If a nutrient is to be regarded as an essential contributor to the diet then that substance or group of substances must meet certain absolute criteria in human nutrition.

1. There is no synthesis or absorption of that substance and the sole source is from the diet.
2. That an insufficiency of this nutrient leads to failure to thrive and ill health at all phases of life and function.
3. Implicit in the concept of essential is that it is possible to define minimal dietary intakes of this nutrient to maintain health, at all stages of life and function.

It is reasonable to regard the following as essential for life:

• Ambient temperature.
• Oxygen
• Water
• Ambient temperature ( between 0 and 30o C), but creates a climate where life is possible and food can grow.
• Energy ( calories)

Essential nutrients are

• Protein nitrogen and essential amino acids
• Essential fatty acids
• Vitamins
• Trace elements.

Energy is derived from a mix of nutrients primarily carbohydrates and fats eaten in varying amounts, dependent upon availability and individual taste and culture. There is a massive number of plant secondary metabolites which have varying properties as antioxidants and other functions but are not seen as essential. Some essential nutrients eg vitamin B12 are synthesised by colonic bacteria but are not absorbed and are lost in the faeces.

Nutrients which only in part meet the criteria of essential —
Where there are limiting factors in the availability of these nutrients during increased demand, for example in the new born baby, growth, pregnancy, and illness. These nutrients have been called indispensable, conditionally –indispensable, conditionally dispensable or dispensable dependent upon the robustness of the provision of the nutrient. Indispensable implies the same as essential, the other terms are in decreasing degree of absolute need. Amino acids and Polyunsaturated fatty acids come into this category.

Nutrients who in no respect meet the criteria as being essential —
The chemical structure of these nutrients in this category are freely exchangeable both from a synthetic and for metabolic needs eg glucose, palmitic acid and glycine. However they may, as carbohydrate or fat be indispensable as contributors to the energy requirements of the body.