NHS website - Can we count on counting calories?
It's a concept at the cornerstone of most diets: counting the calories of your food intake so you don't go over the limit.
But just how accurate are calorie labels? And are some calories more "equal" than others?
There is a seemingly endless stream of media articles focusing on the latest diet wonder, whether it involves intermittent fasting or feasting on fats.
Although they protest otherwise, most miracle weight loss programmes come down to calorie restriction.
Behind the Headlines takes a look at the science behind calorie counting, examining why it may be only one aspect of healthy, sustainable weight loss.
This article is not a systematic review, where a team carries out a synthesis of the relevant medical research on a given subject
What's in a calorie?
A calorie is a unit of measurement of how much energy is stored in a mass of food.
Confusingly, the "calories" we talk about in daily life are officially described as kilocalories, or kcals, and this is how they appear on food labels. One calorie equals one kilocalorie.
A single calorie is defined as having approximately the amount of energy needed to raise the temperature of one kilogram of water by 1C.
An average man needs around 2,500kcal a day. For an average woman, that figure is around 2,000kcal a day.
These values can vary widely depending on levels of physical activity. For example, some Olympic swimmers have reported eating as many as 12,000 calories a day when they are competing.