Human Nutrition and Nutritional Biochemistry
This domain moves from nutrient classes to what the body does with them: digestion and absorption, energy release through glycolysis, the TCA cycle and beta-oxidation, protein quality and nitrogen balance, and the metabolic consequences of deficiency or excess. Calculations appear frequently - energy from a meal, energy requirement, protein quality scores.
Energy
Atwater factors: carbohydrate 4 kcal/g (17 kJ), protein 4 kcal/g (17 kJ), fat 9 kcal/g (37 kJ), alcohol 7 kcal/g (29 kJ). These are metabolisable energy values, not gross heat of combustion. 1 kcal = 4.184 kJ. Total energy expenditure = basal metabolic rate (BMR, roughly Harris–Benedict or Schofield equations) + physical activity + diet-induced thermogenesis + growth in children.
Carbohydrate and lipid metabolism
- Glycolysis (cytosol): glucose → 2 pyruvate, net 2 ATP and 2 NADH.
- Link reaction: pyruvate → acetyl-CoA + CO₂ + NADH, inside the mitochondrion.
- TCA cycle (mitochondria): oxidises acetyl-CoA, produces NADH, FADH₂.
- Oxidative phosphorylation: ATP synthesis via the electron transport chain; roughly 30–32 ATP per glucose overall.
- β-Oxidation (mitochondrial matrix): two-carbon units removed as acetyl-CoA; excess acetyl-CoA gives ketone bodies.
- Gluconeogenesis uses lactate, glycerol and glucogenic amino acids; deamination removes nitrogen, and the urea cycle excretes it.
Protein quality
The nine indispensable amino acids must be supplied. Limiting amino acid: lysine in cereals, methionine/cysteine in legumes - combining maize and beans improves quality. Measures: biological value (BV), net protein utilisation (NPU), protein efficiency ratio (PER, reference casein ≈ 2.5), PDCAAS (truncated at 1.0) and the newer DIAAS. Nitrogen balance = nitrogen intake − nitrogen losses; positive in growth and pregnancy, negative in wasting.
Micronutrients in brief
Iron absorption is enhanced by vitamin C and inhibited by phytates and tannins; vitamin A, D, E, K are fat-soluble and require fat for absorption; iodine and zinc are co-factors in thyroid hormone and enzyme function respectively.
Worked example
A meal contains 60 g carbohydrate, 20 g protein and 15 g fat. Energy = (60 × 4) + (20 × 4) + (15 × 9) = 240 + 80 + 135 = 455 kcal ≈ 1,904 kJ (455 × 4.184). If the protein is maize protein with a BV of 60, the "effective" protein is 20 × 0.60 = 12 g. Reasoning: two steps - Atwater conversion first, then a separate quality adjustment; do not blend them.
Traps
Reversing 4/9/4; using gross energy; assuming all protein is equivalent; placing beta-oxidation in the cytosol; and forgetting that a positive nitrogen balance is normal in children.