How Vitamins and Minerals Work: Deficiency, Function, and Supplementation

What Are Vitamins and Minerals?

Vitamins and minerals are essential micronutrients—compounds required in small amounts that the body cannot synthesize in sufficient quantities to support normal physiological function, and therefore must be obtained through diet or supplementation. They differ from macronutrients (carbohydrates, proteins, and fats) in that they provide no calories, but they are indispensable as cofactors for enzymatic reactions, regulators of gene expression, structural components of tissues, and participants in virtually every metabolic pathway in the body.

Vitamins are organic compounds (containing carbon), while minerals are inorganic elements. There are 13 recognized essential vitamins: A, C, D, E, K, and eight B vitamins (thiamine/B1, riboflavin/B2, niacin/B3, pantothenic acid/B5, pyridoxine/B6, biotin/B7, folate/B9, and cobalamin/B12). Essential minerals are categorized as macrominerals (needed in amounts above 100 mg/day)—including calcium, phosphorus, magnesium, sodium, potassium, chloride, and sulfur—and trace minerals (needed in much smaller amounts)—including iron, zinc, copper, selenium, iodine, manganese, fluoride, and chromium.

Fat-Soluble vs. Water-Soluble Vitamins

The most fundamental distinction among vitamins is their solubility. Fat-soluble vitamins (A, D, E, and K) are absorbed with dietary fat via the lymphatic system and stored in the liver and adipose tissue. Because they accumulate in the body, fat-soluble vitamins can reach toxic levels if consumed in excess—a condition called hypervitaminosis. They need not be consumed daily, as body stores can persist for months. Vitamin A (retinol) is essential for vision (particularly night vision), immune function, and embryonic development. Vitamin D functions as a hormone, regulating calcium absorption and bone metabolism, and has widespread effects on immune function and gene expression. Vitamin E (tocopherol) acts as a fat-soluble antioxidant protecting cell membranes from oxidative damage. Vitamin K is essential for the carboxylation of clotting factors and for bone metabolism.

Water-soluble vitamins (vitamin C and all eight B vitamins) are absorbed directly into the bloodstream, are not significantly stored in the body, and excess amounts are excreted by the kidneys. They must therefore be consumed regularly. Their low storage means toxicity from food is rare, but high-dose supplements of certain water-soluble vitamins (particularly B6 and niacin) can cause harm. The B vitamins function primarily as coenzymes—molecules that assist enzymes in metabolic reactions. Collectively they are essential for energy metabolism, DNA synthesis, red blood cell production, and neurological function. Vitamin C (ascorbic acid) is a powerful water-soluble antioxidant and essential cofactor for collagen synthesis, immune function, and iron absorption.

Key Deficiency Diseases

Vitamin and mineral deficiencies, though less common in developed countries due to food fortification and diverse diets, remain a major global health burden. Vitamin D deficiency—one of the most prevalent nutritional deficiencies globally, affecting an estimated 1 billion people—causes rickets in children (soft, deformed bones) and osteomalacia in adults (bone softening and pain), as well as impaired immune function. Iron deficiency anemia is the world's most common nutritional disorder, affecting approximately 2 billion people, causing fatigue, pallor, impaired cognitive function, and reduced exercise tolerance.

Iodine deficiency causes goiter (thyroid gland enlargement) and cretinism (severe intellectual disability in children born to iodine-deficient mothers)—largely eliminated in many countries through salt iodization programs. Vitamin B12 deficiency causes megaloblastic anemia and, importantly, subacute combined degeneration of the spinal cord—irreversible neurological damage if not corrected. It is particularly prevalent among strict vegans and the elderly (due to reduced gastric acid production impairing B12 absorption). Folate deficiency during early pregnancy is strongly associated with neural tube defects (spina bifida, anencephaly), which is why folic acid supplementation is universally recommended before conception and during the first trimester.

Recommended Dietary Allowances and Tolerable Upper Limits

Nutrient reference values are established by scientific bodies such as the U.S. National Academies of Sciences. The Recommended Dietary Allowance (RDA) is the average daily intake level sufficient to meet the nutrient requirements of 97–98% of healthy individuals in a given age and sex group. The Adequate Intake (AI) is used when insufficient evidence exists to establish an RDA. The Tolerable Upper Intake Level (UL) is the highest average daily intake unlikely to cause adverse health effects in almost all individuals.

The UL concept is critical for understanding supplementation risk. Vitamin A has a UL of 3,000 mcg/day for adults; chronic excess causes liver toxicity, bone fragility, and—dangerously in pregnancy—fetal malformations. Vitamin D's UL is 100 mcg (4,000 IU)/day; excess causes hypercalcemia, kidney stones, and calcification of soft tissues. Iron supplementation without confirmed deficiency can cause gastrointestinal distress and, in hereditary hemochromatosis, accelerate iron overload. The principle 'more is better' does not apply to micronutrients—optimal health is achieved at adequate intake, and excess provides no benefit while increasing toxicity risk.

Food Sources vs. Supplements

The scientific consensus strongly favors obtaining vitamins and minerals from whole foods rather than supplements for most healthy individuals. Foods contain nutrients in forms optimized for absorption and in matrices that include synergistic compounds—fiber, phytonutrients, and other micronutrients—that enhance bioavailability and health effects beyond what isolated supplements can replicate. For example, beta-carotene from food (a vitamin A precursor) is safe at high dietary intakes, while high-dose beta-carotene supplements have been associated with increased lung cancer risk in smokers.

Supplements play an important role in specific clinical situations: folic acid in pregnancy, vitamin D for those with limited sun exposure or malabsorption, vitamin B12 for vegans and the elderly, iron for iron-deficiency anemia, and iodine where dietary sources are insufficient. The supplement industry is a multi-billion-dollar global market, but the majority of clinical trials have not demonstrated meaningful health benefits of multivitamin or antioxidant supplements in well-nourished populations. A high-quality, varied diet—rich in fruits, vegetables, whole grains, legumes, nuts, and lean proteins—remains the most evidence-based foundation for meeting micronutrient needs.