The human body gets energy from the three macronutrients: carbohydrates, proteins, and fats. Energy is released when the chemical bonds in the organic compounds present in the food are broken down. The energy content of food is measured in kilocalories (kcal), defined as the amount of heat required to raise the temperature of one kilogram of water by one degree Celsius. This value is determined by measuring the temperature change of the water surrounding a calorimeter after the complete combustion of a measured food sample.

Each macronutrient provides a different amount of energy. Carbohydrates and proteins provide approximately four kilocalories per gram, while fats provide nine kilocalories per gram. Most foods contain a mixture of these macronutrients, with foods high in fat being more calorie-dense. Excessive consumption of high-fat foods can lead to weight gain.

The body maintains an energy balance when there is an equilibrium between food intake and energy expenditure, which helps maintain a stable body weight. If the energy intake exceeds energy expenditure, the excess energy is stored as fat, resulting in weight gain. Conversely, if the energy expenditure surpasses energy intake, the body uses the stored energy, causing weight loss.