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Nutrients support energy production, structural integrity, or metabolic regulation. The six primary classes are carbohydrates, proteins, lipids, vitamins, minerals, and water. Macronutrients are required in gram quantities and include carbohydrates, proteins, lipids, and water; all except water can provide energy. Micronutrients—vitamins and minerals—are needed in smaller amounts and chiefly serve as coenzymes, cofactors, or regulators. Nutrients are also grouped by essentiality: essential nutrients must come from the diet, conditionally essential nutrients may need dietary supply in particular states, and non-essential nutrients can be synthesized sufficiently by the body. Water has important solvent, transport, and thermoregulatory roles. Nutritional status reflects intake and the body’s handling of nutrients, alongside individual factors; imbalance can present as undernutrition or overnutrition.