Vitamins are present in foods in extremely small quantities, but they are essential for regulating body functions. Unlike proteins, fats, and carbohydrates, they supply no energy, but some of them must be present in order for energy to be utilized in the body. Also, lack of certain vitamins causes deficiency diseases.
Vitamins are classified as water soluble and fat soluble .The water-soluble vitamins (the B vitamins and vitamin C) are not stored in the body and must be eaten every day. Foods containing these vitamins should be handled so the vitamins are not dissolved into the cooking water and lost (as discussed in Chapter 16).
Fats are made up of smaller compounds called fatty acids. Two of these are called essential fatty acids because they cannot be made by the body. The essential fatty acids are linoleic acid and alpha-linolenic acid. Linoleic acid is a member of a group of compounds called omega-6 fatty acids (the term refers to its chemical structure). It is found in vegetable oils and is usually abundant in North American diets.
Alpha-linolenic acid is a member of the group of omega-3 fatty acids. The body can change alpha-linolenic acid into other omega-3 fatty acids called DHA and EPA. These are all important nutrients that play vital roles in growth, in the immune system, in proper eyesight, and in cell structure. Alpha-linolenic acid is found in certain vegetable oils. DHA and EPA, in addition to being made by the body, are also found in some fatty fish such as salmon, mackerel, tuna, and sardines. Unlike omega-6 fatty acids, omega-3 acids are not usually abundant in North American diets, so it is important to be aware of foods that supply them.
MORE CHEMISTRY: LIPOPROTEINS
Lipoproteins are combinations of protein and fat that carry cholesterol and fat through the bloodstream. Two of these compounds are of concern to us. Low-density lipoprotein (LDL) is the most important carrier of cholesterol. Although it performs a needed function, if too much of it is present it deposits excess cholesterol inside arteries, blocking the flow of blood. High-density lipoprotein (HDL), on the other hand, helps remove cholesterol from the blood and eliminate it from the body. Consequently, HDL is seen as a major preventer of heart disease.
Fat-soluble vitamins (A, D, E, and K) can be stored in the body, so they do not need to be eaten every day as long as the total amount eaten over time is sufficient. Consuming too much of a fat-soluble vitamin daily, as sometimes happens when people take too many vitamin supplements, can result in toxic levels of the vitamin stored in the tissues.
More detail about individual vitamins, their functions, and their sources, can be found in Table 6.1.
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