Food provides units of energy in the form of calories that give our bodies fuel to perform all functions from the most basic like breathing to more complicated activities. We need a minimum amount of calories from food to sustain basic metabolic functions and more to carry out physical activities. The more active we are, the more food we need.
Energy comes from the macronutrients -- carbohydrates, fat and protein -- found in all foods. Carbohydrates and protein provide 4 calories of energy per gram while fat yields 9 calories per gram. Carbohydrates are broken down into glucose and absorbed into the bloodstream. Fat is a major source of stored energy and is held as a reserve in the body until we need it. Fat is the preferred source of energy for the heart. Protein is broken down slowly and is a longer-lasting form of energy. Protein is primarily used as energy when total calorie intake is too low and energy cannot be obtained from carbohydrates.
During physical activity, the body uses calories from food in a three-part energy system. Shorter bursts of high-intensity activity, such as a sprint, tap into the immediate or anaerobic systems and draw energy primarily from carbohydrates. During aerobic exercise, which is more moderate in intensity and lasts two minutes or longer, the body burns mostly fat with moderate use of carbohydrate and protein energy.
Eating enough food to maintain your daily metabolic functions is critical for health. Eating too few calories can cause slowed metabolism, nutrient deficiencies and binges that trigger weight gain. People's daily food requirements vary due to weight, height, sex, age and activity level. The USDA recommends obtaining 45 to 65 percent of daily calories from carbohydrates, 20 to 35 percent from fat and about 10 to 15 percent from proteins.
Food for Energy
All foods supply calories for energy, but not all calories are equal. Certain foods provide more vitamins and minerals for the amount of energy they supply, so choose nutrient-dense foods to fuel your daily energy needs. Simple carbohydrates like table sugar are broken down quickly and are a fast source of energy. For a more steady release of energy, consume complex carbohydrates found in whole grains such as wheat pasta and brown rice. Choose lean sources of protein like chicken breast or beans and heart-healthy fats such as nuts and nut and seed oils.
- "Nutrition: Real People, Real Choices"; Susan Hewlings and Denis Medeiros; 2008
- The Merck Manuals: Carbohydrates, Proteins and Fats
- Health.gov: USDA Dietary Guidelines for Americans