938
UNIT 4
Maintenance of the Body
24
Homeostatic Imbalance 24.4
Previously, high cholesterol and LDL:HDL ratios were consid-
ered the most valid predictors of risk for atherosclerosis, cardio-
vascular disease, and heart attack. However, almost half of those
who get heart disease have normal cholesterol levels, while
others with poor lipid profiles remain free of heart problems.
Presently, LDL levels and assessments of other cardiovascular
disease risk factors are believed to be more accurate indicators
of whether treatment is needed, and many physicians recom-
mend dietary changes regardless of total cholesterol or HDL
levels.
Cholesterol-lowering drugs such as
statins
are routinely pre-
scribed for people with LDL levels over 130. It is estimated that
more than 10 million Americans are now taking statins.
Check Your Understanding
25.
If you had your choice, would you prefer to have high blood
levels of HDLs or LDLs? Explain your answer.
26.
What is the maximum recommended cholesterol level for
adults?
27.
What are trans fats and how do they affect LDL and HDL
levels?
For answers, see Appendix H.
Energy Balance
Explain what is meant by body energy balance.
Describe several theories of food intake regulation.
When any fuel is burned, it consumes oxygen and liberates
heat. Te “burning” of food fuels by our cells is no exception. As
we described in Chapter 2, energy can be neither created nor
destroyed—only converted from one form to another. If we ap-
ply this principle (actually the
first law of thermodynamics
) to
cell metabolism, it means that bond energy released as foods
are catabolized must be precisely balanced by the total energy
output of the body. For this reason, a dynamic balance exists
between the body’s energy intake and energy output:
Energy intake
5
energy output
(heat
1
work
1
energy storage)
Energy intake
is the energy liberated during food oxidation.
(Undigested foods are not part of the equation because they
contribute no energy.)
Energy output
includes energy (1) im-
mediately lost as heat (about 60% of the total), (2) used to do
work (driven by A±P), and (3) stored as fat or glycogen. Because
losses of organic molecules in urine, feces, and perspiration are
very small in healthy people, they are usually ignored in calcu-
lating energy output.
A close look at this situation reveals that
nearly all the energy
derived from foodstuffs is eventually converted to heat.
Heat is
lost during every cellular activity—when A±P bonds are formed
and when they are cleaved to do work, as muscles contract, and
through friction as blood flows through blood vessels. Tough
cholesterol deposits are laid down in the artery walls. Te goal
for LDL levels is 100 or less, but new guidelines for those at risk
for cardiac disease recommend 70 mg/dl or less. A good rule of
thumb is that HDL levels can’t be too high and LDL levels can’t
be too low.
Factors Regulating Blood Cholesterol Levels
A negative feedback loop partially adjusts the amount of choles-
terol produced by the liver according to the amount of choles-
terol in the diet. A high cholesterol intake inhibits its synthesis
by the liver, but it is not a one-to-one relationship because the
liver produces a certain basal amount of cholesterol (about 85%
of desirable values) even when dietary intake is high. Con-
versely, severely restricting dietary cholesterol, although helpful,
does not markedly reduce blood cholesterol levels.
However, the relative amounts of saturated and unsaturated
fatty acids in the diet do have an important effect on blood cho-
lesterol levels. Saturated fatty acids
stimulate liver synthesis
of
cholesterol and
inhibit its excretion
from the body. In contrast,
unsaturated (mono- and polyunsaturated) fatty acids (found in
olive oil and in most vegetable oils, respectively)
enhance excre-
tion
of cholesterol and its catabolism to bile salts, thereby reduc-
ing total cholesterol levels.
Te unhappy exception to this good news about unsaturated
fats concerns
trans fats
, “healthy” oils that have been hardened
by hydrogenation to make them more solid, such as some mar-
garines. ±rans fats actually have a worse effect on blood cho-
lesterol levels than saturated fats do. Te trans fatty acids spark
a greater increase in LDLs and a greater reduction in HDLs,
producing the unhealthiest ratio of total cholesterol to HDL.
Te unsaturated omega-3 fatty acids found in especially large
amounts in some cold-water fish (such as salmon) lower the
proportions of both saturated fats and cholesterol. Te omega-3
fatty acids make blood platelets less sticky, thus helping prevent
spontaneous clotting that can block blood vessels. Tey also ap-
pear to lower blood pressure, even in people with normal blood
pressure. In those with moderate to high cholesterol levels, re-
placing lipid- and cholesterol-rich animal proteins in the diet
with soy protein helps lower blood cholesterol.
Factors other than diet also influence blood cholesterol lev-
els. For example, cigarette smoking and stress lower HDL lev-
els, whereas regular aerobic exercise and estrogen lower LDL
levels and increase HDL levels. Interestingly, body shape pro-
vides clues to risky blood levels of cholesterol and fats. “Apples”
(people with upper body and abdominal fat, seen more o²en in
men) tend to have higher levels of cholesterol and LDLs than
“pears” (whose fat is localized in the hips and thighs, a pattern
more common in women).
Most cells other than liver and intestinal cells obtain the bulk of
the cholesterol they need for membrane synthesis from the blood.
When a cell needs cholesterol, it makes membrane receptor pro-
teins for LDL. LDL binds to the receptors, is engulfed in coated pits
by endocytosis, and within 15 minutes the endocytotic vesicles
fuse with lysosomes, where the cholesterol is freed for use. When
excessive cholesterol accumulates in a cell, it inhibits both the cell’s
own cholesterol synthesis and its synthesis of LDL receptors.
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