In our previous article we discovered that:

• On average, exercise does result in some fat loss, but the amount of exercise needs to be high for the results to be meaningful;
• Aerobic exercise seems to be more effective for fat loss than resistance exercise; and
• Some people lose a substantial amount of fat from exercise, but others lose less fat or actually gain fat due to increased appetite.

In this second part we’ll learn that exercise, especially resistance training (lifting weights), helps to preserve muscle mass during weight loss.

Weight loss requires a sustained calorie deficit. That is, the amount of calories consumed must be lower than the amount of calories used by the body. Dieting (reducing calorie intake) is a more effective way of losing weight than exercising (increasing calorie expenditure) (1). Any diet is effective so long as it induces a sustained calorie deficit. Now, that doesn’t mean that all diets are created equal. They’re not. But that’s a topic for a future post.

As we touched on briefly in Part 1, the weight we lose by dieting is made up of fat mass and fat-free mass. Fat-free mass is made up of skeletal muscle, bone, connective tissue, internal organs, and water (2), but weight loss affects mainly skeletal muscle. When under a calorie deficit, the body breaks down fat tissue to make up the energy shortfall. Fat can be used as fuel for most of the body’s energy needs. The brain, unfortunately, still requires glucose, so the body has to break down muscle protein and convert it to glucose for the brain (3). This means that both fat mass and fat-free mass (mainly muscle) are lost during weight loss.

What proportion of weight loss is fat-free mass? A reasonable rule of thumb is 25% (4). So, for every 4 kg of weight loss, around 3 kg is fat mass, and around 1 kg is fat-free mass. But these numbers vary depending on the initial proportion of fat in the body (body fat %), and the severity of the calorie deficit (5). The higher the initial body fat %, the lower the proportion of weight lost as fat-free mass (this is why women tend to lose less fat-free mass than men). The larger the calorie deficit, the more rapid the weight loss, and the higher the proportion of weight lost as fat-free mass.

Notwithstanding the specific numbers, we want to limit the amount of weight lost as fat-free mass. For a few reasons. Firstly, losing fat-free mass drives down our resting metabolic rate, which reduces calorie expenditure, and leads to the infamous weight loss plateau (2). And secondly, declining fat-free mass compromises bone health, physical function, and athletic performance (2, 6).

Thankfully, we can mitigate this loss of fat-free mass. To some extent, at least. One way to do it is to eat more protein, which decreases muscle tissue breakdown, and thereby reduces fat-free mass loss (7, 8). Another way to do it is to exercise. Many of us will have experienced the muscle building effects of exercise. Any form of exercise seems to work, doesn’t it? And the research supports this (9, 10, 11). Via mechanical tension, muscle damage, and metabolic stress, exercise initiates several muscle building mechanisms and signalling cascades (12). And, as any self-respecting bodybuilder will tell you, resistance exercise (i.e. lifting weights) is the most effective for building muscle. And this is because it preferentially stimulates muscle growth, whereas aerobic exercise preferentially stimulates muscle efficiency (13).

The same mechanisms work in a state of calorie deficit. The exercise stimulus signals to the body that muscle is important to retain, even as weight drops. So, we would expect exercise to mitigate losses in fat-free mass during weight loss. We use it, we need it, we keep it. We would also expect resistance exercise to do a better job of retaining fat-free mass than aerobic exercise. But, let’s have a look at the evidence.

What we want is randomised controlled trials (14) that compared the effect of diet only, diet plus aerobic exercise, and diet plus resistance exercise on weight loss and fat-free mass loss. To our knowledge there are nine such trials (15, 16, 17, 18, 19, 20, 21, 22). Two of the trials (15, 16) reported no loss of fat-free mass in the diet only groups, despite overall weight loss. So, we should exclude these as there was no fat-free mass loss for the exercise to protect against. But why was there no fat-free mass loss? We can’t say for sure, but it’s interesting to note that these two trials had the smallest calorie deficit (weight loss was only 0.2 kg to 0.3 kg per week, compared to 0.4 kg to 1.2 kg in the other trials), and included only obese females (relatively high body fat %), both of which would reduce the amount of fat-free mass loss.

Of the remaining seven trials, three (17, 21, 22) showed that resistance exercise preserved, or even increased, fat-free mass during weight loss, but aerobic exercise had little effect. The other four trials (18, 19, 20) showed that aerobic and resistance exercise were equally effective in preserving fat-free mass. The picture is clearer when we pool all the results together. In the diet only groups the average proportion of weight lost as fat-free mass was 27% (close to the 25% rule we mentioned earlier). Adding aerobic exercise to the diets reduced this loss to 9%. A substantial benefit. But resistance training was even more effective, reducing the fat-free mass loss to only 2% of total weight loss. As we might expect, the proportion of weight lost as fat-free mass was much smaller in women than men (10% compared to 43%), and tended to increase with more rapid weight loss (more severe calorie deficit).

So, it looks like both aerobic exercise and resistance exercise will help mitigate muscle loss as you lose weight. But resistance exercise works better, and may even increase muscle mass.

What useful guidance can we glean from all this? Well, if you want to lose weight, you should try to maximise fat loss and minimise muscle loss by doing the following:

• Create a sustained calorie deficit using a high-protein diet (we’ll discuss this in more detail in future posts);
• Target slow and steady weight loss of 0.5 kg to 1.0 kg per week;
• Include 2-3 hours of aerobic exercise and 3-4 hours of resistance exercise per week, for a total of 5-6 hours per week.

Good luck.