590
Chapter 16
exercise to lose weight or to prevent weight gain. Like individ-
uals with anorexia nervosa, those with bulimia manifest a per-
sistent overconcern with body weight, although they generally
remain within 10 percent of their ideal weight. This disease,
too, is accompanied by a variety of physiological abnormali-
ties, but it is unknown in some cases whether they are causal
or secondary.
In addition to anorexia and bulimia, rare lesions or
tumors within the hypothalamic centers that normally regu-
late appetite can result in over- or underfeeding.
What Should We Eat?
In the last few years, more and more dietary factors have been
associated with the cause or prevention of many diseases,
including not only coronary artery disease but hypertension,
cancer, birth defects, osteoporosis, and a variety of other
chronic diseases. These associations come mainly from animal
studies, epidemiologic studies on people, and basic research
concerning potential mechanisms. The problem is that the
fi ndings are often diffi cult to interpret and may be confl ict-
ing. To synthesize all this material in the form of simple, clear
recommendations to the general public is a monumental task,
and all such attempts have been subjected to intense criticism.
One of the most commonly used sets, issued by the National
Research Council, is presented in
Table 16–8
.
Regulation of Body Temperature
In the preceding discussion, it was emphasized that energy
expenditure is linked to our ability to maintain a stable, warm
body temperature. In this section, we discuss the mechanisms
by which the body gains or loses heat in a variety of healthy or
pathological settings.
Birds and mammals, including people, are capable of
maintaining their body temperatures within very narrow
limits despite wide fl uctuations in ambient temperature and
are termed
homeothermic.
The relatively stable and high
body temperature frees biochemical reactions from fl uctuat-
ing with the external temperature. However, the maintenance
of a warm body temperature (approximately 37°C in healthy
persons) imposes a requirement for precise regulatory mecha-
nisms because further, large elevations of temperature cause
nerve malfunction and protein denaturation. Some people
suffer convulsions at a body temperature of 41°C (106°F), and
43°C is considered to be the absolute limit for survival.
Several important generalizations about normal human
body temperature should be stressed at the outset: (1) Oral
temperature averages about 0.5°C less than rectal, which is
generally used as an estimate of internal temperature (also
known as
core body temperature
). Thus, not all parts of
the body have the same temperature. (2) Internal tempera-
ture varies several degrees in response to activity patterns and
changes in external temperature. (3) There is a characteris-
tic circadian fl uctuation of about 1°C (
Figure 16–16
), with
temperature being lowest during the night and highest dur-
ing the day. (4) An added variation in women is a higher tem-
perature during the second half of the menstrual cycle due to
the effects of progesterone.
Temperature regulation can be studied by our usual bal-
ance methods. The total heat content gained or lost by the
Table 16–8
Summary of National Research
Council Dietary Recommendations
1.
Reduce fat intake to 30 percent or less of total calories.
Reduce saturated fatty acid intake to less than 10 percent of
calories and intake of cholesterol to less than 300 mg daily.
2.
Every day eat fi ve or more servings of a combination of
vegetables and fruits, especially green and yellow vegetables
and citrus fruits. Also, increase starches and other complex
carbohydrates by eating six or more daily servings of a
combination of breads, cereals, and legumes.
3.
Maintain protein intake at moderate levels.
4.
Balance food intake and physical activity to maintain
appropriate body weight.
5.
Alcohol consumption is not recommended. For those
who drink alcoholic beverages, limit consumption to the
equivalent of one ounce of pure alcohol in a single day.
6.
Limit total daily intake of sodium to 2.4 g or less.
7.
Maintain adequate calcium intake.
8.
Avoid taking dietary supplements in excess of the RDA
(Recommended Dietary Allowance) in any one day.
9.
Maintain an optimal intake of fl
uoride, particularly during
the years of primary and secondary tooth formation and
growth.
37.0
Rectal temperature (
°
C)
37.5
36.5
36.0
48
48
4
Time of day
Noon
A.M.
A.M.
A.M.
P.M.
P.M.
Midnight
Figure 16–16
Circadian changes in core (measured as rectal) body temperature
in a typical person. This fi gure does not take into account daily
minor swings in temperature due to such things as exercise, eating,
and menstrual cycle, nor are the absolute values on the
y
-axis
representative of all individuals.
Adapted from Scales et al.
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