Nutrient
A
nutrient is any element or compound necessary for or contributing to an organism's
metabolism, growth, or other functioning. Six nutrient groups exist, classifiable as those that provide energy, and as those that otherwise support metabolic processes in the body:Some of them are
essential because they cannot be synthesized in the body and must be obtained from a food source.
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Carbohydrates are compounds made up of
sugars. Carbohydrates are classified by their number of sugar units:
monosaccharides (such as
glucose and
fructose),
disaccharides (such as
sucrose and
lactose),
oligosaccharides, and
polysaccharides (such as
starch,
glycogen, and
cellulose).
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Proteins are organic compounds that consists of
amino acids joined by
peptide bonds. The body does not manufacture certain amino acids (termed essential amino acids); the diet must supply these. In nutrition, proteins are broken down through
digestion back into free amino acids.
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Fats consist of a
glycerin molecule with three
fatty acids attached. Fatty acids are unbranched hydrocarbon chains, connected by single bonds alone (
saturated fatty acids) or by both single and double bonds (
unsaturated fatty acids). Fats are needed to keep cells functioning properly, to insulate body organs against shock, to keep body temperature stable, and to maintain healthy skin and hair. The body does not manufacture certain fatty acids (termed essential fatty acids); the diet must supply these.
Fat has an
energy content of 9 kcal/g (~37.7 kJ/g);
proteins and
carbohydrates 4 kcal/g (~16.7 kJ/g).
Ethanol (
grain alcohol) has an energy content of 7 kcal/g (~29.3 kJ/g).
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Minerals are generally trace elements, salts, or ions such as copper and iron. These minerals are essential to human metabolism.
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Vitamins are organic compounds essential to the body. They usually act as coenzymes for various proteins in the body.
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Water is an essential nutrient and is directly involved in all the chemical reactions of life.
Any classification of "nutrients" is likely to be arbitrary given the status of
nutrition as a developing science. Researchers are becoming more aware of a wider range of nutrients essential for health.
An organism will metabolise any organic compound to use for its energy content, for structural purposes (growth or replacement of living structures), or for participation in chemical reactions necessary for life. Any particular substance can play more than one role in the body, though researchers lack a good understanding of these roles.
The discovery of the group of nutrients called
phytonutrients reinforces the provisional nature of our knowledge. We know little about phytonutrients, organic compounds from plants, which play an essential role in the normal functioning of a body and have complex hormonal effects on health, or play an active role in the amelioration of disease. They do not fit readily into the scheme of the traditional nutrition categories.
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The band of a green alga (Enteromorpha) along this shore indicates that there is a nearby source of nutrients (probably nitrates or ammonia from a small estuary). |
While in essence true to the definition above, the term
nutrients has a more limited meaning within the specialised fields of
water quality and
water pollution, referring specifically to
plant fertilizers. In this context, certain mineral compounds can have an adverse impact on water quality because of their ability to promote plant and
algae growth. An excessive growth of aquatic plants can clog waterways (see
giant salvinia for example), and over-stimulation of algae and
microbes leads to an
ecological process called
eutrophication.
A surprisingly small number of
elements provide interest or concern in this context: really just
nitrogen and
phosphorus in most aquatic systems.
Mineral compounds involved are
ammonia,
nitrites,
nitrates, and
orthophosphates. Organic compounds also may contribute, in as much as they also contain nitrogen and phosphorus. The reason only a few chemicals are of concern has to do with the fact that plants are made up mostly of compounds of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P), and lesser amounts of sulfur (S), potassium (K), magnesium (Mg), and calcium (Ca). These elements constitute the
macronutrients. Many other elements, though necessary for growth, classify as
micronutrients due to the very small quantities required.
Plants obtain carbon, hydrogen, and oxygen (elements most needed for growth) from the air and water, where all three elements occur in great abundance as
water and as
carbon dioxide. Nutrients having greatest potential to influence plant growth in aquatic environments is those elements needed for plant growth in proportionately large amounts (that is, macronutrients) but likely to become
limiting—that is, present in amounts that could be depleted by continued growth. Once used up, further growth will not be possible. Of the nine macronutrients, nitrogen and phosphorus are most likely to become limiting. The others always remain present in great abundance (C, H, O) or usually in amounts that exceed the requirements of aquatic plants or algae.
Farmers apply fertilizer nutrients in the form of nitrogen, phosphorus, and potassium (N, P, and K with perhaps micronutrients) to prevent these elements from becoming limiting in the soil. These elements become concentrated in wastewaters from animal pens and
septic or sewage systems. And these elements (especially N and P) in
runoff or wastewater discharges reaching streams, lakes, or seas will promote aquatic plant growth. Abundant plant growth itself gives cause for concern in assessing water quality. The most abundant "plants" in most aquatic environments are algae. When essential nutrients are plentiful, algae multiply. If these algae are microscopic
phytoplankton, their growth increases the
turbidity of the water. The water then becomes cloudy, colored a shade of green, yellow, or brown (sometimes red; see
algal bloom). A super abundance of algae, or of higher plants, in an aquatic system can signal excessive inputs of nutrients.I am a human
*Donatelle, Rebecca J.
Health: The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005. ISBN 0805328521
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Nutrient density*
Recommendations for human diet
