Humans need vitamin C from food while most animals simply make their own
Most animals and many plants manufacture ascorbic acid for themselves, but people must eat it. That quirk captures the idea of an essential nutrient: something the body requires yet cannot build, at least not in adequate amounts. For humans the list runs to nine amino acids, two fats, thirteen vitamins, fourteen minerals and choline.
A nutrient is any substance an organism uses to survive, grow and reproduce. Cells burn some for energy, breaking carbohydrates, fats and proteins down to water and carbon dioxide, and use others to build tissue or to make structures outside the cell such as hair, scales, feathers and shells. Every organism needs water. Plants draw a wider range of minerals up through their roots and take in carbon dioxide and oxygen through their leaves, while fungi feed on living or dead organic matter.
For animals, nutrients are often split by quantity. Macronutrients, eaten by the gram, are carbohydrates, fats, proteins and water, along with minerals like calcium, sodium and potassium needed in bulk. Micronutrients, measured in milligrams or micrograms, play subtler roles in processes such as nerve signalling. Minerals come ultimately from soil, reaching us through plants or animals that ate them. The importance of each vitamin was discovered when its absence from the diet was found to cause disease.
The essential amino acids are the nine of twenty that human cells cannot make, including tryptophan, lysine and histidine. Only two fatty acids are essential, one omega-3 and one omega-6. Vitamin D is a partial exception, since skin makes it with enough ultraviolet light. Choline was long left off the list because the body produces a little, until people on diets lacking it developed liver and muscle damage. Other molecules, including taurine and glutamine, become conditionally essential in premature babies or during illness or rapid growth.
Some things in food matter without being essential. Fibre escapes digestion, but gut bacteria ferment the soluble kind into short-chain fatty acids. Ethanol supplies about 7 kilocalories per gram while providing no essential nutrients, which is why alcoholic drinks are described as empty calories. Plants also hold thousands of phytochemicals whose effects remain poorly understood.
Source: Nutrient
