Caloric value of carbohydrates, proteins and fats

The three macronutrient classes do not all supply the same amount of usable energy per gram, and the body typically draws around half of its daily energy from carbohydrates, a little over a third from fats, and the remainder from proteins — with the exact daily requirement varying by a person's age, gender, activity level and physiological state (such as pregnancy or lactation). Carbohydrates (sugars and starches) are the main source of everyday cellular fuel, with a caloric value of about 4.1 kilocalories per gram and a physiological fuel value (the energy the body can actually extract, after accounting for digestive losses) of about 4 kilocalories per gram; the body needs roughly 400–500 grams of carbohydrate a day. Lipids are the body's best long-term energy reserve, with a much higher caloric value of about 9.45 kilocalories per gram (physiological fuel value about 9 kcal/g), and a daily requirement of only about 60–70 grams. Proteins, by contrast, are used primarily as a source of amino acids for growth and tissue repair rather than as a fuel reserve — the body stores them only to a limited extent, and any surplus is broken down and excreted as nitrogenous waste; their caloric value is about 5.65 kilocalories per gram but their physiological fuel value …