Metabolism

Metabolism covers all the processes your body uses to take in nutrients, convert them, extract energy from them and excrete waste products. Two directions interlock: building-up metabolism (anabolism) uses protein, fat and carbohydrates to build muscles, cells and stores; breaking-down metabolism (catabolism) breaks down nutrients and reserves to release energy. The whole thing is regulated by hormones such as insulin, thyroid hormones, cortisol and leptin.

“I have a slow metabolism” is something people often say – and it's usually a misunderstanding. Resting energy expenditure depends mainly on height, weight, age, sex and muscle mass. Once these factors are taken into account, healthy people differ by only about 5 to 10%, or 100 to 200 kcal a day. A noticeably slowed metabolism usually has an identifiable cause, such as an underactive thyroid – which should be checked by a doctor, not fought with diets.

Can you “boost” your metabolism? A little. Muscle mass slightly increases resting energy expenditure and makes every movement more energy-intensive. Digesting protein takes more energy than digesting fat or sugar. Everyday activity and sleep keep hormone regulation in balance. Spicy food, green tea or coffee do have a measurable effect, but it's so small that it makes no difference to weight loss. What really slows your metabolism, on the other hand, is very strict dieting, because the body then throttles back its energy use.

In children, metabolism runs at full speed because growing takes energy; in old age, energy expenditure falls mainly because muscle mass is lost and people move less – and both of these can be influenced well.