Vitamins are organic compounds that regulate metabolism, support immunity, and act as antioxidants. The fat-soluble vitamins A, D, E, and K are stored in adipose tissue and liver, so excess intake raises toxicity risk; hypervitaminosis A causes liver damage and birth defects, while excess D leads to hypercalcemia and kidney stones. Vitamin A exists as retinol from animal sources (liver, eggs, dairy) and provitamin carotenoids from plants (carrots, spinach); excess preformed retinol is teratogenic, whereas carotenoid conversion is regulated and safer. Vitamin D is uniquely synthesized when UVB radiation converts 7-dehydrocholesterol in skin to cholecalciferol, then hydroxylated in liver and kidney to active calcitriol; latitude, season, pigmentation, sunscreen use, and age all affect synthesis, and many adults need supplementation to support calcium absorption and prevent rickets or osteomalacia. Vitamin E, found in wheat germ oil, sunflower seeds, and almonds, is a lipid-soluble antioxidant protecting cell membranes at an RDA of 15 mg/day, though high-dose supplements above 400 IU have shown increased mortality risk in meta-analyses.
Vitamin K splits into two functionally distinct forms: K1 (phylloquinone) from leafy greens drives hepatic clotting factor synthesis, while K2 (menaquinone) from fermented foods, egg yolks, and gut bacteria activates extrahepatic proteins like osteocalcin and matrix Gla-protein, directing calcium into bone and away from arteries. Water-soluble vitamins, including the B-complex and C, are not stored but excreted in urine, making daily intake important. Folate is critical for DNA synthesis and methylation, with deficiency in early pregnancy (days 21-28 of gestation) causing neural tube defects such as spina bifida, which is why the RDA rises to 600 µg/day in pregnancy and folic acid supplementation is routinely recommended. Vitamin B12 deficiency causes megaloblastic anemia, neuropathy, and cognitive decline, especially in vegans, the elderly, and users of metformin or proton pump inhibitors. Biotin (B7) is a coenzyme for carboxylases, and its deficiency causes dermatitis, alopecia, and lethargy, though it can interfere with lab tests for troponin and thyroid function, an important clinical caveat. Choline, found abundantly in eggs and liver, supports cell membranes, acetylcholine synthesis, and methylation, with AI of 425 mg/day for women and 550 mg/day for men.
Among minerals, calcium builds bone and teeth, supports muscle contraction and nerve signaling, and pairs with vitamin D for absorption; dairy and leafy greens are top sources, though large calcium doses (>300 mg) taken with iron-rich foods inhibit iron absorption and should be spaced by two or more hours. Iron exists as heme (from animal sources, 15-35% absorption, enhanced by meat factor) and non-heme (from plants, 2-20% absorption, enhanced by vitamin C but inhibited by phytates, tannins, polyphenols, and calcium); the body regulates iron via hepcidin, which rises with repletion or inflammation to sequester iron in macrophages. Magnesium is a cofactor in over 300 enzymatic reactions, with pumpkin seeds, spinach, and almonds as leading sources; deficiency contributes to insomnia, cramps, hypertension, and type 2 diabetes. Iodine, essential for thyroid hormone synthesis, comes mainly from iodized salt, seaweed, and dairy, with needs rising in pregnancy (220-250 µg/day) to support fetal neurodevelopment. Zinc supports immunity and wound healing through thymulin activity and T-cell maturation, though phytates in plants bind zinc and reduce its bioavailability, particularly relevant in vegetarian diets. Selenium, incorporated into selenoproteins including glutathione peroxidase, is found richly in Brazil nuts (one nut contains roughly 68 µg) and protects against oxidative damage, with an UL of 400 µg/day.