Nutrient

Nutrients are chemical substances required by living organisms to sustain life, enable growth, support reproduction, and maintain normal physiological functions. These substances are obtained through dietary intake in animals, plants, fungi and protists, and participate in metabolic processes that generate energy, repair tissues, regulate biochemical pathways and contribute to structural integrity. Nutrients may be incorporated into cellular components or excreted to form non-cellular structures such as hair, feathers, scales or exoskeletons. Some nutrients are broken down into smaller molecules during metabolism, releasing energy and producing end-products such as carbon dioxide and water. Although nutrient requirements differ widely among organisms, all require water as a universal medium for biochemical reactions and homeostasis.

Organic and Inorganic Nutrients

Nutrients are commonly classified as either organic or inorganic substances. Organic nutrients are carbon-containing compounds, including proteins, fats, sugars and vitamins, which participate in energy metabolism or serve as precursors to complex biomolecules. Inorganic nutrients do not contain carbon; they include elements such as iron, zinc and selenium that support enzymatic activity, oxygen transport and structural functions.
Organisms vary in their ability to synthesise certain nutrients. For example, humans require dietary ascorbic acid (vitamin C), whereas most animals and plants are capable of producing it endogenously. This diversity of synthetic capability explains interspecies variations in nutrient essentiality.

Macronutrients

Macronutrients are needed in relatively large quantities, typically gram or ounce amounts, and provide structural components or energy necessary for growth, repair and physiological maintenance. Chemically, humans consume six essential elements in large amounts—carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur (collectively known as CHNOPS). Major macronutrient compounds include carbohydrates, proteins, fats and water. Additionally, several minerals consumed in comparatively high quantities, such as sodium, potassium, magnesium, chloride, phosphorus and sulphur, are often grouped with macronutrients due to their functional importance.
CarbohydratesCarbohydrates are compounds composed of sugar units. They include:

• monosaccharides such as glucose and fructose,
• disaccharides such as lactose and sucrose,
• oligosaccharides, and
• polysaccharides such as starch, cellulose and glycogen.These compounds serve as rapid and long-term energy sources, depending on their molecular complexity.

ProteinsProteins consist of amino acids linked by peptide bonds. Because humans cannot synthesise all amino acids, several must be acquired from food as essential amino acids. Proteins are digested by proteases into constituent amino acids, which are then used for tissue synthesis, enzyme production and regulatory functions.
FatsFats are composed of a glycerol backbone attached to three fatty acids. Fatty acids may be saturated (containing only single bonds) or unsaturated (containing one or more double bonds). Fats provide dense energy reserves, support thermal insulation, and maintain cell membrane integrity. Certain fatty acids, known as essential fatty acids, must be consumed because the human body cannot manufacture them.
AlcoholAlthough not an essential nutrient, alcohol provides energy. Standard nutritional calculations assign a specific caloric value per gram or millilitre of ethanol, contributing to total dietary energy intake.

Micronutrients

Micronutrients are required in much smaller quantities—typically milligrams or micrograms—yet they are vital for metabolic regulation, enzymatic function and cellular maintenance. They include vitamins and mineral elements.
MineralsDietary minerals originate from the Earth’s crust and are absorbed by plants, entering the human diet either directly or through animal products. Essential minerals include elements such as potassium, sodium, calcium, iron, iodine, zinc, copper and selenium. These minerals regulate processes such as nerve conduction, oxygen transport, osmotic balance, and enzyme activation.
VitaminsVitamins are organic compounds required in small amounts to prevent deficiency disorders and maintain metabolic processes. Many exist in multiple forms known as vitamers, which collectively perform the vitamin’s biological role. Humans require thirteen vitamins, including vitamins A, C, D, E and K, and the B-complex group (B₁, B₂, B₃, B₅, B₆, B₇, B₉ and B₁₂). Vitamin D is considered conditionally essential because it can be synthesised in the skin with adequate exposure to ultraviolet light.

Essential Nutrients

Essential nutrients are those that cannot be synthesised by the body in adequate amounts and must therefore be acquired from the diet. Except for water, which is universally essential, these nutrients support fundamental cellular processes and organ function. For humans, essential nutrients include nine amino acids, two fatty acids, thirteen vitamins, fifteen mineral elements and choline. Additional molecules may become conditionally essential in certain developmental stages, diseases or metabolic disorders.
Essential Amino AcidsHumans require nine essential amino acids: phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine and histidine. These amino acids must be ingested regularly to maintain protein synthesis and nitrogen balance.
Essential Fatty AcidsOnly two fatty acids are recognised as essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). These support neurological development, inflammatory regulation and cell membrane composition.
Vitamins and VitamersVitamins perform roles such as regulating metabolism, acting as enzymatic cofactors or functioning as antioxidants. Their absence results in characteristic deficiency diseases. For instance, inadequate vitamin C leads to scurvy, while insufficient vitamin B₁₂ causes pernicious anaemia.
MineralsAlthough elements such as carbon, hydrogen, oxygen and nitrogen are essential for life, they are consumed in such abundance that dietary recommendations do not apply. Instead, mineral requirements relate to trace and ultratrace elements, including molybdenum, chromium, iodine, manganese and others. Cobalt is essential indirectly as a component of vitamin B₁₂.

Nutrient Deficiency and Excess

A deficiency arises when intake of an essential nutrient is insufficient or when absorption is impaired by disease. Deficiency states reduce growth capacity, compromise survival and impair reproductive success. Public health authorities provide dietary guidelines, such as the Dietary Reference Intake in the United States, outlining recommended intakes and tolerable upper limits for both macro- and micronutrients to avoid deficiency and toxicity.
Many countries require food labels to report significant amounts of macro- and micronutrients to guide consumer choices. Excessive intake of nutrients beyond physiological needs may cause adverse effects, including toxicity from fat-soluble vitamins or mineral overload.