Nutrition Science and Nutrient Metabolism
Core concepts
Candidates must be fluent in macronutrient energy values (carbohydrate 4 kcal/g, protein 4 kcal/g, fat 9 kcal/g, alcohol 7 kcal/g), the distinction between essential and conditionally essential nutrients, and the logic of digestion and absorption: carbohydrates as monosaccharides via brush-border enzymes and SGLT1/GLUT transporters, proteins as amino acids and di/tripeptides via PepT1, and lipids packaged into chylomicrons that enter lymph before circulation. Know the roles of coenzymes (B vitamins), the fat-soluble vitamin absorption dependency on bile and micelles, and the basics of energy expenditure (BMR/RMR, thermic effect of food, activity).
Metabolic integration matters: glycolysis, the TCA cycle, oxidative phosphorylation, gluconeogenesis, lipogenesis, ketogenesis, and transamination/deamination. Understand nitrogen balance, protein quality (limiting amino acids, complementary proteins), and glycemic response.
Traps in questions
- Confusing energy density with nutrient density.
- Assuming all fiber is fermentable or that insoluble fiber lowers cholesterol (soluble/viscous fibers do).
- Mixing up fat-soluble (A, D, E, K) with water-soluble vitamin transport and storage.
- Treating the Atwater factors as exact for all foods (fiber, alcohol, and food matrix alter usable energy).
- Confusing RDA (covers ~97–98% of healthy people) with EAR (median requirement) or AI (used when EAR is unavailable).
Rule of thumb
For label math, multiply grams by the Atwater factors and compare to the stated calories; a large mismatch often signals rounding, fiber, or sugar-alcohol effects.
Worked example
A snack has 20 g carbohydrate, 5 g protein, 8 g fat. Energy = (20×4)+(5×4)+(8×9) = 80+20+72 = 172 kcal. If the label claims 150 kcal, suspect fiber or rounding-not a metabolic anomaly.