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Macronutrients—the major calorie sources—are carbohydrates, proteins, and fats. Each macronutrient serves a particular function.

Functions of Macronutrients

Carbohydrates are major sources of energy. They should be our major source of energy, 50 to 70 percent of total calories. They provide current and stored (glycogen) energy. They are commonly classified as simple sugars, starches, or fiber. Carbohydrate-rich foods include whole grains, pasta, bread, rice, cereal, fruits, and vegetables.

Proteins are important for cell structure, especially the muscles. All enzymes are proteins. Proteins also serve as a minor energy source. Proteins are made up of amino acids. There are 21 of these amino acids, and every protein is made up of some combination of them—hence their nickname “building blocks.” Our bodies can produce only about half of the necessary amino acids it needs on its own, the remaining half— essential amino acids—must be obtained through our diet. Protein sources include meat, fish, beans, nuts, and dairy.

Fats are major sources of energy. They have vital roles in cell structure. Fats also prolong digestion and give food more taste. Some fat components cannot be produced by our bodies and must be obtained through diet—so-called essential fatty acids. Fat is classified as saturated or unsaturated. Fat sources include meat, butter, milk, nuts, and oils.

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