Nutrition for USMLE Step 1: what to know in 2026

How nutrition appears in a question

A nutrition item rarely asks "which vitamin is missing?" in so many words. It gives you a patient whose diet, drug, surgery or disease explains a deficiency, then asks for the mechanism, the lab finding or the next step. Read the social history and medication list closely: alcohol use, a strict diet, bariatric surgery, a malabsorption disorder or a drug like isoniazid or metformin is usually the clue.

Water-soluble vitamins

• B1, thiamine. Cofactor for pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, branched-chain α-keto acid dehydrogenase and transketolase. Deficiency in alcohol use disorder or malnutrition causes Wernicke encephalopathy (confusion, ophthalmoplegia, ataxia), Korsakoff syndrome, and dry or wet beriberi. Give thiamine before glucose.
• B2, riboflavin. Part of FAD and FMN. Deficiency: angular cheilitis, stomatitis, corneal vascularization.
• B3, niacin. Part of NAD and NADP, made partly from tryptophan. Deficiency causes pellagra: dermatitis in sun-exposed skin, diarrhea, dementia. Think of it with Hartnup disease, carcinoid syndrome (tryptophan diverted to serotonin) and isoniazid.
• B6, pyridoxine. Cofactor for transamination, decarboxylation, heme synthesis (ALA synthase) and neurotransmitter synthesis. Isoniazid causes deficiency: peripheral neuropathy, convulsions, and sideroblastic anemia.
• B7, biotin. Cofactor for carboxylases. Deficiency is rare; classic causes are long-term antibiotics or eating raw egg whites (avidin binds biotin). Dermatitis, alopecia.
• B9, folate. Needed for DNA synthesis. Deficiency causes megaloblastic anemia without neurologic signs, with high homocysteine and normal methylmalonic acid. Causes: poor diet, alcohol, pregnancy, methotrexate, trimethoprim, phenytoin. Supplementation before conception lowers neural tube defect risk.
• B12, cobalamin. Cofactor for methionine synthase and methylmalonyl-CoA mutase. Deficiency causes megaloblastic anemia with neurologic signs (subacute combined degeneration of the dorsal columns and lateral corticospinal tracts) and high homocysteine and methylmalonic acid. Causes: pernicious anemia, ileal resection or Crohn disease, strict vegan diet, long-term metformin, fish tapeworm.
• C, ascorbic acid. Needed to hydroxylate proline and lysine in collagen, and helps iron absorption. Deficiency causes scurvy: gum bleeding, perifollicular hemorrhages, corkscrew hairs, poor wound healing.

The folate and B12 pair is one of the most tested contrasts. Folate is trapped as methyl-folate when B12 is missing, so both cause megaloblastic anemia; only B12 deficiency raises methylmalonic acid and damages the spinal cord. Giving folate alone can correct the anemia while the neurologic damage continues.

Fat-soluble vitamins (A, D, E, K)

Fat-soluble vitamins need bile and pancreatic enzymes to be absorbed, so deficiency follows fat malabsorption: cystic fibrosis, chronic pancreatitis, cholestasis, celiac disease, bariatric surgery or orlistat.

• Vitamin A. Deficiency: night blindness, dry eyes with Bitot spots, keratomalacia, poor immunity. Excess: idiopathic intracranial hypertension, liver toxicity; isotretinoin and high-dose vitamin A are teratogenic.
• Vitamin D. Deficiency: rickets in children, osteomalacia in adults, with low calcium and phosphate and high PTH. Risk: little sun, dark skin at high latitude, malabsorption, chronic kidney or liver disease.
• Vitamin E. Antioxidant protecting membranes. Deficiency: hemolytic anemia with acanthocytes, and spinocerebellar and dorsal column degeneration that can look like B12 deficiency but without megaloblastic anemia.
• Vitamin K. Needed for γ-carboxylation of factors II, VII, IX and X and proteins C and S. Deficiency: bleeding with a prolonged PT. Newborns get vitamin K at birth because they have little gut flora and low placental transfer. Long antibiotic courses and warfarin also matter.

Minerals

• Iron. Microcytic anemia with low ferritin. In adults, look for blood loss first, especially GI loss in older patients.
• Zinc. Deficiency: poor wound healing, loss of taste and smell, alopecia, and a rash around the mouth and perineum. Acrodermatitis enteropathica is the inherited absorption defect.
• Copper. Deficiency can follow bariatric surgery or excess zinc intake: anemia, neutropenia, and a myelopathy that mimics B12 deficiency.
• Iodine. Deficiency causes goiter and hypothyroidism; in pregnancy it can cause congenital hypothyroidism.

Malnutrition and refeeding

• Kwashiorkor: protein deficiency with relatively adequate calories. Edema from low albumin, a fatty liver, skin and hair changes, a swollen belly.
• Marasmus: overall calorie deficiency. Severe muscle and fat wasting without edema.
• Refeeding syndrome: restarting nutrition in a severely malnourished patient shifts phosphate, potassium and magnesium into cells. Hypophosphatemia is the classic finding; it can cause heart failure, arrhythmia and respiratory weakness. Restart slowly and replace electrolytes.

Metabolism links that get tested

• Alcohol raises the NADH/NAD⁺ ratio in the liver, which favors lactate and blocks gluconeogenesis (fasting hypoglycemia) and promotes fatty liver.
• Fasting states: glycogenolysis for the first day, then gluconeogenesis, then ketone bodies as the brain's main fuel in prolonged starvation.
• Diet in chronic disease: protein and potassium in chronic kidney disease, sodium in heart failure, carbohydrate consistency in diabetes, and gluten in celiac disease.