Metabolism and exercise: what really changes your energy use, and what does not

What your metabolism is made of

The energy you use in 24 hours has three parts.

• Resting expenditure: what keeps you alive at rest, breathing, heart, brain, liver, temperature. It is the largest part for most people.
• The cost of digestion: the energy used to process food. It is commonly estimated at about 10% of daily expenditure, the value researchers such as Ostendorf and colleagues used in their 2019 study of people maintaining weight loss.
• Activity: from structured training to walking between meetings. This is the most variable part, between people and from one day to the next.

When Mifflin and St Jeor built their equation in 1990 from 498 adults, they found that fat-free mass, meaning everything except fat, was the best single predictor of resting expenditure. The calculator on this page uses their equation, based on weight, height, age and sex. For an illustrative man of 55, 90 kilograms and 182 centimetres, exercising lightly, it gives about 1,770 kilocalories at rest and about 2,430 for the day.

Does metabolism slow down with age?

Much less than people think, and later. Pontzer and colleagues pooled measurements by doubly labelled water, the reference method for daily energy expenditure, from people aged 8 days to 95 years. Adjusted for fat-free mass, expenditure was stable in adulthood, from 20 to 60, even during pregnancy, and declined only after 60.

That changes the diagnosis. If you weigh more at 45 than at 30, the main suspects are less daily movement and a gradual loss of muscle, both of which you can act on. The guide to strength after 40 explains why muscle becomes the priority.

What a kilogram of muscle is really worth

Fitness marketing often promises that muscle will "burn fat while you sleep". The figures are more modest. A study by Wang and colleagues, using organ and tissue masses measured by MRI, confirmed the classic estimates of specific metabolic rates: about 13 kilocalories per kilogram per day for skeletal muscle and 4.5 for fat tissue. The organs are far more expensive: about 200 for the liver, 240 for the brain and 440 for the heart and kidneys.

So gaining 2 kilograms of muscle adds roughly 26 kilocalories a day at rest. That is not nothing, but it will not offset a pastry. The real value of muscle lies elsewhere: strength, glucose control, ageing well, and keeping your resting expenditure from falling when you lose weight. Strength training for executives gives a two-session structure.

Does exercise raise metabolism?

Yes, by the energy it costs, which you can estimate with the method in calories burned during exercise. But the total is not a simple sum. In 332 adults across five populations, Pontzer and colleagues found that daily energy expenditure rose with physical activity at lower levels, then levelled off at higher levels. The body seems to make room for training by spending a little less elsewhere.

The practical reading is reassuring for most people: moving from inactive to moderately active clearly increases what you use. Pushing towards very high training volumes yields smaller returns for weight, while still bringing other benefits.

Adaptation after weight loss

When you lose weight, your needs fall because there is less of you. But Rosenbaum and Leibel, reviewing the evidence in 2010, describe an additional drop: metabolic, hormonal and behavioural responses that make the body use less energy than its new size would predict, in lean and obese people alike. They link it to the difficulty of keeping lost weight off.

The most striking example is extreme. Fothergill and colleagues followed 14 contestants of a televised weight-loss competition who had lost about 58 kilograms in 30 weeks. Six years later, their resting expenditure was still about 700 kilocalories a day below their starting level, and about 500 below what their body composition predicted. Such adaptation is not typical of gradual, moderate weight loss, but it shows the direction of the effect.

What follows for you is mostly practical: recalculate your needs as your weight changes, and expect the pace of loss to slow. See weight loss plateau.

The levers you actually control

1. Keep your muscle. The ACSM position stand on weight loss found that resistance training may increase fat-free mass and the loss of fat mass.
2. Eat enough protein. A meta-analysis of 24 trials by Wycherley and colleagues found that, at the same calorie level, higher-protein diets limited the drop in resting expenditure during weight loss, by about 140 kilocalories a day on average. See protein for weight loss.
3. Move every day. Walking, stairs and standing breaks are the most flexible part of your expenditure.
4. Choose a moderate deficit. It is easier to hold and gentler on training quality.
5. Sleep. Short nights tilt weight loss towards muscle rather than fat, as Nedeltcheva and colleagues showed.

Unexplained weight gain or loss, persistent fatigue, feeling cold, or changes in heart rate can have a medical cause, such as a thyroid disorder. If they appear, see your doctor rather than adjusting your diet on your own.

Build the habit that protects your metabolism, with Never Skip Club

Muscle and daily movement are built by repetition, not by a single heroic month. Create your free account on Never Skip Club, launch guided strength or cardio sessions built for your level and equipment, and join a crew of friends who hold the same weekly rhythm: the ranking rewards regularity, which is exactly what your metabolism responds to. For a programme designed around your age and goals, find a coach from our network, or sign in to continue.