Macronutrients are nutrients required in relatively large quantities: carbohydrates, lipids and proteins. Their chemical energy becomes available through metabolic reactions and is transferred to adenosine triphosphate (ATP), which powers processes such as muscle contraction.
How Macronutrients Supply Energy
During digestion, macronutrients are broken into smaller molecules that can be absorbed: carbohydrates into glucose, lipids into fatty acids and glycerol, and proteins into amino acids. These molecules may then be oxidized during cellular respiration, with some of their energy captured in ATP.
| Macronutrient | Energy yield | Main role in exercise |
|---|---|---|
| Carbohydrate | Approximately , or | Glucose is used in aerobic and anaerobic respiration. Stored glycogen is especially important during moderate- to high-intensity exercise. |
| Lipid | Approximately , or | Fatty acids undergo beta-oxidation and mainly support lower-intensity, longer-duration activity when oxygen is available. |
| Protein | Approximately , or | Primarily supports growth and tissue repair. Amino acids may contribute energy during prolonged exercise or when carbohydrate availability is low. |
Carbohydrate is stored as glycogen in the muscles and liver, whereas lipid is stored mainly as triglycerides in adipose tissue. Protein is not a dedicated energy store; before amino acids can be used for energy, their amino group must be removed through deamination.
For example, a meal containing carbohydrate, protein and lipid provides:
A common misconception is that all nutrients provide energy. Micronutrients such as vitamins and minerals do not supply energy directly, although many assist energy metabolism.
Exam Technique
For an IB SEHS explain question, connect digestion, oxidation and ATP production rather than only listing energy values. For a calculate question, multiply the mass of each macronutrient by its energy yield and show every step with units.