A calorie is a calorie, right? Fixed and unchanging, like a gram, or a mile? Well … no, not necessarily. You see, what a straightforward calorie count on a restaurant menu or food packet can’t tell you is how your individual body will use those calories. This comes down to multiple factors including genetics, sex, age, hormones, gut microbes, sleep patterns, the time of day we are eating, how active or sedentary we are, our body fat and muscle mass, and – crucially – what sort of food the calorie is in. Our bodies are much better at absorbing the energy from a calorie of low-fibre, processed food (like a potato chip) than they are at taking in calories from whole foods, like an apple.
Calories are a measure of the heat (energy) given off when a food is completely burned away in a pressurised bomb calorimeter. “But we don’t eat calories. We eat food,” says Dr Giles Yeo, an obesity researcher at Cambridge University and author of Why Calories Don’t Count. That energy is used differently by different bodies.
