Does my metabolism really slow down as I get older?

Yes, your resting metabolism really does decline. Large cohort studies in American, Italian and Japanese populations confirm this: resting energy expenditure decreases gradually, and that decline continues into old age. Part of the explanation lies in the reduced functioning of the energy centres inside your cells.

The main cause is muscle loss. In people who are not very active, muscle mass falls by roughly 15% between the ages of thirty and eighty. Because muscle is the biggest driver of your resting metabolic rate, that loss translates directly into lower energy expenditure. At the cellular level, a protein that regulates calcium transport in muscle cells plays a role here. That protein becomes less active with age, which disrupts energy production in muscle cells.

Your endurance also declines in a measurable way. Your maximum oxygen uptake decreases progressively. Up to late middle age this is driven mainly by changes in heart function. At very old age, the reduced capacity of the muscle cells themselves also plays a large role: in older adults that contribution can account for around 50% of the total difference compared with young adults.

For women there is an additional layer: menopause brings changes in body composition that are separate from ordinary ageing. The rate at which fat mass increases doubles during that period, while muscle mass decreases more quickly. Notably, total body weight does not rise faster during that period. It is therefore a redistribution, from muscle to fat, not simply a matter of weight gain.

Finally, there is animal research into oleuropein, a compound found in olives, which in mice activated energy production in muscle cells and improved endurance. That sounds promising, but all data come exclusively from cell and mouse studies. There are no studies in humans yet, so no practical conclusions can be drawn at this stage.