Nutritional science fundamentals: nutrients, energy balance, digestion and metabolism
Domain 1 tests whether you can move confidently between a nutrient, its energy value, its digestion site and the DGE reference value for it. Tasks typically combine a short calculation with an explanatory part, so a single mistaken factor (kcal instead of kJ, or grams instead of energy percentage) costs several marks.
Energy values and energy balance
- Atwater factors as used in German textbooks and in labelling: protein 17 kJ (4 kcal)/g, fat 37 kJ (9 kcal)/g, carbohydrate 17 kJ (4 kcal)/g, alcohol 29 kJ (7 kcal)/g, dietary fibre 8 kJ (2 kcal)/g.
- Total energy expenditure = basal metabolic rate (BMR) × PAL, plus diet-induced thermogenesis of roughly 10 % of intake. PAL runs from about 1.2 (sedentary, bedridden) to 2.4 (heavy physical labour).
- BMR is estimated (Harris-Benedict, or the simpler Mifflin-St Jeor equation), never measured, in exam tasks - say so when you evaluate a result.
Digestion in one line each
- Starch: salivary α-amylase in the mouth, halted by stomach acid (pH ~2), resumed by pancreatic amylase in the duodenum, completed at the brush border by maltase, sucrase and lactase.
- Protein: pepsinogen to pepsin below pH 2, then trypsin, chymotrypsin and carboxypeptidase from the pancreas; absorbed as free amino acids and di-/tripeptides.
- Fat: bile salts emulsify, pancreatic lipase with colipase hydrolyses triglycerides, absorption in the jejunum, transport as chylomicrons via the lymph.
- Vitamin B12 requires intrinsic factor from parietal cells and is absorbed in the terminal ileum - the classic differentiation item.
DGE reference values for adults
Protein 0.8 g/kg body weight; fat about 30 % of energy with saturated fatty acids ≤10 %; carbohydrate over 50 %; fibre 30 g/day; vitamin C 95 mg (women)/110 mg (men); folate 300 µg; calcium 1000 mg; vitamin D 20 µg (800 IU) assuming no endogenous synthesis.
Worked example
A 250 g portion supplies 20 g protein, 30 g fat and 60 g carbohydrate. Energy = 20×4 + 30×9 + 60×4 = 80 + 270 + 240 = 590 kcal, about 2 470 kJ. Fat contributes 270/590 = 45.8 % of energy, far above the 30 % guideline; carbohydrate contributes 240/590 = 40.7 %, below the recommended >50 %. Verdict: acceptable occasionally, but not compatible with a daily preventive diet.
Traps
- Mixing kcal and kJ (1 kcal = 4.184 kJ) and forgetting that fibre is listed under carbohydrate but counted at 2 kcal/g.
- Giving grams when the question asks for energy percentage, or the reverse.
- Treating BMR as total expenditure, or ignoring PAL in a sedentary office worker.
- Claiming a precise BMR from an equation without flagging the estimation error.