Nutrition Science and Biochemistry
What to know
Candidates must connect macronutrient metabolism to clinical practice: glycolysis, the citric acid cycle, gluconeogenesis, ketogenesis, and the fed/fasted transition (insulin vs glucagon). Know the energy yields (carbohydrate and protein ~4 kcal/g, fat ~9 kcal/g, alcohol ~7 kcal/g), the roles of the B vitamins as coenzymes (thiamin in pyruvate dehydrogenase, niacin in NAD/NADP, riboflavin in FAD, B6 in transamination, B12/folate in one-carbon metabolism), and the essential amino and fatty acids. Understand protein quality (limiting amino acids, PDCAAS/DIAAS), nitrogen balance, and the distinction between fat-soluble and water-soluble vitamin handling.
Traps
Questions often test the rate-limiting step or the coenzyme rather than the pathway name. Watch for confusion between ketosis and ketoacidosis, between essential and conditionally essential nutrients, and between energy values of food (Atwater factors) versus physiological fuel value. Another common trap: attributing a deficiency sign to the wrong vitamin (e.g., peripheral neuropathy to thiamin, not B12 alone; megaloblastic anemia to folate or B12, but neurological signs point to B12).
Rule of thumb
For any "which nutrient/enzyme/cofactor" question, first classify the nutrient (water- vs fat-soluble; macro- vs micronutrient), then recall its primary biochemical role, then its classic deficiency syndrome. If the stem mentions a metabolic pathway bottleneck, suspect a vitamin coenzyme.
Worked example
A patient with chronic alcohol use presents with confusion and ataxia. The biochemical link is thiamin deficiency impairing pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, reducing ATP production in glucose-dependent tissue. Answer: thiamin, not niacin.