Micronutrients, Biochemistry, and Physiology
What this domain tests
This domain tests micronutrient function, deficiency and toxicity patterns, and the interpretation of laboratory markers in clinical nutrition.
Core rules
- Fat-soluble vitamins A, D, E, K are stored and toxic in excess; water-soluble B vitamins and vitamin C are not stored appreciably. Vitamin A as retinal supports rhodopsin; retinoic acid regulates gene transcription; deficiency causes night blindness and xerophthalmia.
- 25-hydroxyvitamin D is the storage and exposure marker; 1,25-dihydroxyvitamin D is the active hormone and is not the routine deficiency test. Vitamin D regulates calcium absorption and PTH.
- Vitamin K drives gamma-carboxylation of clotting factors; warfarin and broad-spectrum antibiotics alter status. Vitamin E is an antioxidant; alpha-tocopherol is the reference form.
- Thiamine is cofactor for pyruvate dehydrogenase and transketolase; deficiency causes beriberi and Wernicke encephalopathy. Niacin equivalents use 60 mg tryptophan = 1 mg NE.
- Vitamin B12 and folate are required for DNA synthesis; B12 deficiency raises methylmalonic acid and homocysteine, while folate deficiency raises homocysteine only.
- Iron status is assessed with ferritin, transferrin saturation, and hemoglobin; hepcidin falls in iron deficiency and rises in inflammation. Zinc supports immunity and wound healing; iodine is required for thyroid hormone synthesis.
Worked example
A 32-year-old vegan has MCV 108 fL, low serum B12, normal folate, elevated methylmalonic acid, and elevated homocysteine. The macrocytosis could be folate or B12, but elevated methylmalonic acid is specific for B12 deficiency. Normal folate makes folate deficiency unlikely. Treat B12 deficiency and do not give folate alone, because folate can mask hematologic signs while neurologic damage progresses.
Traps
Calling serum ferritin low in every inflammatory state; ordering 1,25-dihydroxyvitamin D for deficiency screening; treating macrocytosis with folate before B12 is excluded; prescribing high-dose vitamin A in pregnancy because of teratogenicity; assuming zinc deficiency from taste alone.