464 SECTION V Gastrointestinal Physiology
intake of 1 g/kg body weight to supply the eight nutritionally
essential amino acids and other amino acids is desirable. The
source of the protein is also important. Grade I proteins, the
animal proteins of meat, fish, dairy products, and eggs, con-
tain amino acids in approximately the proportions required
for protein synthesis and other uses. Some of the plant pro-
teins are also grade I, but most are grade II because they sup-
ply different proportions of amino acid and some lack one or
more of the essential amino acids. Protein needs can be met
with a mixture of grade II proteins, but the intake must be
large because of the amino acid wastage.
Fat is the most compact form of food, since it supplies 9.3
kcal/g. However, often it is also the most expensive. Indeed,
internationally there is a reasonably good positive correlation
between fat intake and standard of living. In the past, Western
diets have contained large amounts (100 g/d or more). The evi-
dence indicating that a high unsaturated/saturated fat ratio in
the diet is of value in the prevention of atherosclerosis and the
current interest in preventing obesity may change this. In Cen-
tral and South American Indian communities where corn (car-
bohydrate) is the dietary staple, adults live without ill effects for
years on a very low fat intake. Therefore, provided that the
needs for essential fatty acids are met, a low-fat intake does not
seem to be harmful, and a diet low in saturated fats is desirable.
Carbohydrate is the cheapest source of calories and pro-
vides 50% or more of the calories in most diets. In the average
middle-class American diet, approximately 50% of the calo-
ries come from carbohydrate, 15% from protein, and 35%
from fat. When calculating dietary needs, it is usual to meet
the protein requirement first and then split the remaining cal-
ories between fat and carbohydrate, depending on taste,
income, and other factors. For example, a 65-kg man who is
moderately active needs about 2800 kcal/d. He should eat at
least 65 g of protein daily, supplying 267 (65 × 4.1) kcal. Some
of this should be grade I protein. A reasonable figure for fat
intake is 50 to 60 g. The rest of the caloric requirement can be
met by supplying carbohydrate.
MINERAL REQUIREMENTS
A number of minerals must be ingested daily for the mainte-
nance of health. Besides those for which recommended daily di-
etary allowances have been set, a variety of different trace
elements should be included. Trace elements are defined as ele-
ments found in tissues in minute amounts. Those believed to be
essential for life, at least in experimental animals, are listed in Ta-
ble 27–3. In humans, iron deficiency causes anemia. Cobalt is
part of the vitamin B 12 molecule, and vitamin B 12 deficiency
leads to megaloblastic anemia (see Chapter 32). Iodine deficien-
cy causes thyroid disorders (see Chapter 20). Zinc deficiency
causes skin ulcers, depressed immune responses, and hypogo-
nadal dwarfism. Copper deficiency causes anemia and changes
in ossification. Chromium deficiency causes insulin resistance.
Fluorine deficiency increases the incidence of dental caries.
Conversely, some minerals can be toxic when present in the
body in excess. For example, severe iron overload causes hemo-
chromatosis, copper excess causes brain damage (Wilson dis-
ease), and aluminum poisoning in patients with renal failure
who are receiving dialysis treatment causes a rapidly progressive
dementia that resembles Alzheimer disease (see Chapter 19).
Sodium and potassium are also essential minerals, but list-
ing them is academic, because it is very difficult to prepare a
sodium-free or potassium-free diet. A low-salt diet is well tol-
erated for prolonged periods because of the compensatory
mechanisms that conserve Na+.VITAMINS
Vitamins were discovered when it was observed that diets
adequate in calories, essential amino acids, fats, and miner-
als failed to maintain health. The term vitamin has now
come to refer to any organic dietary constituent necessary
for life, health, and growth that does not function by supply-
ing energy.
Because there are minor differences in metabolism between
mammalian species, some substances are vitamins in one spe-
cies and not in another. The sources and functions of the major
vitamins in humans are listed in Table 27–4. Most vitamins
have important functions in intermediary metabolism or the
special metabolism of the various organ systems. Those that are
water-soluble (vitamin B complex, vitamin C) are easily
absorbed, but the fat-soluble vitamins (vitamins A, D, E, and K)
are poorly absorbed in the absence of bile or pancreatic lipase.
Some dietary fat intake is necessary for their absorption, and in
obstructive jaundice or disease of the exocrine pancreas, defi-
ciencies of the fat-soluble vitamins can develop even if their
intake is adequate. Vitamin A and vitamin D are bound to
transfer proteins in the circulation. The α-tocopherol form of
vitamin E is normally bound to chylomicrons. In the liver, it is
transferred to very low density lipoprotein (VLDL) and distrib-
uted to tissues by an α-tocopherol transfer protein. When this
protein is abnormal due to mutation of its gene in humans,
there is cellular deficiency of vitamin E and the development of
a condition resembling Friedreich ataxia. Two Na+-dependent
L-ascorbic acid transporters have recently been isolated. One is
found in the kidneys, intestines, and liver, and the other in the
brain and eyes.TABLE 27–3 Trace elements believed essential for life.
Arsenic Manganese
Chromium Molybdenum
Cobalt Nickel
Copper Selenium
Fluorine Silicon
Iodine Vanadium
Iron Zinc