What is energy?

Many of us are consuming more energy (calories) than we need from food and drinks. This article will explain the concept of energy and how it impacts on bodyweight.

What is energy density?

We all need energy to grow, stay alive, keep warm and be active. Energy is provided by the carbohydrate, protein and fat in the food and drinks we consume. It is also provided by alcohol. Different food and drinks provide different amounts of energy. You can find this information on food labels when they are present.

Energy is measured in units of kilocalories (kcal) or kilojoules (kJ).

One kilocalorie (1 kcal) is equal to 4.18 kilojoules (4.18 kJ).

• Fat contains 9 kcal (37 kJ) per gram

• Alcohol contains 7 kcal (29 kJ) per gram

• Protein contains 4 kcal (17 kJ) per gram

• Carbohydrate contains 3.75 kcal (16 kJ) per gram (for the purposes of food labelling this is rounded up to 4 kcal per gram)

The amount of energy a food contains per gram is known as its energy density so we can describe fat as more energy dense than protein or carbohydrate.

You can find out more about why we need carbohydrate, protein and fat in our diet and in which proportions on our pages on healthy, sustainable diets.

How is energy content calculated?

The total energy content of a food can be found by burning it and measuring how much heat is released.

Foods with fewer calories per gram such as fruits, vegetables, low-fat soups, lean protein and fibre-rich foods have a relatively low energy density.

Foods with high fat and/or low water contents such as chocolate, cakes, biscuits, deep-fried foods and snacks, butter and oils, have a relatively high energy density.

Basing your diet on foods which are lower in calories (or have a lower energy density) and eating foods which are high in calories (or have a higher energy density) less often and in small amounts, can help to control your overall calorie intake. Some foods with a higher energy density such as oily fish, cheese, nuts, seeds and avocados contain healthier types of fat and other important nutrients meaning they can be consumed in moderate amounts as part of a healthy, balanced diet.

Energy requirements

Carbohydrate is the most important source of energy for the body because it is the main fuel for both your muscles and brain. Sources of carbohydrates include starchy foods, like bread, rice, potatoes, pasta, pulses and breakfast cereals. Choose higher fibre and wholegrain versions of these where possible. Higher intakes of fibre have been linked with a lower risk of getting diseases such as colorectal cancer, type 2 diabetes and heart disease. We also get energy from protein and fat and these form an important part of our diet too but we don't need to eat as much of these as carbohydrates.

The Eatwell Guide shows the ideal make-up of a healthy, balanced diet. You can find out more about the Eatwell Guide on our page on a healthy, balanced diet.

Different people need different amounts of energy. The amount needed to maintain a healthy weight depends on your basal metabolic rate (BMR), which is the minimum amount of energy your body uses to maintain basic bodily functions like breathing and your heartbeat. BMR varies from person to person depending on your age, body size, gender and genes. But we also use energy to digest food and for physical activity.

Some activities use more energy than others. For example, running will use more energy than a gentle walk and rowing will use more energy than typing! The more active you are, the more energy your body uses up.

Energy balance

Your weight depends on the balance between how much energy you consume from food and drinks, and the total amount of energy that is used by your body. When you eat or drink more energy than you use, you put on weight; if you consume less energy from your diet than you use, you lose weight; but if you eat and drink the same amount of energy as you use up, you are in energy balance and your weight remains the same. It is important for your health to maintain a healthy weight.

For more information on the sources used in this text, please contact us.

Last reviewed November 2016. Revised October 2022.