Nutrition Science Fundamentals
Domain 2 tests the biochemistry and physiology underneath every recommendation: energy-yielding nutrients, requirement frameworks, bioavailability, and energy expenditure.
Energy and macronutrients
Atwater factors: carbohydrate 4 kcal/g, protein 4, fat 9, alcohol 7. Adult AMDRs: carbohydrate 45–65% of energy, fat 20–35%, protein 10–35%; added sugars <10% of energy; fibre 14 g per 1,000 kcal (≈25 g/day women, 38 g/day men).
The DRI framework
EAR (median requirement) → RDA = EAR + 2 SD, covering ~97–98% of healthy people → AI when data are insufficient → UL (highest intake unlikely to cause harm) → AMDR. The RDA is not a minimum and not a therapeutic target; the UL is not a single-dose toxicity threshold.
Protein and fat quality
Nine indispensable amino acids; PDCAAS/DIAAS score protein quality, so 0.8 g/kg of mixed plant protein is not equivalent to the same grams of animal protein. Two essential fatty acids: linoleic (omega-6) and alpha-linolenic (omega-3). Protein RDA is 0.8 g/kg for adults, 1.0–1.2 g/kg for healthy older adults, and 1.2–2.0 g/kg is a common evidence-based range in acute illness.
Bioavailability and interactions
Heme iron is absorbed far better than non-heme; vitamin C enhances non-heme absorption, while phytates, oxalates, calcium, and polyphenols inhibit it. Fat-soluble vitamins require dietary fat; vitamin K intake must stay consistent on warfarin.
Energy expenditure
BMR/RMR ≈60–70% of total, thermic effect of food ~10%, physical activity the remainder. Mifflin–St Jeor: men 10W + 6.25H − 5A + 5; women the same − 161 (W in kg, H in cm, A in years).
Worked example
Woman, 70 kg, 165 cm, 55 y, sedentary. RMR = 700 + 1,031.25 − 275 − 161 = 1,295 kcal; ×1.3 activity factor ≈ 1,684 kcal, round to 1,700. Protein 0.8 g/kg = 56 g = 224 kcal = 13% of energy (inside 10–35%). Fat at 20–35% = 375–655 kcal = 42–73 g. Carbohydrate fills the rest, ≈210–280 g. If she were 78 and frail, protein would rise toward 1.2 g/kg (84 g) at similar energy.
Traps
Using 7 kcal/g for alcohol inconsistently or omitting it from a "healthy diet" calculation; confusing AI with RDA; keeping protein at 0.8 g/kg in a catabolic patient; assuming a low-GI food is low in glycaemic load and vice versa.