Thermodynamics

198 | ThermodynamicsTABLE 4–3The range of healthy weight foradults of various heights (Source:National Institute of Health)English Units SI UnitsHealthyHealthyHeight, weight, Height, weight,in. lbm* m kg*58 91–119 1.45 40–5360 97–127 1.50 43–5662 103–136 1.55 46–6064 111–146 1.60 49–6466 118–156 1.65 52–6868 125–165 1.70 55–7270 133–175 1.75 58–7772 140–185 1.80 62–8174 148–195 1.85 65–8676 156–205 1.90 69–90*The upper and lower limits of healthy rangecorrespond to mass body indexes of 19 and 25,respectively.person should consume no more calories than he or she can burn. Note thatin people with high metabolic rates, the body dissipates the extra calories asbody heat instead of storing them as fat, and thus there is no violation of theconservation of energy principle.In some people, a genetic flaw is believed to be responsible for theextremely low rates of metabolism. Several studies concluded that losingweight for such people is nearly impossible. That is, obesity is a biologicalphenomenon. However, even such people will not gain weight unless they eatmore than their body can burn. They just must learn to be content with littlefood to remain slim, and forget about ever having a normal “eating” life. Formost people, genetics determine the range of normal weights. A person mayend up at the high or low end of that range, depending on eating and exercisehabits. This also explains why some genetically identical twins are not so identicalwhen it comes to body weight. Hormone imbalance is also believed tocause excessive weight gain or loss.Based on his experience, the first author of this book has also developed adiet called the “sensible diet.” It consists of two simple rules: eat whateveryou want whenever you want as much as you want provided that (1) you donot 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 tosee this unscientific diet advertised anywhere since there is nothing to besold and thus no money to be made. Also, it is not as easy as it sounds sincefood is at the center stage of most leisure activities in social life, and eatingand drinking have become synonymous with having a good time. However, itis comforting to know that the human body is quite forgiving of occasionalimpositions.Being overweight is associated with a long list of health risks from highblood pressure to some forms of cancer, especially for people who have aweight-related medical condition such as diabetes, hypertension, and heartdisease. Therefore, people often wonder if their weight is in the properrange. Well, the answer to this question is not written in stone, but if youcannot 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 theother hand, some people who are obsessed with the weight issue try to losemore weight even though they are actually underweight. Therefore, it is usefulto have a scientific criterion to determine physical fitness. The range ofhealthy weight for adults is usually expressed in terms of the body massindex (BMI), defined, in SI units, asBMI W 1kg2H 2 1m 2 2 withBMI 6 19underweight19 BMI 25healthy weightBMI 7 25overweight(4–39)where W is the weight (actually, the mass) of the person in kg and H is theheight in m. Therefore, a BMI of 25 is the upper limit for the healthy weightand a person with a BMI of 27 is 8 percent overweight. It can be shown thatthe formula above is equivalent in English units to BMI 705 W/H 2 whereW is in pounds and H is in inches. The proper range of weight for adults ofvarious heights is given in Table 4–3 in both SI and English units.