person should consume no more calories than he or she can burn. Note that
in people with high metabolic rates, the body dissipates the extra calories as
body heat instead of storing them as fat, and thus there is no violation of the
conservation of energy principle.
In some people, a genetic flaw is believed to be responsible for the
extremely low rates of metabolism. Several studies concluded that losing
weight for such people is nearly impossible. That is, obesity is a biological
phenomenon. However, even such people will not gain weight unless they eat
more than their body can burn. They just must learn to be content with little
food to remain slim, and forget about ever having a normal “eating” life. For
most people, genetics determine the range of normal weights. A person may
end up at the high or low end of that range, depending on eating and exercise
habits. This also explains why some genetically identical twins are not so iden-
tical when it comes to body weight. Hormone imbalanceis also believed to
cause excessive weight gain or loss.
Based on his experience, the first author of this book has also developed a
diet called the “sensible diet.” It consists of two simple rules: eat whatever
you want wheneveryou want as much asyou want provided that (1) you do
not eat unless you are hungry and (2) you stop eating before you get stuffed.
In other words,listen to your body and don’t impose on it. Don’t expect to
see this unscientific diet advertised anywhere since there is nothing to be
sold and thus no money to be made. Also, it is not as easy as it sounds since
food is at the center stage of most leisure activities in social life, and eating
and drinking have become synonymous with having a good time. However, it
is comforting to know that the human body is quite forgiving of occasional
impositions.
Being overweight is associated with a long list of health risks from high
blood pressure to some forms of cancer, especially for people who have a
weight-related medical condition such as diabetes, hypertension, and heart
disease. Therefore, people often wonder if their weight is in the proper
range. Well, the answer to this question is not written in stone, but if you
cannot see your toes or you can pinch your love handles more than an inch,
you don’t need an expert to tell you that you went over your range. On the
other hand, some people who are obsessed with the weight issue try to lose
more weight even though they are actually underweight. Therefore, it is use-
ful to have a scientific criterion to determine physical fitness. The range of
healthy weight for adults is usually expressed in terms of the body mass
index(BMI), defined, in SI units, as(4 –39)where Wis the weight (actually, the mass) of the person in kg and His the
height in m. Therefore, a BMI of 25 is the upper limit for the healthy weight
and a person with a BMI of 27 is 8 percent overweight. It can be shown that
the formula above is equivalent in English units to BMI 705 W/H^2 where
Wis in pounds and His in inches. The proper range of weight for adults of
various heights is given in Table 4 –3 in both SI and English units.198 | Thermodynamics
BMIW 1 kg 2
H^2 1 m^22¬with
underweight
healthy weight
overweightBMI 619 ¬¬
19
BMI
25 ¬¬
BMI 725 ¬¬TABLE 4 –3The range of healthy weight for
adults of various heights (Source:
National Institute of Health)
English Units SI UnitsHealthy Healthy
Height, weight, Height, weight,
in. lbm* m kg*58 91–119 1.45 40–53
60 97–127 1.50 43–56
62 103–136 1.55 46–60
64 111–146 1.60 49–64
66 118–156 1.65 52–68
68 125–165 1.70 55–72
70 133–175 1.75 58–77
72 140–185 1.80 62–81
74 148–195 1.85 65–86
76 156–205 1.90 69–90*The upper and lower limits of healthy range
correspond to mass body indexes of 19 and 25,
respectively.