711
CHAPTER EIGHTEEN
Nutrition and Metabolism
cholesterol that has been metabolized to provitamin D by
intestinal enzymes, then stored in the skin and exposed to
ultraviolet light (see chapter 13, p. 485).
Like other fat-soluble vitamins, vitamin D resists the effects
of heat, oxidation, acids, and bases. It is primarily stored in the
liver and is less abundant in the skin, brain, spleen, and bones.
Vitamin D stored in the form of hydroxycholecalciferol
is released as needed into the blood. When parathyroid hor-
mone is present, this form of vitamin D is converted in the
kidneys into an active form of the vitamin (dihydroxychole-
calciferol). This substance, in turn, is carried as a hormone
in the blood to the intestines where it stimulates production
of calcium-binding protein. Here, it promotes absorption of
calcium and phosphorus, ensuring that adequate amounts of
these minerals are available in the blood for tooth and bone
formation and metabolic processes.
deficiency. Vitamin A deficiency also causes degenerative
changes in certain epithelial tissues, and the body becomes
more susceptible to infection.
PRACTICE
32
What chemical in the body is the precursor to vitamin A?
33
What conditions destroy vitamin A?
34
Which foods are good sources of vitamin A?
Vitamin D
is a group of steroids that have similar prop-
erties. One of these substances, vitamin D
3
(cholecalciferol),
is found in foods such as milk, egg yolk, and F sh liver oils.
Vitamin D
2
(ergocalciferol) is commercially produced by
exposing a steroid obtained from yeasts (ergosterol) to ultra-
violet light. Vitamin D can also be synthesized from dietary
that you like and distribute them into three or four
balanced meals of 250 to 500 calories each.
Appetite is an important consideration in
dieting to control weight. Many people in the
1990s, following advice from the U.S. govern-
ment, followed low-fat diets, which caused
weight gain if the dieters compensated by eating
more highly reF
ned carbohydrates. These foods
escalate the rise and fall of blood glucose fol-
lowing a meal, which stimulates hunger sooner
than if the meal contained more protein and fat.
Substituting whole grains for “white” carbohy-
drates slows the rate of entry of glucose into the
bloodstream (the glycemic index), and this can
better control the urge to eat.
Ideally, weight loss can be accomplished by
changing diet and exercise habits. However, realis-
tically, two-thirds of those who lose weight regain
it within five years. Physicians are increasingly
regarding obesity as a chronic illness that for some
people may require more drastic measures than
dieting and exercising.
Drug Therapy
Some physicians recommend drug therapy if the
BMI exceeds 30 or if it exceeds 27 and the person
also has hypertension, type 2 diabetes mellitus,
or hyperlipidemia. Several types of “diet drugs”
are no longer in use because they are dangerous.
Amphetamines, for example, carried the risk of
addiction, and the combination of fen±
uramine
and phentermine damaged heart valves.
term, but undernourished as fetuses, are at high
risk of obesity. Physiological changes that coun-
tered starvation in the uterus cause obesity when
they persist.
Certain genes encode proteins that con-
nect sensations in the gastrointestinal tract with
centers in the hypothalamus that control hunger
and satiety. It is how we satisfy those signals—
what we eat—that provides the environmental
component to body weight. A certain set of gene
variants may have led to a trim F
gure in a human
many thousands of years ago, when food had to
be hunted or gathered—and meat was leaner.
Today those same gene variants do not foster
slimness in a person who takes in many more
calories than he or she expends.
Treatments for Obesity
Diet and Exercise
A safe goal for weight loss using dietary restric-
tion and exercise is 1 pound of fat per week. A
pound of fat contains 3,500 calories of energy,
so that pound can be shed by an appropriate
combination of calorie cutting and exercise. This
might mean eating 500 fewer calories per day or
exercising o²
500 calories each day. More than a
pound of weight will drop because water is lost
as well as fat.
Dieting should apply to the energy-providing
nutrients (carbohydrates, proteins, and fats) but
never to the vitamins and minerals. Choose foods
Newer antiobesity drugs target fat in
diverse ways. Tetrahydrolipostatin, marketed
as Orlistat and Xenical, inhibits the function
of pancreatic lipase, preventing the digestion
and absorption of about a third of dietary fat,
which is eliminated in loose feces. This effect is
not disruptive as long as the person follows a
low-fat diet. ³uture weight control drugs may
manipulate appetite-control hormones, such as
ghrelin and leptin.
Surgery
³or people with BMIs above 40, or above 35 in
addition to an obesity-related disorder, bariatric
(weight loss) surgery can lead to great weight
loss. Two major types of procedures are done. In
laparoscopic adjustable gastric banding, a sili-
cone band ties o²
part of the stomach, limiting
the capacity of the organ to hold food. The band
can be in±
ated or de±
ated in a doctor’s o´
ce by
adding or removing saline. The second type of
bariatric surgery is gastric bypass, in which part of
the stomach is stapled shut, forming a pouch sur-
gically connected to the jejunum, bypassing the
duodenum. Both procedures lead to decreased
hunger, drastically reduced food intake, and
some decrease in the absorption of nutrients. A
special diet, liquid at F
rst, must be followed. Many
patients who have had bariatric surgery report
improvement in or disappearance of type 2 dia-
betes, back pain, arthritis, varicose veins, sleep
apnea, and hypertension.
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