Think back to your high school physics textbooks: energy can’t be destroyed, it can only be moved around. It’s the same in your body. You put energy in, and what you don’t expel, you store as fat.
Your metabolic rate is the level of efficiency and speed at which you use up that energy. Measured in joules and kilojoules, most of our day’s energy intake (50% to 80%) is used simply to stay alive: to breathe, pump blood and keep other bodily functions working. We then need a little extra on top to fuel our daily physical activity. Add these together and you know roughly how many kilojoules you can eat without putting on weight.
Most people who claim to have a slow metabolism – a low metabolic rate – in fact don’t: they just have an imbalance between what goes into their mouths and how much exercise they do.
Why can some people eat what they like and never gain weight?
The most metabolically active component of your body is lean muscle. After you eat your breakfast, 80% of the glucose content goes straight there. So it makes sense that people with a higher percentage of lean muscle mass will require more energy just to sit still than people with a higher fat percentage.
In the lab, a person’s basal metabolic rate (BMR) is measured with sophisticated equipment. After a night’s fast, subjects are asked to lie still while the oxygen they exhale is recorded. This enables researchers to figure out how much energy they’re using while in repose.
“We see vast differences in people’s BMRs,” says Professor John Hawley, head of the Exercise Metabolism Research Group at RMIT. “Multiply these over a day, a month or a year and you can end up with a 10kg difference between people.”