An essential nutrient is a nutrient required for normal body functioning that can not be synthesized by the body. Categories of essential nutrient include vitamins, dietary minerals, essential fatty acids, phytochemicals and essential amino acids.
Different species have very different essential nutrients. Most essential nutrients are substances that are metabolically necessary but cannot be synthesized in sufficient quantity by the organism. Minerals, for example, cannot be synthesized at all in biological systems, so (for example) a human must obtain the iron they need to build hemoglobin from their diet (of course, this iron is recycled as long as possible, but some is inevitably lost, for example during menstruation).
Most essential nutrients are needed only in small quantities, and are stored and reused by the body. As a result, unlike absence of air or water for humans, absence of essential nutrients usually leads only gradually to the development of a deficiency disease .
Modern nutritional wisdom is that it is diffficult, if not impossible to isolate all of the elements in our food which are essential to good nutrition. Therefore nutritionists advise us to eat as wide a range of foods which are as little processed as possible. This gives us the best chance of regularly obtaining small amounts of a wide range of essential nutrients, and reduces the chance of poor nutrition and the ill health that may result. Further advice on essential nutrition will be based on the circumstances of the person, e.g. their age, occupation, even their location.
Some essential nutrients may be toxic in large doses; for example, an iron intake that is too high can lead to the production of free radicals beyond what the body's antioxidant system can handle. Others, including ascorbic acid, are very efficiently flushed from the body when not needed, and can safely be taken in enormous doses up to the point where the excretory system is overloaded (excess acidity in the kidneys and bladder, for example).