Nutrition is the process by which organisms obtain, digest, absorb, distribute, metabolise and utilise substances required for energy, growth, maintenance and physiological function. In human health, nutrition includes the intake of foods and beverages and the subsequent biological handling of their constituents. It encompasses energy balance, macronutrients, micronutrients, water, fibre and numerous bioactive food compounds. Nutrition is broader than simply counting calories because the composition, quality, quantity and timing of dietary intake influence different physiological processes. Nutritional requirements vary with age, body size, activity level, pregnancy, illness, genetics and other circumstances.
Human Nutrition provides energy primarily through metabolised carbohydrates, fats and proteins, although alcohol can also provide substantial energy when consumed. Carbohydrate and fat are major fuels for many tissues, while protein supplies amino acids required for tissue synthesis and numerous other functions. Energy balance describes the relationship between energy intake and energy expenditure over time, but body mass changes are not determined by a simple fixed daily equation because physiological adaptation can alter expenditure and substrate utilisation. Nutrition therefore involves both energy quantity and nutrient adequacy. A diet can provide sufficient energy while still being inadequate in particular vitamins, minerals or other essential components.
Nutritional requirements include essential substances that the body cannot produce in adequate amounts. Essential amino acids must be obtained through dietary protein, while essential fatty acids must be obtained through food because humans cannot synthesise them in sufficient quantities. Vitamins and minerals perform diverse roles in enzymatic reactions, cellular structures, fluid regulation, oxygen transport, bone metabolism and other processes. Water is also essential and participates in temperature regulation, circulation, digestion and cellular reactions. Requirements are generally expressed as population reference values rather than exact personal prescriptions because individual needs vary and cannot be determined solely from a single universal number.
Nutrition has a direct relationship with physical performance and exercise adaptation. Carbohydrate availability can influence performance during prolonged or high-intensity activity because carbohydrate metabolism can generate energy rapidly. Protein provides amino acids used for repair and adaptation after exercise, while dietary fat supports energy provision, cell membranes and the absorption of fat-soluble vitamins. Hydration affects thermoregulation and cardiovascular function, particularly during prolonged exercise or heat exposure. The nutritional demands of an endurance athlete, strength athlete, sedentary individual and person recovering from illness can therefore differ substantially even though all require the same broad classes of essential nutrients.
Dietary patterns can be evaluated at several levels, from individual foods to overall eating patterns. A nutritious diet generally needs sufficient energy and essential nutrients while remaining compatible with personal requirements and circumstances. Variety can increase the likelihood of obtaining a broad range of nutrients, while the nutritional value of individual foods depends on preparation, portion size and the rest of the diet. Nutrition also interacts with cultural, economic, environmental and behavioural factors. Food availability, cooking facilities, preferences, knowledge, budget and social context can all influence what people eat, meaning that nutritional recommendations must distinguish physiological principles from assumptions about how individuals live.
Nutrition science uses several concepts that should not be conflated. Nutrient density describes the quantity of useful nutrients relative to energy content, while energy density describes energy relative to food mass or volume. Adequacy concerns obtaining enough essential nutrients, whereas excess concerns intake beyond physiological requirements and sometimes beyond safe levels. A food being processed does not automatically make it nutritionally poor, and an unprocessed food is not automatically appropriate in unlimited quantities. Similarly, a single nutrient rarely determines the healthfulness of an entire dietary pattern. Evaluating overall intake is usually more informative than classifying individual foods as universally good or bad.
Nutritional deficiencies occur when dietary intake, absorption or utilisation fails to provide enough of a required substance. Excessive intake can also cause harm, particularly for certain vitamins, minerals and other bioactive substances. Some conditions alter absorption or metabolism and therefore change nutritional requirements even when food intake appears adequate. Supplements can be useful when a deficiency exists or when obtaining a particular nutrient through food is difficult, but supplementation is not synonymous with good nutrition. Nutritional interventions should therefore consider the specific nutrient, dose, duration, underlying cause and individual context rather than assuming that more intake is always beneficial.
Nutrition is also an interdisciplinary field involving physiology, biochemistry, epidemiology, food science, public health and behavioural science. Nutritional research can investigate molecular mechanisms, controlled dietary interventions, population patterns and clinical outcomes, and these approaches answer different questions. Associations observed in population studies do not automatically establish causal effects, while short-term controlled studies may not capture long-term behavioural realities. Understanding Nutrition accurately therefore requires distinguishing nutrient mechanisms, dietary patterns, individual interventions and population-level evidence rather than treating all nutrition claims as equivalent forms of evidence.
