Nutritive Supplementation
Supplement questions reward a disciplined recommendation: indication, form, dose, timing, and stop rule. Candidates who jump to a product name without an indication lose marks.
Establish the indication first
- Documented deficiency or biochemical insufficiency.
- Inadequate intake: B12 for vegans, folate 600 mcg DFE in pregnancy, iron when needs exceed diet.
- Malabsorption: bariatric surgery (B12, iron, calcium, fat-soluble vitamins), short bowel, pancreatic insufficiency.
- Drug-induced depletion: metformin and B12; proton-pump inhibitors and B12 plus magnesium; corticosteroids and calcium/vitamin D; cholestyramine and fat-soluble vitamins.
Form and bioavailability
- Iron: alternate-day dosing of 60–120 mg elemental iron improves fractional absorption by avoiding repeated hepcidin surges; bisglycinate is better tolerated than ferrous sulfate.
- Calcium: citrate suits low gastric acid or PPI use; carbonate needs acid and should be taken with food.
- Vitamin D3 raises 25(OH)D more effectively than D2; give with a fat-containing meal.
- Magnesium: oxide is poorly absorbed; citrate, malate, or glycinate are better tolerated.
- Prefer third-party-verified products (USP, NSF) to reduce mislabeling risk.
Dose ceilings and interactions
Adult tolerable upper limits: preformed vitamin A 3000 mcg RAE; vitamin D 4000 IU; zinc 40 mg; iron 45 mg; calcium 2000–2500 mg. Minerals compete for absorption, so separate calcium, iron, and zinc doses. Levothyroxine must be separated from calcium and iron by about 4 hours; tetracyclines and fluoroquinolones chelate with di- and trivalent cations; vitamin K intake must stay consistent on warfarin.
Worked example
A 46-year-old woman on levothyroxine, metformin, and omeprazole has fatigue and B12 of 180 pg/mL. Plan: oral cyanocobalamin 1000 mcg daily, because the PPI and metformin both impair absorption and passive diffusion requires a high oral dose; methylmalonic acid can confirm tissue deficiency. Take levothyroxine on an empty stomach and separate any calcium or iron by 4 hours. Use calcium citrate if supplementation is needed. Recheck B12 in 8–12 weeks.
Traps
High-dose fat-soluble vitamins without monitoring; assuming natural equals safe; long-term zinc without copper; treating D2 and D3 as interchangeable; supplementing a lab value while ignoring symptoms or diet.