Macronutrients and Energy Metabolism
Macronutrients and Energy Metabolism tests how carbohydrate, protein, fat, alcohol, and fiber are digested, absorbed, used for energy, and reflected in reference intakes. Expect calculations, metabolic pathway questions, and practical interpretation of macronutrient distribution.
Core rules
- Adult acceptable macronutrient distribution ranges commonly taught are carbohydrate 45–65% of energy (US) or 45–60% (EFSA), protein 10–35%, and fat 20–35%. Protein requirement is about 0.8–0.83 g/kg/day for healthy adults.
- Total energy expenditure includes basal or resting metabolic rate, thermic effect of food, and physical activity; resting metabolism usually dominates.
- Glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation generate ATP from carbohydrate and fat. Beta-oxidation converts fatty acids to acetyl-CoA; excessive acetyl-CoA can form ketones. Protein contributes amino acids for gluconeogenesis and energy when intake exceeds needs or during fasting.
- Fiber promotes laxation, modulates glycemia and lipids, and supports gut microbiota; common adult targets are about 25–30 g/day.
Worked example
A 70 kg adult needs about 0.8 g/kg protein = 56 g/day. During recovery from illness, a clinician may target 1.2 g/kg = 84 g/day. The difference is 28 g protein, or 112 kcal from protein, but the higher target must fit total energy and renal status.
Traps
Do not call protein the body's preferred primary fuel. Do not confuse glycemic index with glycemic load. Do not assume all fat grams are equivalent; fatty acid type matters. Do not apply athlete or clinical protein targets to the general population without justification.