Macronutrients as Energy Sources
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Carbohydrates, fats, and proteins are the energy-containing macronutrients whose energy is expressed in kcal and metabolized to fuel the body.
Detail
Total caloric intake comes from the consumption of foods and drinks. The carbohydrates, fats, and proteins in these foods contain energy that is expressed in terms of kcal (see Kilocalorie (kcal)).
In energy metabolism, these macronutrients are used differently depending on activity. At rest, the body gets one-third of its needed ATP from carbohydrate (CHO) and two-thirds from fats. During maximum physical exercise, muscle metabolizes nearly 100% CHO if sufficient CHO stores are available. During long exercise (2+ hours), up to 10% of energy can come from metabolizing protein. The aerobic system can metabolize carbohydrates, fats, and even proteins to resynthesize ATP (see Oxygen (Aerobic) Energy System).
Key facts
- Energy-containing macronutrients: carbohydrates, fats, proteins.
- Their food energy is expressed in kcal.
- At rest: ~1/3 of ATP from CHO, ~2/3 from fats.
- Maximum exercise: nearly 100% CHO (if stores allow).
- Long exercise (2+ hours): up to 10% of energy from protein.
Connections
- Kilocalorie (kcal) — the unit of food energy.
- Caloric Balance (Energy Balance) — total intake from these macronutrients.
- Macronutrient Distribution Guidelines — recommended proportions.
- Oxygen (Aerobic) Energy System — aerobic breakdown of these fuels for ATP.
Sources
See how this applies in practice, with honest limits on what the plan changes.
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