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Nutrients are substances in foods that the body needs to function, grow, repair tissue, reproduce, and maintain health. They supply energy, build and repair structures, regulate metabolism, and support immune and nervous system function.

Nutrients are commonly divided into macronutrients, which include carbohydrates, proteins, fats, and water, and micronutrients, which include vitamins and minerals. Macronutrients provide energy or serve as building blocks for the body's tissues, while micronutrients act as cofactors and regulators in numerous biological processes. Some nutrients are essential, meaning the body cannot synthesize them in adequate amounts and they must be obtained from the diet.

Certain nutrients are especially important for particular processes or life stages. For example, calcium and vitamin D support bone health, iron is critical for oxygen transport, folate is essential during pregnancy, and omega-3 fatty acids contribute to neuronal development and cardiovascular health. The relative importance of specific nutrients can vary with age, sex, health status, and activity level, making dietary emphasis often context dependent.

Dietary sources and intake guidelines are designed to help people meet nutritional needs. A varied diet based on fruits, vegetables, grains, proteins, and dairy or fortified alternatives typically provides most essential nutrients. Bioavailability, preparation methods, and interactions between nutrients can influence how much of a nutrient is absorbed and utilized. Official recommendations, such as recommended dietary allowances or adequate intakes, are issued by health authorities and differ by country to reflect population needs. Deficiency can lead to disease, while excess intake of certain nutrients can cause toxicity or other adverse effects.