Micronutrients are vitamins and minerals required by the human body in relatively small quantities compared with macronutrients. Despite their small quantitative requirements, they are essential for numerous biological processes, including energy metabolism, DNA synthesis, immune function, nervous-system activity, bone maintenance, blood formation and antioxidant defence. Vitamins are organic compounds, whereas minerals are inorganic elements. Micronutrients generally do not provide meaningful amounts of energy themselves, but they enable and regulate processes through which energy-yielding nutrients are metabolised and used.
Vitamins are commonly divided into fat-soluble and water-soluble groups. Fat-soluble vitamins include A, D, E and K and can be stored in body tissues, particularly the liver and adipose tissue. Water-soluble vitamins include vitamin C and the B-group vitamins, which generally have more limited storage, although exceptions exist. These differences affect absorption, transport, excretion and the potential consequences of excessive intake. Mineral micronutrients include elements such as iron, calcium, magnesium, zinc and iodine, each of which has distinct physiological functions and recommended intake levels.
Micronutrients frequently function as enzyme cofactors or components of enzymes and other proteins. B vitamins participate in numerous metabolic reactions, iron is central to haemoglobin and several electron-transfer proteins, iodine is required for thyroid hormone synthesis, and calcium contributes to bone structure and cellular signalling. Zinc participates in many enzymes and regulatory proteins, while magnesium is involved in numerous reactions involving ATP and nucleic acids. These examples demonstrate why a micronutrient can be required in a very small quantity yet have effects throughout multiple physiological systems.
Deficiency can occur when intake is inadequate, absorption is impaired, requirements increase or losses become excessive. Iron deficiency can impair haemoglobin production and oxygen transport, while inadequate vitamin D can contribute to impaired bone mineralisation under appropriate circumstances. Severe deficiencies can produce characteristic clinical syndromes, whereas marginal deficiency may be less obvious and can manifest through changes in physiological function. Nutritional deficiencies should not be diagnosed solely from symptoms because many symptoms are non-specific. Appropriate assessment may require dietary history, laboratory testing and clinical evaluation.
Excess micronutrient intake can also be harmful. Toxicity is particularly relevant when concentrated supplements are consumed at high doses because obtaining excessive amounts from ordinary foods is generally more difficult for many nutrients. Fat-soluble vitamins can accumulate, while some minerals can cause adverse effects when intake greatly exceeds physiological needs. Water-soluble vitamins are not universally harmless at high doses either, because some can produce toxicity or other adverse effects despite substantial renal excretion. Consequently, micronutrient supplementation should be based on a specific nutritional rationale rather than the assumption that higher intake necessarily improves health.
Bioavailability is especially important when evaluating mineral intake. The chemical form of a mineral, food composition and interactions with other dietary compounds can alter absorption. Some nutrients enhance the absorption of others, while compounds in foods can reduce absorption under particular circumstances. The body's regulatory mechanisms can also alter absorption according to nutritional status. Thus, nutrient content tables provide important information but do not completely describe physiological availability. This distinction is relevant when interpreting diets and when assessing why an apparently adequate intake may not always result in adequate nutritional status.
Micronutrient needs change across the lifespan and under particular physiological conditions. Growth, pregnancy, lactation, ageing and some illnesses can alter requirements or metabolism. Population reference values are therefore often separated by age and sex and may include specific values for pregnancy or lactation. These values are designed to cover the needs of most healthy people within a defined population rather than predict the exact requirement of every individual. Clinical conditions can require further adjustment, particularly where absorption, renal function or metabolism is altered.
Micronutrients are best considered within the context of the entire diet. Fruits, vegetables, grains, legumes, dairy products, meat, fish, eggs, nuts and seeds can each contribute different combinations of vitamins and minerals. Fortified foods can also provide substantial amounts of particular micronutrients, and supplementation can be appropriate in defined circumstances. However, micronutrients do not compensate for inadequate protein, carbohydrate, fat, water or total energy. Their importance lies in enabling normal physiological function, meaning that adequate nutrition requires both sufficient micronutrient supply and an appropriate overall dietary pattern.
