666 Chapter 19
(B 2 ), niacin (B 3 ), pyridoxine (B 6 ), pantothenic acid, biotin, folic
acid, vitamin B 12 , and vitamin C (ascorbic acid). Recommended
dietary allowances for these vitamins are listed in table 19.2.Water-Soluble Vitamins
Derivatives of water-soluble vitamins serve as coenzymes in
the metabolism of carbohydrates, lipids, and proteins. Thia-
mine, for example, is needed for the activity of the enzyme
that converts pyruvic acid to acetyl coenzyme A. Riboflavin
and niacin are needed for the production of FAD and NAD,
respectively. FAD and NAD serve as coenzymes that transfer
hydrogens during cell respiration (chapter 4; see fig. 4.17).
Pyridoxine is a cofactor for the enzymes involved in amino
acid metabolism. Deficiencies of the water-soluble vitamins
can thus have widespread effects in the body ( table 19.3 ).
Free radicals are highly reactive molecules that carry an
unpaired electron. They can damage tissues by removing an
electron from, and thus oxidizing, other molecules. Vitamin C
(a water-soluble vitamin) and vitamin E (a fat-soluble vitamin)
function as antioxidants through their ability to inactivate free
radicals. These vitamins may afford protection against some of
the diseases that may be caused by free radicals.proteins can be reused in protein synthesis, a person needs only
about 35 g/day of protein in the diet. It should be noted that these
are average figures and will vary in accordance with individual
differences in size, sex, age, genetics, and physical activity. The
average daily turnover for fat is about 100 g/day, but very little
is required in the diet (other than that which supplies fat-soluble
vitamins and essential fatty acids), since fat can be produced
from excess carbohydrates.
The minimal amounts of dietary protein and fat required to
meet the turnover rate are adequate only if they supply sufficient
amounts of the essential amino acids and fatty acids. These mol-
ecules are termed essential because they cannot be synthesized
by the body and must be obtained in the diet. The nine essential
amino acids are lysine, tryptophan, phenylalanine, threonine,
valine, methionine, leucine, isoleucine, and histidine. The essential
fatty acids are linoleic acid and alphalinolenic acid.
Unsaturated fatty acids—those with double bonds between
the carbons—are characterized by the location of the first dou-
ble bond. Linoleic acid, found in corn oil, contains 18 carbons
and two double bonds. It has its first double bond on the sixth
carbon from the methyl (CH 3 ) end, and is therefore designated
as an n-6 (or omega-6) fatty acid. Alpha-linolenic acid also has
18 carbons, but it has three double bonds. More significantly
for health, its first double bond is on the third carbon from the
methyl end. Fatty acids that have their first double bond on the
third carbon from the methyl end are described as omega-3 (or
n-3 ) fatty acids. Mammals lack the enzymes needed to insert
the double bond at the n-6 or n-3 positions and thus must eat
linoleic and alpha-linolenic acids in their diets.
FITNESS APPLICATION
Omega-3 fatty acids are believed to promote cardiovas-
cular health. Alpha linolenic acid (with 18 carbons) can be
obtained from such foods as soy and rapeseed oil, walnuts,
and many green vegetables. Eicosapentaenoic acid ( EPA,
with 20 carbons) and docosahexaenoic acid ( DHA, with
22 carbons) are found in cold-water fatty fish such as salmon,
mackerel, trout, and herring. The omega-3 fatty acids may
inhibit thrombus formation, the progression of atheroscle-
rosis, and/or ventricular arrhythmias. Because of this, the
American Heart Association recommends that people eat at
least two fish meals per week.CLINICAL APPLICATION
Vitamin B 1 ( thiamine ) deficiency is rare outside of devel-
oping nations (where it can cause beriberi ) unless a person
suffers from alcoholism, or from conditions such as Crohn’s
disease or anorexia that interfere with absorption of nutrients.
A dire consequence of thiamine deficiency in alcoholics is
Wernicke-Korsakoff syndrome, or wet brain. This consists of
the brain damage of Wernicke’s encephalopathy, often com-
bined with Korsakoff syndrome involving memory impair-
ments and psychiatric disturbances.Fat-Soluble Vitamins
Vitamin E has important antioxidant functions, as will be
described shortly. It also acts on the immune system to block the
release of pro-inflammatory cytokines and thereby reduce dam-
age caused by inflammation. Some fat-soluble vitamins have
highly specialized functions. Vitamin K, for example, is required
for the production of prothrombin and for clotting factors VII, IX,
and X. Vitamins A and D also have functions unique to each, but
these two vitamins overlap in their mechanisms of action.
Vitamin A is a collective term for a number of molecules
that include retinol (the transport form of vitamin A), retinal (also
known as retinaldehyde, used as the photo pigment in the retina),
and retinoic acid. Most of these molecules are ultimately derived
from dietary b -carotene, present in such foods as carrots, leafy veg-
etables, and egg yolk. The b -carotene is converted by an enzyme
in the intestine into two molecules of retinal. Most of the retinal is
reduced to retinol, while some is oxidized to retinoic acid. It is the
retinoic acid that binds to nuclear receptor proteins (see chapter 11;Vitamins and Minerals
Vitamins are small organic molecules that serve as coenzymes
in metabolic reactions or that have other highly specific func-
tions. They must be obtained in the diet because the body either
doesn’t produce them, or produces them in insufficient quanti-
ties. (Vitamin D is produced in limited quantities by the skin, and
the B vitamins and vitamin K are produced by intestinal bacte-
ria.) There are two classes of vitamins: fat-soluble and water-
soluble. The fat-soluble vitamins include vitamins A, D, E, and
K. The water- soluble vitamins include thiamine (B 1 ), riboflavin