Nutrition Science and Macronutrient/Micronutrient Metabolism
Core concepts
Candidates must be fluent in energy balance (Atwater factors: carbohydrate 4 kcal/g, protein 4 kcal/g, fat 9 kcal/g, alcohol 7 kcal/g), the fate of macronutrients in fed and fasted states, and the regulation of glycemia, lipemia, and nitrogen balance. Know the hormonal axes: insulin (anabolic, promotes glycogen, lipogenesis, protein synthesis, potassium and phosphate intracellular shift), glucagon (glycogenolysis, gluconeogenesis, ketogenesis), cortisol (proteolysis, gluconeogenesis), and thyroid hormone (basal metabolic rate). Understand protein quality (DIAAS/PDCAAS, limiting amino acids, complementary proteins), essential fatty acids (linoleic, alpha-linolenic), fiber physiology (soluble vs insoluble, viscosity, fermentation to short-chain fatty acids), and micronutrient function-especially B vitamins as coenzymes, vitamin C in collagen hydroxylation and iron absorption, vitamin K in gamma-carboxylation, and the calcium-phosphate-vitamin D axis.
Traps
Questions exploit unit confusion (mg vs mcg; retinol activity equivalents vs IU), the difference between RDA and AI, and the fact that energy factors differ for specific foods (fiber ~2 kcal/g; alcohol 7 kcal/g). Watch for statements that confuse water-soluble with fat-soluble vitamin toxicity, or that attribute folate's role to B12. A frequent trap is assuming enteral/parenteral provision equals absorption.
Worked example
A 70 kg patient receives 1.5 g/kg protein and 30 kcal/kg. Protein = 105 g = 420 kcal; total energy 2100 kcal; nonprotein energy = 1680 kcal; nitrogen = 105/6.25 = 16.8 g; NPC:N = 1680/16.8 = 100:1. Rule of thumb: nitrogen (g) = protein (g) ÷ 6.25.
Rule of thumb
For weight-stable adults, ~25-30 kcal/kg and 0.8-1.0 g/kg protein; stress and critical illness raise protein needs but rarely exceed 2.0-2.5 g/kg.