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What's a Calorie? Learn From an Expert

What is a calorie, and how many do we need every day? At its most simple level, calories are a measure of how much energy food contains - so the aim is to consume about as many calories as our bodies use each day.

If we put in more calories than we take out, we'll gain weight - if we put in fewer calories than we take out, we'll lose weight. The amount of energy we use depends on what we're doing - someone doing a lot of physical activity will use more calories than someone doing very little.

A calorie (also known as a large calorie, or kilocalorie) is defined as the amount of energy needed to raise the temperature of 1kg of water by 1 degree Celsius.

This is what calorie counts on food labels tell us - but this can be very different from the energy that our bodies can extract from food. Different foods require more or less energy to digest, and people can have very different levels of enzymes, gut bacteria and even intestine length - meaning some people will get more calories than others out of the same food.

In addition, calorie counts only tell us about energy - not anything about other nutrients, such as protein and vitamins. So working out how many calories someone needs depends on a lot of different factors.