Download Chapter 10 Summary

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 B 6 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 B 6
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 watersoluble 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.