Nutrition

Nutrition is the means by which organisms obtain and use nutrients. Nutrition is also the determination of the kinds and quantities of substances (nutrients) needed by organisms to sustain life. Some organisms such as plants require only a supply of light, water, and a few other molecules and ions in order to thrive, and are known as autotrophs, or self nourishers, for they literally build their own molecules and capture energy in the process. There are a few other non-plant autotrophic organisms in the deep oceans near geothermal vents that are able to build their own nutrients without using sunlight.

While green plants get the energy they need directly from sunlight, animals must get the energy that they need for life functions from plants.

The major types of molecules found in organisms are water, carbohydrates, proteins, lipids, and nucleic acids.

Proteins are large molecules built from different combinations of large numbers of amino acids. Of the 20 different amino acids that make up our body's proteins, we can build 12 from other foods, but there are eight amino acids that humans also need and which they cannot make, called essential amino acids. One of the eight essential amino acids (methionine) is found in corn but not in beans, and two others (lysine and tryptophan) are found in beans but in only small amounts in corn. Therefore, the combination of beans and corn, found in many Mexican foods, supplies a balance of the essential amino acids.

The present focus is primarily on human nutrition. The base of the U. S. Department of Agriculture Food Guide pyramid, the part with the greatest volume, consists of the cereal foods: bread, pasta, and rice. Data from around the world indicate that people whose diets have a large proportion of these foods tend to be healthier. This food group, mostly carbohydrates, should provide most of the energy needed. The recommendations are given in servings. A serving varies from food to food, but is in the range of 1-2 oz (30-60 g) per serving. Depending on physical size, age, gender, and activity, 6–11 servings per day are recommended from the cereal group.

The second level of the food pyramid consists of fruits and vegetables. These are especially important in supplying micronutrients. Micronutrients are elements that help regulate physiological pathways. A second benefit derived from this group comes from indigestible fiber, which is correlated with better functioning and health of the large intestine. Five to nine servings a day are suggested from this group.

The third level of the pyramid consisting of proteins in the form of meats, eggs, beans, nuts, and milk products is smaller than the first and second levels to emphasize that the percentage of these foods should be smaller in comparison to the total intake. The major function of A healthy diet includes a variety of foods. Photograph by Michael A. Keller. The Stock Market. Reproduced by permission. proteins is to repair or build new tissue and to supply enzymes and hormones.

At the tip of the pyramid are the lipids, representing a small volume, demonstrating that fats and oils should be consumed in small quantities for optimum health.

Micronutrients are subdivided according to the quantities needed in the human diet. If more than 100 milligrams (mg) per day of an element is needed, it is classed as a mineral. The seven essential minerals are calcium, magnesium, phosphorus, sodium, potassium, sulfur, and chlorine. Substances that are essential but needed in amounts of less than 100 mg per day are called trace elements. These are iron, copper, iodine, manganese, zinc, molybdenum, selenium, and chromium. There are likely others (such as boron) that are yet to be identified.

Calcium and phosphorus are both used structurally to form bones and teeth. Lesser known but vital functions of calcium include its uses as an enzyme activator and as a regulator of nerve and muscle activity. Phosphorous is also a component of nucleotide molecules that are structural components of the nucleic acids, DNA and RNA, and of the energy transfer molecules such as ATP, NAD, and FAD.

Magnesium is found in bones too, but, more importantly, it is needed in adequate amounts to work with calcium in regulating nerve and muscle activity. In fact, an early symptom of magnesium shortage can be irregular heart action. Sodium is necessary for the proper functioning of the nerves and muscles. Sodium is more concentrated in the fluids outside of the cells than the intraocular fluids. Sodium is more often consumed in excess because it is easy to enhance the flavors of foods by adding table salt (sodium chloride). Potassium ions are most concentrated in the fluid inside cells. There is evidence that high blood pressure can be prevented or controlled by increasing one's intake of potassium. Chloride functions as an electrolyte balance for the sodium and other positive ions in cell and tissue fluids and is necessary for the salivary enzyme ptyalin to help digest starch. Sulfur is a component of three amino acids and is part of the enzyme molecules active in the oxidation of fatty acids.

These minerals must be in their ionic form so as to be soluble and absorbable. Many minerals are often chelated, that is, attached to a larger organic particle such as an amino acid, or anions such as gluconate, lactate, or citrate to be properly observed. Some trace elements function in cooperation with an enzyme or a vitamin molecule to bring about physiological responses.

More and more, people are choosing foods for their vitamin or mineral content, because the foods provide specific nutrients that may otherwise be missing. Adding daily supplements of vitamins C, E, and A (or its precursor beta carotene) to diets may promote good health and fight disease.