A dietary supplement is a product intended to supplement the diet by providing nutrients or other substances with nutritional or physiological effects. Depending on the jurisdiction, products may contain vitamins, minerals, amino acids, fatty acids, fibre, plant-derived substances, microorganisms or other ingredients. They are commonly supplied as tablets, capsules, powders, liquids, gummies or similar preparations. Dietary supplements are different from ordinary foods because they are concentrated products designed to deliver specific ingredients in measured amounts. They are also distinct from medicines in regulatory classification, although some supplement ingredients have pharmacological activity and can influence bodily processes. Definitions and regulatory requirements vary between countries, so the legal meaning of dietary supplement is jurisdiction-dependent.
Vitamins and minerals are among the most familiar supplement ingredients. Examples include vitamin D, vitamin B12, folic acid, iron, calcium, magnesium and zinc. Other products contain omega-3 fatty acids, protein, creatine, probiotics, botanical extracts or combinations of many ingredients. A supplement can be used to correct or prevent a nutrient deficiency, meet increased nutritional requirements in particular circumstances or provide an ingredient that is difficult to obtain in sufficient quantity from the diet.
Some supplements have well-established medical or public-health uses, while others are marketed for broad purposes such as energy, immunity, weight management, athletic performance or general wellbeing with varying levels of evidence. The presence of a nutrient in a product does not itself establish that supplementation is beneficial for a person who already receives enough from food.
The distinction between nutrient adequacy and supplementation is fundamental. Essential nutrients are required for normal physiological function, but the relationship between intake and health is not always linear. If intake is insufficient, increasing it can correct deficiency and restore normal physiological processes. Once adequate levels are reached, additional amounts may provide little or no extra benefit, and excessive amounts can sometimes cause harm.
Fat-soluble vitamins such as vitamins A, D, E and K can accumulate in the body more readily than many water-soluble vitamins, although water-soluble vitamins can also cause toxicity at sufficiently high doses. Minerals can likewise be harmful in excessive quantities. Supplementation should therefore be based on nutritional need, risk of deficiency, clinical circumstances and evidence rather than the assumption that more nutrients necessarily produce better health.
Dietary supplements can be useful in clearly defined situations. Examples include vitamin B12 supplementation for people with inadequate intake or impaired absorption, folic acid supplementation around pregnancy to reduce the risk of neural-tube defects, vitamin D supplementation in circumstances where intake or sunlight exposure is insufficient, and iron treatment for confirmed iron deficiency. Some people have increased requirements or absorption problems because of particular medical conditions, diets, medications or physiological states.
In such situations, supplementation can be an important part of healthcare. However, treatment of a deficiency may require a specific dose and duration that differs substantially from the amount found in an ordinary multivitamin. Laboratory testing may also be appropriate when deficiency is suspected, particularly for nutrients where excess supplementation has meaningful risks.
Supplement quality is an important consideration. The amount stated on a label may not fully capture differences in chemical form, bioavailability, stability or contamination. Herbal products can contain numerous active compounds and may vary according to species, cultivation, extraction and processing. Some products marketed as supplements have been found in various markets to contain undeclared pharmaceutical ingredients or contaminants. Quality-assurance schemes and regulatory controls can reduce but not necessarily eliminate these concerns. Storage conditions can also affect stability. Consumers should therefore consider the ingredient, dose, formulation, manufacturer, regulatory status and reason for use rather than assuming that all supplements are equivalent.
Supplements can interact with medicines and medical conditions. Iron can interfere with absorption of certain medicines, calcium can affect absorption of some drugs, vitamin K can interact with anticoagulation management, and several herbal products can alter drug metabolism. High-dose supplementation can also affect laboratory tests or mimic disease-related abnormalities. Some supplements are inappropriate in particular situations such as pregnancy, kidney disease, liver disease or disorders of mineral metabolism unless professionally supervised. People preparing for surgery may also need to disclose supplement use because some products can influence bleeding, blood pressure, sedation or other physiological processes. A complete medication history should therefore include non-prescription supplements.
Dietary supplements should not generally be regarded as substitutes for a balanced diet. Foods provide mixtures of nutrients, fibre and other bioactive compounds and can contribute to healthy dietary patterns in ways that an isolated nutrient cannot reproduce. Supplements can be valuable when diet alone does not provide sufficient amounts or when a medically established indication exists, but taking multiple products can increase cost and the risk of excessive intake or interactions. Multivitamins are not automatically necessary for everyone, and supplement use should be guided by evidence rather than marketing claims. Statements such as natural, pharmaceutical-grade, detoxifying or immune-boosting do not independently establish clinical benefit.
The term dietary supplement therefore describes a broad class of concentrated nutritional or physiological products rather than a single type of treatment. Their value depends on the ingredient, dose, person's nutritional status, underlying health, evidence for the intended purpose and potential risks. Some supplements have clear and important roles in preventing or treating deficiency, while others provide little demonstrated benefit for people who are already nutritionally adequate. Responsible use involves checking whether supplementation is actually needed, choosing an appropriate preparation and dose, considering interactions and contraindications, and recognising the distinction between correcting deficiency and attempting to enhance health beyond normal nutritional adequacy.
