Chapter 10 Summary - Cengage



Chapter 10 Summary

Water-soluble vitamins tend to be absorbed in the small intestine and circulated to the liver in the blood. The absorption and use, or bioavailability, of many vitamins is influenced by a variety of factors, and although there are exceptions, water-soluble vitamins tend not to be stored in the body. Water-soluble vitamins play important functions as coenzymes, but many also have noncoenzyme roles. Water-soluble vitamins are often added to foods (fortification), and when added in certain amounts these foods can be labeled as “enriched.” The water-soluble vitamins in foods are easily destroyed or lost during cooking and storage. However, decreasing cooking times and protecting foods during storage can help prevent this loss.

There are three forms of thiamin in the body: free thiamin, thiamin pyrophosphate (TPP), and thiamin triphosphate (TTP). TPP functions as a coenzyme, catalyzing reactions that enable the body to use glucose, amino acids, and fatty acids for energy. Thiamin is also involved in the synthesis of DNA, RNA, and NADPH. TTP also has noncoenzyme roles important for nerve function. Good sources of thiamin include pork, peas, whole grains, fish, enriched cereal products, and other fortified foods. Thiamin deficiency causes beriberi, of which there are four forms. High thiamin consumption has no known toxic effects. There are three forms of riboflavin: free riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). Riboflavin functions as a coenzyme in a variety of reduction-oxidation reactions in the body, enabling it to use glucose, amino acids, and fatty acids for energy. Riboflavin is also needed for the activation or synthesis of vitamin A, folate, niacin, vitamins B6 and K, and some neurotransmitters. Good sources of riboflavin include liver, meat, dairy products, whole-grain products, and enriched cereals. Severe riboflavin deficiency causes ariboflavinosis.

There are two forms of niacin—nicotinic acid and nicotinamide. Both can be converted to NAD+ and NADP+, which are coenzymes that catalyze a variety of redox reactions related to energy metabolism. These reactions enable the body to use glucose, amino acids, and fatty acids for energy (ATP). Niacin is also needed for synthesizing fatty acids, cholesterol, steroid hormones and DNA, and for metabolizing vitamin C and folate. NAD+ also has functions unrelated to its role as a coenzyme, including protein synthesis, maintenance, replication and repair of DNA, glucose homeostasis, and cholesterol metabolism. Niacin can be made from tryptophan in the body. Good sources of niacin or tryptophan include liver, chicken, fish, pork, mushrooms, and lamb. Enriched grain products also provide niacin. The bioavailability of niacin in some plant-based foods can be increased by exposure to alkaline solutions. Niacin deficiency causes pellagra, whereas high doses of nicotinic acid can cause skin lesions, gastrointestinal upset, increased plasma glucose, and liver damage. Caution is advised when consuming large doses of niacin from supplements or fortified foods.

Pantothenic acid is a component of coenzyme A (CoA), which is needed to make acetyl-CoA. Acetyl-CoA is required for energy metabolism and ATP production. It is also required for synthesizing heme, cholesterol, bile salts, fatty acids, phospholipids, and steroid hormones. Good food sources of pantothenic acid include fortified cereals, mushrooms, organ meats (such as liver), and sunflower seeds, although heat can destroy it. Severe pantothenic acid deficiency causes burning feet syndrome, characterized by tingling feet, weakness, and gastrointestinal distress. High doses of the vitamin have been reported to cause nausea.

Vitamin B6 takes three forms: pyridoxine, pyridoxal, and pyridoxamine, all of which are converted to their coenzyme form, pyridoxal phosphate (PLP). PLP is involved in many reactions related to the metabolism of amino acids. Vitamin B6 is also needed for synthesizing neurotransmitters and heme, converting tryptophan to niacin, and breaking down glycogen to glucose. Severe vitamin B6 deficiency causes microcytic hypochromic anemia, because the body cannot produce hemoglobin. Because the vitamin is stored, toxicity can occur, resulting in neurological problems. Good sources of vitamin B6 include chickpeas (garbanzo beans), fish, liver, and potatoes, as well as fortified breakfast cereals and bakery products.

Biotin acts as a coenzyme for enzymes catalyzing carboxylation reactions. These enzymes allow the body to use glucose, amino acids, and fatty acids to produce ATP. Biotin is also needed for the synthesis of fatty acids, the breakdown of the amino acid leucine, and cell growth and development. Good sources of biotin include nuts, eggs, mushrooms, and tomatoes. Avidin, found in raw egg whites, can decrease biotin bioavailability, and biotin in foods can be destroyed by extreme heat. Biotin deficiency causes a variety of neurological problems and can be severe, especially in infants. There are no known toxic effects of biotin.

The active form of folate in the body is tetrahydrofolate acid (THF), which is involved in single-carbon transfer reactions. Many of these are needed for amino acid metabolism. For example, 5-methyl THF is converted to THF during the conversion of homocysteine to methionine. This reaction also requires vitamin B12. Folate is also required for DNA synthesis and therefore growth, maintenance, and repair of all tissues. Good sources of folate include liver, legumes, mushrooms, and green leafy vegetables. Enriched cereal products and other fortified foods are also good sources. Folate deficiency causes macrocytic anemia because of the inability of red blood cells to produce DNA and divide properly. Folate deficiency also increases the risk for some women of giving birth to children with neural tube defects.

Vitamin B12, also called cobalamin, is required as a coenzyme in two important reactions. The first allows some amino acids and fatty acids to enter the citric acid cycle. The second reaction catalyzes the conversion of homocysteine to methionine and regenerates the active form of folate (THF). Good sources of vitamin B12 include shellfish, liver, fish, meat, fortified breakfast cereals, and bakery products. Vitamin B12 deficiency can be caused by inadequate vitamin B12 intake or lack of intrinsic factor, the latter being called pernicious anemia. Vitamin B12 deficiency is characterized by macrocytic anemia because it causes secondary folate deficiency. Vitamin B12 deficiency can also lead to severe neurological complications. There are no known toxic effects of high vitamin B12 intake.

Vitamin C provides important antioxidant functions in the body. Many of these reactions involve the regeneration of reduced mineral components of enzymes, modulating the synthesis of important compounds such as collagen, carnitine, and neurotransmitters. Other vitamin C-related reactions stabilize free radicals and repair damage caused by free radical oxidation. Vitamin C is found in many foods, including a variety of fruits and vegetables, and is well absorbed. Vitamin C deficiency causes scurvy, which is characterized by bleeding gums and poor wound healing. Although high consumption of vitamin C may have beneficial effects on the immune system, some people have reported unpleasant effects of large doses of vitamin C intake from supplements.

Choline is a water-soluble compound that has been recently deemed an essential nutrient—at least in men. Choline is needed for synthesizing several phospholipids, including lecithin, and is required for producing acetylcholine, a neurotransmitter. Choline is also needed for muscle control and a variety of metabolic reactions. The functions of choline are still being studied. Choline is found in many foods but is especially high in eggs, liver, legumes, and pork. It is also added to foods as lecithin. Choline deficiency may cause liver damage in some people. Very high intakes of choline cause an unpleasant and characteristic fishy body odor.

Although carnitine is not an essential nutrient for adults, many scientists believe it is conditionally essential for the newborn infant. Carnitine is important for fatty acid transport across biological membranes and is therefore needed to obtain energy from lipids. Carnitine is found most abundantly in meat and milk products. There are no dietary recommendations for carnitine at the present.

We can use many tools and guidelines to choose foods rich in the water-soluble vitamins. For example, the Dietary Guidelines for Americans and MyPyramid were specifically devised to help us consume adequate amounts of all the micronutrients. However, at times getting adequate amounts of the water-soluble vitamins may be difficult, and consuming dietary supplements may be prudent. Clinicians recommend that we be careful to not exceed UL levels and that we keep a record of dietary supplements that we are taking.

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