Thermodynamics

196 | Thermodynamicsdays.) Although the desire to get rid of the excess fat in a thin world may beoverwhelming at times, starvation diets are not recommended because thebody soon starts to consume its own muscle tissue in addition to fat. Ahealthy diet should involve regular exercise while allowing a reasonableamount of calorie intake.The average metabolizable energy contents of various foods and theenergy consumption during various activities are given in Tables 4–1 and4–2. Considering that no two hamburgers are alike, and that no two peoplewalk exactly the same way, there is some uncertainty in these values, as youwould expect. Therefore, you may encounter somewhat different values inother books or magazines for the same items.The rates of energy consumption listed in Table 4–2 during some activitiesare for a 68-kg adult. The energy consumed for smaller or larger adults canbe determined using the proportionality of the metabolism rate and the bodysize. For example, the rate of energy consumption by a 68-kg bicyclist islisted in Table 4–2 to be 639 Calories/h. Then the rate of energy consumptionby a 50-kg bicyclist is639 Cal>h150 kg2 470 Cal>h68 kgFor a 100-kg person, it would be 940 Cal/h.The thermodynamic analysis of the human body is rather complicatedsince it involves mass transfer (during breathing, perspiring, etc.) as well asenergy transfer. As such, it should be treated as an open system. However,the energy transfer with mass is difficult to quantify. Therefore, the humanbody is often modeled as a closed system for simplicity by treating energytransported with mass as just energy transfer. For example, eating is modeledas the transfer of energy into the human body in the amount of the metabolizableenergy content of the food.DietingMost diets are based on calorie counting; that is, the conservation of energyprinciple: a person who consumes more calories than his or her body burnsTABLE 4–1Approximate metabolizable energy content of some common foods(1 Calorie 4.1868 kJ 3.968 Btu)Food Calories Food Calories Food CaloriesApple (one, medium) 70Baked potato (plain) 250Baked potato with cheese 550Bread (white, one slice) 70Butter (one teaspoon) 35Cheeseburger 325Chocolate candy bar (20 g) 105Cola (200 ml) 87Egg (one) 80Fish sandwich 450French fries (regular) 250Hamburger 275Hot dog 300Ice cream (100 ml,10% fat) 110Lettuce salad withFrench dressing 150Milk (skim, 200 ml) 76Milk (whole, 200 ml) 136Peach (one, medium) 651Pie (one –8 slice, 23 cmdiameter) 300Pizza (large, cheese,1one –8 slice) 350