dinner several hours prior to the gym and had finished my meal with two half-coated chocolate biscuits. When I returned home I consulted the biscuit packet to find out the number of calories per biscuit. My 30 sweaty minutes on the treadmill had exactly burned off the two biscuits. If I wanted to burn off the rest of my dinner I would have to run all night!

Humans as a species are not designed to lose weight easily with exercise. This is only to be expected. Food was hard to get over our long evolutionary history. Our ancestors, on average, had to undertake much heavy exercise to catch their dinner and had no guarantee that the next day’s hunt would catch anything at all. So, evolution equipped us to hold on tenaciously to ingested calories that were so hard-won.

John Cloud reviews much of the research on the effectiveness of exercise in reducing weight. One large study, published by JS Church and colleagues in the peer- reviewed journal PLoS One, took 464 overweight women who didn't take regular exercise and randomly divided them into four groups.

Three of the groups were required to undertake supervised exercise lasting 72 minutes, 136 minutes and 194 minutes per week respectively for six months. The women in the fourth group just maintained their usual activities. The women in all groups were asked not to change their eating habits.

THE SURPRISING RESULTwas that the women in each of the four groups lost weight over the six months and no significant differences were noted between the groups. I wouldn't expect dramatic differences between the groups, but I would expect the heaviest exercise group to lose a little weight compared with the control group. The likely reason for the result is that heavy exercise tends to make most people hungry and they reward themselves later with a little treat – one muffin will negate the calorie-burning effect of an hour's heavy exercise. Another possible explanation is that many people, after a hard bout of exercise, feel a bit stiff and "achey" and don't move about much for the rest of the day.

Also, the women in the control group, because they were participating in a weight-control study, may have been more abstemious in their eating habits than normal, even though they promised to maintain their usual eating habits.

Let me finish by emphasizing that I am certainly not advising against exercise. It is well established that regular aerobic exercise (walking, running, cycling, swimming and so on) is very good for general health, protecting against heart disease, bowel disorders, diabetes, depression and a host of other problems. I recommend exercise highly. However, it is not an effective way to reduce weight.

William Reville is associate professor of biochemistry and public awareness of science officer at University College Cork - http://understandingscience.ucc.ie