| 1 g of carbohydrates | 4 kcal | 17 kJ |
| 1 g of protein | 4 kcal | 17 kJ |
| 1 g fat | 9 kcal | 38 kJ |
| 1 g of alcohol | 7 kcal | 29 kJ |
The body needs only a few nutrients from the diet. The body can produce carbohydrates and fats on its own if necessary. If energy intake exceeds the body’s needs, substances are stored in the liver and muscles. Once these are full, storage continues in fat cells, which swell when fat is stored and shrink when fat is used. However, obesity depends not only on excessive energy intake but also on the number of fat cells in the body, heredity, and how much fat we store as children.
The three main sources of the energy humans need are carbohydrates, protein, and fat. Vitamins and minerals do not provide energy but are essential for the body to function.
The amount of energy a food provides is measured in kcal or kJ. The same unit of measurement is used to estimate how much energy is expended during various activities.
1 kcal = 4.2 kJ.
Energy = the heat released during the oxidation of nutrients in the body.