Is your metabolism really slow? What dieting does, and how to fight back

What is metabolism?

Metabolism is the energy your body uses each day. Most of it, about two-thirds, goes on simply keeping you alive at rest: breathing, heartbeat, brain and organs. This is your resting metabolic rate. The rest goes on movement and on digesting food.

Resting metabolismMovement and exerciseDigesting food

65%

25%

10%

Sources: Westerterp 2004; Pontzer et al. 2021.

Is mine slow or fast?

People do differ. In one study, about 63% of the differences in resting metabolism between people came down to how much lean tissue, mostly muscle and organs, they had. Fat, age and other factors explained a little more, and about a quarter was unexplained, so some people genuinely burn a bit more or less than others of the same size.

A large 2021 study of over 6,000 people found that, once body size is accounted for, metabolism stays roughly stable from age 20 to 60, and only starts to fall after that. So middle-age weight gain is usually more about lifestyle than a slowing metabolism.

Conditions such as an underactive thyroid can slow metabolism. If you're tired, cold and gaining weight without a clear reason, ask your doctor for a simple blood test.

What dieting does to your metabolism

When you lose weight, you burn less, partly because there's less of you. But studies show the drop is often bigger than weight loss alone explains. Researchers call this metabolic adaptation. Your body is trying to protect its energy stores.

The most extreme example comes from contestants on a TV weight-loss show who lost about 58 kg each through very intense diet and exercise. Six years later, their resting metabolism was still around 700 calories a day lower than before. That's an extreme case, but it shows why crash diets often backfire.

Does fasting protect your metabolism?

In the first few days of a fast, energy use rises slightly, driven by a stress hormone, noradrenaline. But in trials lasting weeks to a year, fasting didn't lead to more weight loss or better protection of metabolism than eating less every day. In one trial, fasting led to less fat loss than daily calorie cutting, with more of the weight lost coming from lean tissue.

So fasting is a fine tool if it helps you eat less, but it isn't a way to boost your metabolism.

What actually helps

• Strength exercise: in one trial, women who did resistance training while losing weight kept their muscle and their resting metabolism didn't drop, unlike those who did only aerobic exercise or none
• Enough protein: higher-protein diets during weight loss protect lean tissue and limit the fall in metabolism. Protein also takes more energy to digest than carbs or fat
• Lose weight steadily, not with crash diets
• Move more during the day: walking, stairs and standing all add up
• Sleep well, as poor sleep increases hunger

Our physiotherapist can build a safe strength programme for you, and our nutritionist can help you get enough protein while losing weight.