Why do you gain weight more easily after 40, even if you eat the same amount?

Muscles are the engine of your metabolism. After the age of forty you gradually lose muscle mass, and muscles burn more energy at rest than fat tissue does. This means your body needs fewer calories per day, even if you exercise and eat the same as before. This muscle loss (sarcopenia) has been robustly demonstrated and is considered one of the most important causes of weight gain in later life.

On top of that muscle loss, your resting metabolism also falls for a second reason: organs shrink slightly and use less energy per kilogram of body weight than they used to. At the same time, on average you move a little less as you get older. This has both biological causes, such as changes in brain regions that prompt you to be active, and practical environmental factors. The result is that your total daily energy expenditure drops considerably.

Something is also happening inside your cells. A substance that cells need for their energy management, NAD+, declines throughout life. In mouse studies, restoring NAD+ led to less weight gain and improved fat metabolism. In humans the evidence is preliminary: small studies show cautiously positive signals, but a proven causal relationship has not yet been established.

Less well known is the role of fat cells in the bone marrow. These increase progressively with age and, via signalling molecules, influence other tissues, including muscles and fat depots. This is a relatively new area of research and the precise importance for weight gain in humans is still unclear. Finally, mouse studies suggest that epigenetic changes, whereby certain genes are switched on or off differently as you age, also contribute to the tendency to gain weight. Whether this applies equally strongly in humans still needs to be investigated.