In order to live, animals must consume nutrients, of which there are five major classes: carbohydrates, proteins, lipids or fats, vitamins, and minerals. In addition to these constituents, of course, animal life requires other materials for its sustenance-water, oxygen, and fiber, which aids in the digestive processing of foods-but these components usually are not regarded as nutrients.
Nutrition itself is the series of processes by which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their processing in the bodies of organisms. Here the general term organism has been used, but for the most part the present essay is concerned with animal nutrition, or at least the nutrition of primary consumers (animals that eat plants) and secondary consumers (animals that eat other animals).
By contrast, plants and a few other types of organism are autotrophs, or primary producers in the food web. Autotroph means "self-feeder," and these organisms are distinguished by the fact that they do not depend on other organisms as a source of energy. Instead, plants obtain energy from the Sun and carbon dioxide from the atmosphere, and from these materials they build the large organic molecules that they need to survive.
Though plants are the most obvious example of an autotroph, they are not the only ones. In the deep oceans, far from any plant life, primary consumers depend on phytoplankton, which are microscopic organisms that encompass a range of bacteria and algae. Nonplant autotrophs may use means different from those employed by plants in generating their own food. For example, there are certain nonplant autotrophic organisms that live in the deep oceans near hydrothermal vents, which are cracks in the ocean floor caused by volcanic activity. These organisms, unlike most autotrophs, do not need sunlight to survive. Instead, they build their own nutrients in a sunless world, using sulfur compounds found near the vents.
An element is a chemical substance made of only one kind of atom, whereas in a compound, atoms of more than one element are chemically bonded to one another. Unlike compounds, elements cannot be broken chemically into other substances. There are approximately 90 elements that occur in nature, and many of these elements—but not nearly all—are important to nutrition.
Even when we rule out obviously harmful elements, such as lead or uranium, there are still numerous chemical elements that play a part in the nutrition of living things. This can be illustrated by a glance at the abundance of various chemical elements in the human body, which include oxygen, carbon, and hydrogen. Oxygen alone accounts for a whopping 65% of the human body's mass, and carbon (18%), hydrogen (10%), and oxygen together make up 93% of the mass in the human body.
A great deal of oxygen and hydrogen, of course, is found in that most useful of all chemical compounds, water. In this vein, it should be noted that all the elements that take part in biogeochemical cycles, which are essential to the functioning of Earth, appear in relatively large proportions within the human body. These elements are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. (For more about biogeo-chemical cycles and the elements involved in them, including their proportion within the human body's mass, see The Biosphere.)
Carbon is present in all living things, and its presence in certain forms is key to distinguishing organic from inorganic substances. Contrary to popular belief, organic substances are not just living things, their parts, and their products. Something that has never been living still can be considered organic, provided that it contains compounds that include carbon. (The only exceptions would be carbonates and carbon oxides, two groups of carbon-based compounds that are excluded from the ranks of organic substances.) As we shall see, carbon, along with oxygen and hydrogen, plays a key role in nutrition.
Most of the remaining 7% of the body's mass is composed of ten other elements. Among these elements are the other three involved in biogeochemical cycles, whose names are italicized: nitrogen (3%), calcium (1.4%), phosphorus (1.0%), magnesium (0.50%), potassium (0.34%), sulfur (0.26%), sodium (0.14%), chlorine (0.14%), iron (0.004%), and zinc (0.003%). Note that many of these elements are found in vitamin and mineral supplements that people might take on a daily basis to augment the essential nutrients in their bodies. There are exceptions, however, such as sodium, of which most people already ingest too much in the form of salt.
Generally speaking, it is safe to assume that any element that appears naturally in the human body is healthful as a nutritional component. This rule of thumb goes only so far, however: chlorine, for instance, is poisonous in large quantities, whereas in the very small proportions found in the human body, it can be essential to health and well-being. It is certainly possible to ingest some elements in unhealthy quantities, a fact that is particularly true of trace elements.
Copper is an example of a trace element, so named because only traces of them are present in the human body. In tiny quantities, copper is beneficial to human health, but if that small amount is exceeded, the effects can run the gamut from sneezing to diarrhea. In the proper proportions, however, trace elements are essential: without enough iodine, for instance, goiter, a large swelling of the thyroid gland in the neck area, can develop. Chromium helps the body metabolize sugars, which is why people concerned with losing weight or toning their bodies through exercise may take a chromium supplement. Even arsenic, which is lethal in large quantities, is a trace element in the human body, and medicines for treating such illnesses as "sleeping sickness" contain tiny amounts of arsenic. Other trace elements include cobalt, fluorine, manganese, molybdenum, nickel, selenium, silicon, and vanadium.
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