Human nutrition and digestion

Nutrition 
The body requires many different types of organic mole­cules and various types of inorganic ions and molecules in the diet each day. Laypersons tend to think of the term or­ganic as meaning that the food was grown without the use of pesticides, but it actually refers to molecules associated with living things. Our bodies primarily contain carbohy­drates, fats, proteins, and nucleic acids, the organic mole­cules. Inorganic molecules are associated with nonliving things; for example, the ocean contains salt (sodium chloride). We all know, however, that sodium chloride plays a significant role in the body.
Nutrition involves an interaction between food and the living organism, and a nutrient is a substance that the body uses to maintain health.
Following digestion, nutrients enter the blood in the circulatory system, which distributes them to the tissues, where they are utilized by the body's cells. Mitochondria use glucose to produce a constant supply of ATP for the cell. In other words, glucose is the body's immediate en­ergy source. Since the brain's only source of energy is glu­cose, it needs a constant supply.
The liver is able to chemically alter ingested fats to suit the body's needs, with the exception of linoleic acid, a fatty acid that it is unable to produce. Since linoleic acid is re­quired for construction of plasma membranes, it is consid­ered an essential fatty acid. Essential molecules must be present in food because the body is unable to manufacture them.
If glucose is not available, fats can be metabolized into their components, which are then used as an energy source. Therefore, fats are said to be a long-term energy source. When adipose tissue cells store fats, the body increases in weight. Cells have the capability of converting excess sugar molecules into fats for storage, which accounts for the fact that carbohydrates can also contribute to weight gain.
Amino acids from protein digestion are used by the cells to construct their own proteins, including the en­zymes that carry out metabolism. Protein formation re­quires 20 different types of amino acids. Of these, nine are required in the diet because the body is unable to produce them. These are termed the essential amino acids. The body produces the other amino acids by simply transform­ing one type into another type. Some protein sources, such as meat, are complete in the sense that they provide all the different types of amino acids. Vegetables supply the body with amino acids, but they are incomplete sources because at least one of the essential amino acids is absent. A com­bination of certain vegetables, however, can provide all of the essential amino acids.
Vitamins 
Vitamins are vital to life because they play essential roles in cellular metabolism. Since the body is unable to pro­duce them, vitamins must be present in the diet. Vita­mins are organic molecules, but they differ radically from carbohydrates, fats, and proteins. They are much smaller in size and are not broken down to be used as building blocks or as a source of energy. Instead, the body protects them and provides many of them with protein carriers that transport them in the blood to the cells. In the cells, vitamins become helpers in metabolic processes that break down or synthesize other organic molecules. Be­cause vitamins can be used over and over again, they are required in very small amounts only.
Vitamins fall into two groups: fat-soluble vitamins (vi­tamins A, D, E, and K) and water-soluble vitamins (the B complex vitamins and vitamin C). Most of the water-soluble vitamins are coenzymes, or enzyme helpers, that help speed up specific reactions. The functions of the fat-soluble vitamins, some of which have been previously discussed, are more specialized. Vitamin A, as noted in chapter 9, is used to synthesize the visual pigments. Vita­min D is needed to produce a hormone that regulates cal­cium and phosphorus metabolism. Vitamin E, as discussed in the Medical Focus reading on this page, is an antioxidant. Vitamin K is required to form prothrombin, a substance necessary for normal blood clot­ting.
Minerals 
In contrast to vitamins, minerals are inorganic elements. An element, you will recall, is one of the basic substances of matter that cannot be broken down further into simpler substances. Minerals sometimes occur as a single atom, in contrast to vitamins, which contain many atoms, and carbohydrates, such as starch, which contain thousands of atoms. Minerals cannot lose their identity, no inatter how they are handled. Because they are indestructi­ble, no special precautions are needed to preserve them when cooking.
Minerals are divided into macro nutrients, which are needed in gram amounts per day, and micronutrients (trace elements), which are needed in only microgram amounts per day. The macro nutrients sodium, magne­sium, phosphorus, chlorine, potassium, and calcium serve as constituents of cells and body fluids, and as structural components of tissues. The micronutrients have very spe­cific functions. As research contin­ues, more elements will be added to the list of those considered essential.