Our Mysterious Metabolism

Eight truths about how we burn calories.

- Julia Belluz

1 YOUR METABOLISM IS IN EVERY CELL IN YOUR BODY.

A lot of people talk about their metabolism like it’s a muscle or organ they can somehow control. But in reality, the term refers to a series of chemical processes in each cell that turn the calories you eat into fuel to keep you alive.

“It’s the culmination of different tissues with different needs and how many calories it takes to keep them functioning,” says Michael D. Jensen, a physician-scientist who studies obesity and metabolism at the Mayo Clinic in Rochester, Minn.

The body’s major organs—the brain, liver, kidneys and heart—account for over half of the energy burned at rest, while fat, the digestive system and especially the body’s muscles account for the remainder.

2 MOST OF THE ENERGY YOU BURN IS FROM YOUR RESTING METABOLISM.

There are three main ways you burn energy: a) the basal metabolism, which is the energy used for your body’s basic functioning while at rest; b) the energy used to break down food, also known as the thermic effect of food; and c) the energy used in physical activity.

One very underappreciated fact about the body is that your resting metabolism accounts for a huge amount of the total calories you burn each day. Physical activity, on the other hand, accounts for a tiny part of your total energy expenditure—about 10 to 30 per cent (unless you’re a professional athlete or have a highly physically demanding job). Digesting food accounts for about 10 per cent.

“It’s generally accepted that for most people, the basal metabolic rate accounts for 60 to 80 per cent of total energy expenditure,” says Alexxai Kravitz, a neuroscientist and obesity researcher at Washington University in St. Louis.