Vitamins and Minerals

Fat-Soluble Vitamins: Stored and Accumulative

Vitamins A, D, E, and K travel with dietary fat, require bile acids for micelle formation, and absorb through the same pathway as lipids. Because the body stores them in liver and adipose tissue, deficiency develops slowly but toxicity is also possible with excess intake — unlike water-soluble vitamins, there is no simple "spill the excess into urine" safety valve.

Vitamin A supports vision, epithelial integrity, and immune function, but is also a potent teratogen: excess intake in pregnancy causes birth defects, which is why isotretinoin and other retinoid drugs derived from vitamin A carry strict pregnancy-prevention requirements and mandatory contraception counseling.

Vitamin D governs intestinal calcium absorption and bone mineralization. Deficiency is common — from limited sun exposure, malabsorption, dark skin, obesity, or aging skin that synthesizes it less efficiently — and produces bone softening and muscle weakness. Excess supplementation instead causes hypercalcemia, with confusion, constipation, kidney stones, and cardiac rhythm disturbance.

Vitamin E is an antioxidant with mild antiplatelet activity; at high supplemental intake it raises bleeding risk, an effect that compounds dangerously with warfarin or other anticoagulants.

Vitamin K is essential for hepatic synthesis of clotting factors II, VII, IX, and X, and is the direct antidote for warfarin toxicity, restoring the liver's ability to produce functional clotting factors (though the effect takes time, unlike faster-acting reversal agents). Newborns are born with low vitamin K stores and sterile gut flora that hasn't yet begun producing it, so a vitamin K injection is given routinely at birth to prevent hemorrhagic disease of the newborn.

Water-Soluble Vitamins: Reactive and Depleted Quickly

B vitamins and vitamin C are not meaningfully stored, so deficiency appears faster and repletion is often more urgent.

Thiamine (B1) deficiency, classically from alcohol use disorder or malnutrition, impairs cerebral glucose metabolism. The critical rule is to give thiamine before glucose to any patient with suspected deficiency, because glucose alone can precipitate or worsen Wernicke encephalopathy by consuming remaining thiamine stores during carbohydrate metabolism.

Niacin (B3) at higher intake causes cutaneous flushing, an uncomfortable but harmless prostaglandin-mediated reaction, and a well-known limiting side effect of niacin used for lipid management.

Pyridoxine (B6) is depleted by isoniazid, an antitubercular drug, so it is co-administered to prevent isoniazid-induced peripheral neuropathy. Paradoxically, excessive B6 intake itself can also cause peripheral neuropathy — proof that a "protective" vitamin is not automatically safe at any dose.

Folate and vitamin B12 are metabolically linked, and folate supplementation can mask the hematologic signs of B12 deficiency (correcting the anemia) while its underlying neurologic damage progresses undetected. Folate is also specifically recommended before conception and in early pregnancy to reduce neural tube defect risk.

Vitamin C aids nonheme iron absorption, and its own absorption can be affected by other dietary and supplemental factors; high-dose supplementation for illness prevention has limited evidence.

Minerals and Practical Supplementation Patterns

Zinc supports immune function and wound healing but can impair copper absorption in excess. Selenium is a trace antioxidant cofactor. Iodine is essential for thyroid hormone synthesis, and both deficiency and excess can cause thyroid dysfunction.

Certain populations and exposures predictably need attention: pregnancy (folate, iodine, vitamin D), bariatric surgery (fat-soluble vitamins, B12, iron, and other minerals, from reduced absorptive surface), malabsorptive conditions and restrictive diets (fat-soluble vitamins, B12), older adults (vitamin D, B12), and alcohol use disorder (thiamine and other B vitamins). Known drug-nutrient interactions include metformin lowering B12 with long-term use, proton pump inhibitors reducing B12 and magnesium absorption by altering gastric acidity, isoniazid depleting B6, and long-term phenytoin affecting vitamin D metabolism. Dietary supplements are regulated far more loosely than prescription drugs, so potency, purity, and even labeled content can vary, and they can still interact meaningfully with prescribed medications. The governing principle is that correcting a genuine deficiency helps, but supplementing beyond need is not automatically harmless — for fat-soluble vitamins especially, more is not better.