Nutrients are substances obtained from food and other dietary sources that the body requires for energy, structural development, physiological regulation or maintenance. They are commonly classified as macronutrients and micronutrients according to the quantities generally required, with water and dietary fibre often discussed separately because of their distinctive physiological roles. Nutrients include carbohydrates, fats, proteins, vitamins and minerals, while some nutritional frameworks also consider water an essential nutrient. Each nutrient has specific biochemical functions, although nutrients rarely operate independently. Physiological processes depend on networks of nutrients interacting through metabolic pathways, cellular structures and regulatory systems.
Macronutrients are required in relatively large quantities and include carbohydrate, protein and fat. They supply energy in different circumstances and also serve structural and functional purposes. Micronutrients, principally vitamins and minerals, are required in smaller quantities but can be indispensable because they participate in enzyme reactions, gene regulation, antioxidant systems, mineralised tissue formation, oxygen transport and many other processes. The distinction between macro and micro refers primarily to quantity required rather than importance. A micronutrient deficiency can have severe consequences even though the absolute amount required is small, while excessive intake of a micronutrient can also be harmful.
Nutrients can be essential, conditionally essential or non-essential depending on the body's capacity to synthesise them and the circumstances. Essential nutrients cannot be produced in sufficient quantities and must therefore be supplied through diet. Some substances become conditionally essential during growth, illness or physiological stress when normal synthesis is insufficient. Non-essential does not mean biologically irrelevant; it generally means that the body can synthesise enough under ordinary conditions. The distinction is therefore about dietary necessity rather than whether a substance participates in human physiology.
Nutrients undergo digestion, absorption, transport, storage and metabolism. Digestive processes break many food components into absorbable molecules, although certain compounds are absorbed differently or are incompletely digested. Once absorbed, nutrients can enter circulation, be stored in tissues, incorporated into structural molecules or metabolised for energy. Vitamins and minerals can act as enzyme cofactors, signalling molecules or structural components without necessarily providing energy themselves. The body's handling of nutrients is regulated by hormonal, neurological and cellular mechanisms that respond to nutritional status and physiological demand.
Nutrient bioavailability describes the proportion of an ingested nutrient that is absorbed and available for physiological use. It can be affected by food composition, chemical form, digestion, intestinal function and interactions with other substances. For example, the absorption of some minerals varies according to the food matrix and the presence of compounds that enhance or inhibit absorption. Fat-soluble vitamins are absorbed differently from water-soluble vitamins and can accumulate in body tissues, which contributes to different deficiency and toxicity patterns. Consequently, the amount listed on a food label does not always correspond directly to the amount ultimately available to cells.
Nutrients interact extensively. Protein metabolism depends on adequate energy availability, mineral cofactors are required for numerous enzymes, and vitamins can participate in pathways involved in macronutrient metabolism. Calcium and vitamin D are interconnected in bone and mineral metabolism, while iron availability affects haemoglobin synthesis and oxygen transport. Such interactions mean that nutritional adequacy cannot always be inferred from isolated nutrient quantities. A diet containing adequate calories can still produce deficiency if particular nutrients are consistently absent, while a diet high in one nutrient does not necessarily compensate for inadequate intake of another.
Nutrient deficiencies can arise from insufficient intake, impaired absorption, increased physiological requirements, excessive losses or altered metabolism. Symptoms vary according to the nutrient and severity, and some deficiencies develop gradually because body stores buffer short-term reductions in intake. Conversely, excessive consumption can cause toxicity, particularly with nutrients that accumulate in tissues or are consumed in concentrated supplemental forms. Nutritional assessment therefore considers both deficiency and excess. Blood tests, dietary assessment, clinical signs and other investigations can be used depending on the nutrient and clinical question, but no single method universally measures nutritional status.
Understanding Nutrients requires distinguishing nutrients from foods and dietary patterns. A food is a complex mixture containing many nutrients and other compounds, while a nutrient is a particular substance or group of substances with defined biological functions. Nutritional science consequently studies both individual nutrient mechanisms and how nutrients are delivered through whole diets. Public-health recommendations generally aim to promote adequate nutrient intake without requiring people to calculate every nutrient precisely. The practical significance of nutrient knowledge lies in understanding why dietary composition matters, how deficiencies and excesses develop and why nutritional requirements differ among individuals and circumstances.
