Biochemistry and Nutrition: fuel logic, cofactors, and diet calculations
Biochemistry and Nutrition tests molecular logic: how fuels become ATP, how enzymes are regulated, and how deficient cofactors produce recognizable syndromes. PLE items often combine a laboratory value or diet history with a pathway step.
Core rules
- ATP yields: 1 FADH2 ≈ 1.5 ATP, 1 NADH ≈ 2.5 ATP; glycolysis nets 2 ATP and 2 NADH; pyruvate dehydrogenase produces NADH and acetyl-CoA; the TCA cycle yields 3 NADH, 1 FADH2, and 1 GTP per acetyl-CoA.
- Respiratory quotient: carbohydrate 1.0, fat ≈0.7, protein ≈0.8, mixed diet ≈0.8.
- Essential amino acids are classically remembered by PVT TIM HALL; arginine is conditionally essential. Essential fatty acids are linoleic and alpha-linolenic acids.
- Enzyme kinetics: Km is the substrate concentration at half Vmax; competitive inhibitors raise Km with unchanged Vmax; noncompetitive inhibitors lower Vmax.
- Vitamins: thiamine pyrophosphate serves pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase; B12 and folate drive one-carbon transfer; vitamin C supports collagen hydroxylation.
Worked example
For a 70-kg adult, prescribe 30 kcal/kg, protein 1.2 g/kg, and fat 25% of calories. Energy = 70 × 30 = 2100 kcal. Protein = 84 g × 4 = 336 kcal. Fat = 0.25 × 2100 = 525 kcal ÷ 9 = 58 g. Carbohydrate = 2100 − 336 − 525 = 1239 kcal ÷ 4 = 310 g. Reasoning: use Atwater factors 4-9-4 and reserve protein for nitrogen balance, not just calories.
Traps
Do not use 9 kcal/g for protein or carbohydrate. Do not give glucose before thiamine in a malnourished patient because this can precipitate Wernicke encephalopathy. Do not treat B12 deficiency with folate alone, which may worsen neurologic injury.