In order to lose weight, we’re told to go on a diet and do some exercise (1). Eat less, and move more. Sounds logical and sensible, doesn’t it? But we’re not told what role exercise actually plays in weight loss. Does it actually help? And if so, how does it help? Over the course of this two-part article, we’ll see that:

• Weight loss is made up of both fat loss and muscle loss. So, ‘weight loss’ is the wrong health goal. We should be targeting fat loss and muscle preservation;
• On average, aerobic exercise does result in some fat loss, but the amount of exercise needs to be high for the results to be meaningful;
• Some people lose a substantial amount of fat from exercise, but others lose less fat or actually gain fat;
• Exercising, especially resistance training, helps to preserve muscle mass during weight loss from dieting

Being overweight or obese (2) is associated with an increased risk of most cancers, most cardiovascular diseases, type II diabetes, and musculoskeletal problems (3). In 2015, high BMI accounted for around four million deaths worldwide, and nearly 40% of these were in overweight (not obese) people (4). But when we say ‘overweight’, what we really mean is ‘overfat’. Total body weight includes the weight of both fat mass and fat-free mass (muscle, bone, organs, connective tissue, and water). A high proportion of fat-free mass is positive for health. Excess fat is the problem.

The main health and fitness goal for many people is to ‘lose weight’. The problem is, when we lose weight, by dieting for example, a substantial amount of the weight lost is fat-free mass (mainly due to muscle loss) (5). So, rather than lose weight, we should be trying to lose fat while preserving muscle. What is the role of exercise in all this? In Part 1, we’ll explore whether exercising (on its own) results in fat loss. And, in Part 2, we’ll see whether exercise can preserve muscle during weight loss. But before we get into that, we need to understand what drives fat gain and fat loss.

When we eat more calories (energy) than we burn, we store the excess calories in the body. So our body weight increases. When we eat fewer calories than we burn, we use stored calories to cover the shortfall. So our body weight decreases. We can express this as follows (assuming water weight is constant):

• Change in body weight = calorie intake – calorie expenditure (6)

We know that any change in body weight is mostly attributable to a change in fat mass (5). Therefore, body fat can be reduced by decreasing calorie intake, increasing calorie expenditure, or both. Exercising increases calorie expenditure, which should lead to fat loss. But, does it?

A common view seems to be that exercise is not very effective for fat loss. And there is a substantial amount of research to support this. Compared with diet (reducing calorie intake), exercise results in only modest fat loss (7). But the problem with most of these studies is that people were just prescribed the exercise. Adherence wasn’t enforced, and, in many cases, not even measured.

What we should look at is the studies where the exercise sessions were supervised. Then we know that people actually did the exercise that was prescribed. We also want studies with sufficient numbers of people, enough time to see any effects, and where calorie intake wasn’t restricted (otherwise any fat loss could be due to calorie restriction).

When we do that we see a clearer picture. Aerobic exercise (such as walking, running, and cycling) can result in up to 6.5 kg of fat loss in 3 months (8, 9), but that requires burning over 4,000 Calorie per week. Equivalent to working out for an hour a day, 7 days a week, at a relatively high intensity. At more achievable levels of exercise, say 3-4 hour-long sessions a week, people only lose around 2 kg over 6 months (10, 11, 12, 13, 14, 15). Hardly worth the effort. And doing only 1-2 hour-long sessions a week appears to yield no fat loss at all (16, 17). But total fat loss is not the only metric we should consider. Even the studies that resulted in no total fat loss, showed a significant decrease in waist circumference (16, 17), which is beneficial to health independent of fat loss (18). Resistance training (e.g. lifting weights), doesn’t appear to impact total body fat very much at all, but it does reduce waist circumference (19, 20).

So, we now know that aerobic exercise can result in fat loss, but the amount of exercise needs to be high for the results to be meaningful. We need to remember, however, that these results are based on population averages. For any amount of exercise, some people lose more fat than others. And this ‘variability’ can be large.

Researchers at the University of Leeds (21) showed that 3 months of supervised aerobic exercise 5 days a week resulted in fat loss of 3.7 kg. On average. Some people lost nearly 10 kg of fat, while some people actually gained fat. To figure out what was going on, the researchers split the group into people who lost less fat than predicted (compensators) and people who lost as much fat as predicted or more fat than predicted (non-compensators). The average fat loss for compensators was only 2.1 kg, while for non-compensators was 5.3 kg. How did this happen? Fortunately, the researchers also measured calorie intake and perceptions of hunger. It turned out that compensators were hungrier than non-compensators. As a result, compensators increased calorie intake, whereas non-compensators decreased it. So, it looks like exercise made some people eat less and lose weight, and made others eat more and lose less weight, or even gain weight.

More recent studies have shown a similar variability in response to exercise, and the effect gets bigger as the amount of exercise increases (22, 23). Again, the most likely underlying driver of this is appetite. Aerobic exercise appears to suppress hunger on average, but this effect is highly variable across different people (24, 25).

So, what can we take away from all this? Well, if you want to lose fat, aerobic exercise may help. The more calories you burn, the more likely you are to lose fat. Some of you will lose fat from exercise. But some of you will compensate (completely) for the lost calories, by eating more, and not lose any fat, or possibly even gain some fat. Resistance exercise may also help with fat loss for some, but the effect is likely to be smaller. Even if you don’t lose fat overall, you may still shrink your waistline.

Experiment a bit. And see what happens.