Nutrition and Body Composition
The Nutrition and Body Composition domain tests how energy intake, macronutrients, hydration, and body-composition methods interact with exercise performance and health. Expect calculations, method assumptions, and practical interpretation rather than diet prescriptions.
Core rules
- Energy balance determines weight change; macronutrient distribution affects performance and satiety. Carbohydrate availability supports high-intensity and prolonged exercise; protein supports repair and adaptation; fat supports essential functions and energy density.
- Common sport-nutrition ranges: protein about 1.2–2.0 g/kg/day for active adults, with higher intakes during energy restriction or heavy training; fat 20–35% of energy; carbohydrate scaled to training load.
- Hydration: aim to limit body-mass loss to roughly 2% during exercise; replace fluids and sodium after long or hot sessions. Overhydration with hypotonic fluid can cause hyponatremia.
- Body-composition methods: hydrostatic weighing and DXA are criterion-ish laboratory methods; BIA and skinfolds are field methods. All rely on assumptions (constant fat-free mass density, hydration, distribution of fat).
- Fat mass = body mass × body-fat percentage. Fat-free mass = body mass − fat mass. “Lean body mass” is not identical to fat-free mass because it includes essential lipids.
- Weight-loss goals should preserve fat-free mass, use modest energy deficits, and include resistance training and adequate protein.
Worked example
An 80 kg client has 20% body fat. Fat mass = 80 × 0.20 = 16 kg; fat-free mass = 64 kg. If the target is 15% body fat while preserving all fat-free mass, target body mass = 64 ÷ 0.85 = 75.3 kg. The required loss is 80 − 75.3 = 4.7 kg. This assumes no fat-free-mass loss, which is optimistic; the plan should monitor strength and protein intake. If the client lost 4.7 kg of mixed tissue, the result would differ.
Traps
- Treating BIA as unaffected by hydration, food, or exercise.
- Assuming skinfold measurements are interchangeable between technicians.
- Confusing fat-free mass with lean body mass.
- Prescribing very low-calorie diets without medical supervision.
- Believing spot reduction is possible.
- Ignoring that protein needs are per kilogram of body mass, not a fixed percentage of calories.