Essential and Non-essential Nutrients
Nutrients are frequently categorized as essential or nonessential. Essential nutrients are unable to be synthesized internally (either at all, or in sufficient quantities), and so must be consumed by an organism from its environment. Nonessential nutrients are those nutrients that can be made by the body, they may often also be absorbed from consumed food. The majority of animals ultimately derive their essential nutrients from plants, though some animals may consume mineral-based soils to supplement their diet.
For humans, these include essential fatty acids, essential amino acids, vitamins, and certain dietary minerals. Oxygen and water are also essential for human survival, but are generally not considered "food" when consumed in isolation.
Humans can derive energy from a wide variety of fats, carbohydrates, proteins, and ethanol, and can synthesize other needed amino acids from the essential nutrients.
Non-essential substances within foods can still have a significant impact on health, whether beneficial or toxic. For example, most dietary fiber is not absorbed by the human digestive tract, but is important in digestion and absorption of otherwise harmful substances. Interest has recently increased in phytochemicals, which include many non-essential substances which may have health benefits.
Read more about this topic: Nutrient
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