Nutrition and Metabolism
they eat whole for a rapid infusion of carbohydrates. Adding drinks that replace
electrolytes and provide sugar brings the intake to 300 to 400 calories per
hour on the bike. A rider nibbles or sips about every F
fteen minutes. After rid-
ing, bikers get new musettes that provide drinks that are one-F
fth protein and
fths carbohydrate to aid in recovery, because the muscles soak up the car-
bohydrates in the hour after riding. In the bus, they continue their recovery by
drinking fruit smoothies.
The 7,000-calorie diet plan works for Lance Armstrong, but he is not
average. In fact, University of Texas at Austin researcher Ed Coyle probed
Armstrong’s physiology from ages 21 to 28 for clues to his prowess—even in
the face of testicular cancer during that period. Armstrong has an exception-
ally strong heart and extensive vascular system, but he also works hard. ±or
example, he reduced his weight by 10 pounds before each Tour de ±rance vic-
tory, which increased power per kilogram of body weight by 18%. Armstrong
uses a machine in the Human Performance Laboratory at the university
to measure his muscle power at given oxygen intakes. His training and diet
enable him to ride twice as fast as the average cyclist. Armstrong’s athletic
abilities are eclectic: he ran the New York City and Boston marathons in under
three hours.
or an elite athlete such as seven-time Tour de ±rance winner
Lance Armstrong, carbohydrates are essential for powering
muscles. His diet of 70% carbohydrates and 15% each of protein
and fat equals 6,000 to 7,000 calories per day and rises to 9,000
calories on the most grueling days of competition.
Diet is crucial to the success of endurance athletes like Armstrong. The
goals are simple: fuel and recovery. At most levels of e²
ort, fuel comes from
both carbohydrates and fat, but as the e²
ort intensiF
es, most energy comes
from carbohydrates. The body can store only about 1,800 calories worth of
carbohydrates, which an endurance athlete’s body will burn through in a few
hours, so the athlete needs to eat often to fuel the muscles and to maintain
the supply of glucose to the brain. Protein is required for recovery, to repair
microscopic muscle tears.
Coaches have the diet of Tour de ±rance participants down to a precise
science. The athletes get their food in three meals and many snacks. Meals
include rice, pasta, cereal, potatoes, fruits, vegetables, and whole grains for
carbohydrates, and protein from lean meats, eggs, and yogurt. ±ats come
from cheese, butter, and olive oil. While biking, the riders carry bags, called
“musettes,” that contain power bars, meat sandwiches, and potatoes, which
The human body requires fuel as well as materials to
develop, grow, and heal. Nutrients from food ful±
ll these
tre-ents) are chemicals supplied
from the environment that an organism requires for sur-
vival. There are two major classes of nutrients. The
needed in bulk, are the carbohydrates, proteins,
and fats.
are essential in small daily doses
and include vitamins and minerals. The body also requires
In countries with adequate food supplies, most healthy
individuals can obtain nourishment by eating a variety of
foods and limiting fat intake. People who do not eat meat
products can also receive adequate nutrition but must pay
more attention to food choices to avoid developing nutrient
ciencies. For example, eliminating red meat also means
eliminating an excellent source of iron, copper, zinc, and
vitamin B
. The ±
ber that often makes up much of a veg-
etarian’s diet, although very healthful in many ways, also
decreases absorption of iron. Therefore, a vegetarian must
obtain suf±
cient iron from nonmeat sources. This is easily
done, providing proper
(adequate nutrients) when
sources, actions, and interactions of nutrients are consid-
ered. Forti± ed foods, green leafy vegetables, and especially
whole grains provide many of the nutrients also in meat.
Table 18.1
lists types of vegetarian diets.
Digestion breaks down nutrients to sizes that can be
absorbed and transported in the bloodstream.
refers to the ways that nutrients are altered chemically and
used in anabolism (building up or synthesis) and catabo-
lism (breaking down) of chemical compounds to support the
activities of life. (Chapter 4, pp. 120–124, introduced metab-
olism of carbohydrates.) Nutrients that human cells cannot
synthesize, such as certain amino acids, are particularly
important and are therefore called
essential nutrients.
We eat to obtain the nutrients that power the activities
of life. Eating is a complex, ± nely tuned homeostatic mech-
anism that balances nutrient intake with nutrient use. Too
few nutrients, and disorders associated with malnutrition
result. Too many nutrients, and obesity is the consequence.
Several factors infl uence food intake, including smell, taste,
and texture of food; neural signals triggered by stretch recep-
tors in the stomach; stress; and hormones. Several types of
interacting hormones control appetite by affecting part of the
hypothalamus called the
arcuate nucleus
(table 18.2)
Insulin, secreted from the pancreas, regulates fat stores by
stimulating adipocytes to take up glucose and store fat, and
by stimulating certain other cells to take up glucose and form
Types of Vegetarian Diets
Food Restrictions
No animal foods
Eggs allowed; no dairy or meat
Dairy allowed; no eggs or meat
Dairy and eggs allowed; no meat
Dairy, eggs, and F
sh allowed; no other meat
Dairy, eggs, chicken, and F
sh allowed; no other meat
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