Chapter 6 the Vitamins

Chapter 6 – The Vitamins

Chapter 6 Summary

Turning Back the Clock

Though vitamin deficiency diseases were familiar to people in ancient times, they were not connected with consumption of certain foods until the 16th century or later. Over time, people identified which foods were lacking in the diets of people with deficiency diseases, and then began to analyze food constituents in the mid-19th century. It was not until the 20th century, however, that scientists were able to identify the particular substances—named vitamines—that would prevent these diseases if consumed.

The Two Classifications of Vitamins

Vitamins fall into two categories: those that dissolve in water, or water soluble; and those that dissolve in fat, or fat soluble. To date, scientists have identified 13 vitamins, each with its own special roles to play. There are nine water-soluble vitamins: eight B vitamins and vitamin C. Each of the major food groups supplies a number of vitamins. The body excretes water-soluble vitamins if the blood levels rise too high. As a result, they rarely reach toxic levels in the body. In contrast, the four fat-soluble vitamins—A, D, E, and K—are stored in the liver and in body fat, making it possible for megadoses of the fat-soluble vitamins to build up to toxic levels in the body.

Water-Soluble Vitamins

The B vitamins serve as coenzymes, assisting many enzymes in the body. Thiamin, riboflavin, niacin, and pantothenic acid are especially important in the reactions that release energy from carbohydrate and fats. Vitamin B6 facilitates protein metabolism. Folate is involved in pathways leading to the synthesis of new cells and plays a crucial role in a healthy pregnancy. Vitamin B12 works closely with folate, enabling it to make new red blood cells. Biotin is involved in a number of body processes, including energy metabolism. Deficiency diseases involving the B vitamins include beriberi (thiamin), pellagra (niacin), and anemia (folate or vitamin B12).

Thiamin is widely distributed in foods; a balanced and varied diet of nutritious foods will best ensure an adequate intake. Riboflavin is concentrated in milk products and leafy green vegetables. Niacin is found wherever protein is found and can also be made from the amino acid tryptophan. Vitamin B6 is most abundant in meats, vitamin B12 is found only in animal products, and folate is supplied by green, leafy vegetables, legumes, and foods fortified with folic acid. Pantothenic acid and biotin are widespread in the food supply.

Vitamin C acts as an antioxidant, helps to enhance the absorption of iron from plant foods, and promotes the formation of the protein collagen. Deficiency of vitamin C causes scurvy. The best food sources of vitamin C are citrus fruits, strawberries, cantaloupe, broccoli, and other members of the cabbage family. The antioxidant vitamins (vitamin C, vitamin E, and the carotenoids—including beta-carotene) and many phytochemicals found in plant foods serve the body by protecting it from damaging compounds known as free radicals that can promote heart disease and cancer, among other conditions.

Fat-Soluble Vitamins

Vitamin A as a part of visual pigments is essential for vision. It is involved in maintaining the integrity of mucous membranes throughout the internal linings of the body, and thus in promoting resistance to infection. It helps maintain the skin and is essential for the remodeling of bones during growth or mending. The recommended intake for vitamin A is easily met by consuming the vitamin’s precursor form—beta-carotene—from food sources such as dark green, leafy vegetables, carrots, cantaloupe, or broccoli.

Vitamin D promotes intestinal absorption of calcium, mobilization of calcium from bone stores, and retention of calcium by the kidneys, and it is therefore essential for the mineralization of bones and teeth. Deficiency of vitamin D causes rickets in children and osteomalacia in adults. The recommended adult intake is best met by consuming fortified products such as milk or breakfast cereals.

The best-substantiated role of vitamin E in human beings is as an antioxidant that protects vitamin A and the polyunsaturated fatty acids (PUFA) from destruction by oxygen. The human requirement for vitamin E is known to vary with polyunsaturated fatty acid intake; since the vitamin occurs with PUFA in foods, it is normally supplied in the needed amounts.

Vitamin K promotes normal blood clotting; deficiency causes hemorrhagic disease. Accumulating evidence supports a role for vitamin K in maintenance of bone health. The vitamin is synthesized by intestinal bacteria and is available from foods such as leafy green vegetables and milk.

Nonvitamins

Choline is classified as a conditionally essential nutrient because a person who consumes none (very rare) will develop a deficiency. Nonvitamins are essential nutrients for bacteria or certain animals, but not for humans, because the human body can manufacture them—these include inositol and carnitine. Some substances marketed in dietary supplements, such as PABA or vitamin B17, are not necessary for humans.

Color Your Plate with Vitamin-Rich Foods—and Handle Them with Care

This chapter’s Savvy Diner feature looks at the importance of adding a variety of colorful fruits and vegetables to the diet and discusses the effects of cooking and processing on a food’s nutrient content. In storing and cooking vitamin-rich foods, the principles to remember are to exclude air, chill, minimize vitamin losses in water, and not to overcook.

Phytonutrients in Foods: The Phytochemical Superstars

The chapter discusses the major classes of phytochemicals found in foods and the research that links these substances with health. It includes a discussion of regulatory issues regarding supplements in general, and tips and resources for evaluating the safety and efficacy of herbal remedies. The Eat Well Be Well feature introduces the topics of functional foods, discusses the factors driving the increasing interest in functional foods, and offers tips on incorporating them into the diet.

Nutrition and Cancer Prevention

Though research has been unable to consistently and clearly demonstrate links between a particular diet and cancer, diet is considered one of the many lifestyle/behavioral factors that affect cancer risk. Authorities recommend the following for cancer prevention: maintain a healthy weight, keep fat intakes moderate, consume produce rich in beta-carotene and vitamin C as well as cruciferous vegetables, and meet recommended intakes for calcium and fiber. Other positive behaviors include exercising regularly and avoiding alcohol and tobacco products.