I’ve gained weight since I started exercising, so it must be muscle mass

For a recent study, 81 women walked on a treadmill for half an hour three times a week for twelve weeks. Measurements of their body fat showed that 55 of the women gained weight during that period — and the gain was in fat, not muscle mass (Sawyer et al., 2014). This is a common pattern because we tend to seriously overestimate the number of calories we burn by exercising, and sporting activity stimulates your appetite.

The results are corroborated by studies carried out by Finlayson et al. (2011) and King & Blundell (1995), in which participants in exercise programs reported having a highly increased appetite for fatty foods right after exercising. It seems people fall into different types when it comes to exercise: some are less hungry after an exercise session, while about half the population find their appetite enormously stimulated after exercising, leading them to replace the energy they have burned (or to consume even more).

Physical trainers say it’s possible for an average man to build up as much as 23 kg of muscle mass over his lifetime using natural methods(!), with many years of continuous weight training. For women, it’s possible to build up around 11 kg of muscle mass. As it happens, that also means it’s basically impossible for a woman with a low proportion of body fat to be in the overweight BMI range because of naturally gained muscle mass.

The speed at which muscles grow is also surprisingly slow: according to the fitness expert Lyle McDonald, men can build up around 1 kg of muscles per month in the first year, under optimum conditions. In the second year, they can build around 500 g per month of muscle, in the third year, around 250 g per month, and in the fourth only 2 to 3 kg for the whole year. For women, the figures are about half those of men.

The nutritionist and sports educator Alan Aragon gives a guiding estimate for beginner male weight trainers, telling them to hope to gain around 1 to 1.5 per cent of body weight in muscle per month. Once again, the figures are about half that for women.

This means that if a man puts on more than 1 kg during a month of intensive weight training, and a woman puts on more than half a kilo, the gain is probably due to fat, and it’s time for them to take a closer look at their calorie intake.

But there’s no need to panic right away. Muscles can sustain many tiny injuries at the beginning of an exercise program, and that can cause the tissue to swell up and therefore weigh more. So if you suddenly find that you weigh 1 to 2 kg more the day after starting to exercise, there’s no reason to worry or to give up sport immediately: that additional weight will disappear again, and it’s nothing more than a sign that your muscles have been put under strain.

What’s more, the pattern of putting on fat after exercise described here will only kick in if you eat intuitively. If you are alert to the appetite-stimulating effect of exercising, you can avoid falling into this typical trap. Just because you have an appetite, it doesn’t mean you have to eat something. And if you know you’re likely to get hungry after exercising, you can plan your day so that you can eat a meal after your sports session.