days.) Although the desire to get rid of the excess fat in a thin world may be
overwhelming at times, starvation diets are not recommended because the
body soon starts to consume its own muscle tissue in addition to fat. A
healthy diet should involve regular exercise while allowing a reasonable
amount of calorie intake.
The average metabolizable energy contents of various foods and the
energy consumption during various activities are given in Tables 4 –1 and
4 –2. Considering that no two hamburgers are alike, and that no two people
walk exactly the same way, there is some uncertainty in these values, as you
would expect. Therefore, you may encounter somewhat different values in
other books or magazines for the same items.
The rates of energy consumption listed in Table 4 –2 during some activities
are for a 68-kg adult. The energy consumed for smaller or larger adults can
be determined using the proportionality of the metabolism rate and the body
size. For example, the rate of energy consumption by a 68-kg bicyclist is
listed in Table 4 –2 to be 639 Calories/h. Then the rate of energy consump-
tion by a 50-kg bicyclist isFor a 100-kg person, it would be 940 Cal/h.
The thermodynamic analysis of the human body is rather complicated
since it involves mass transfer (during breathing, perspiring, etc.) as well as
energy transfer. As such, it should be treated as an open system. However,
the energy transfer with mass is difficult to quantify. Therefore, the human
body is often modeled as a closed system for simplicity by treating energy
transported with mass as just energy transfer. For example, eating is modeled
as the transfer of energy into the human body in the amount of the metabo-
lizable energy content of the food.Dieting
Most diets are based on calorie counting; that is, the conservation of energy
principle: a person who consumes more calories than his or her body burns1 50 kg 2639 Cal>h
68 kg470 Cal>h196 | Thermodynamics
TABLE 4 –1Approximate metabolizable energy content of some common foods
(1 Calorie 4.1868 kJ
3.968 Btu)
Food Calories Food Calories Food Calories
Apple (one, medium) 70
Baked potato (plain) 250
Baked potato with cheese 550
Bread (white, one slice) 70
Butter (one teaspoon) 35
Cheeseburger 325
Chocolate candy bar (20 g) 105
Cola (200 ml) 87
Egg (one) 80Fish sandwich 450
French fries (regular) 250
Hamburger 275
Hot dog 300
Ice cream (100 ml,
10% fat) 110
Lettuce salad with
French dressing 150Milk (skim, 200 ml) 76
Milk (whole, 200 ml) 136
Peach (one, medium) 65
Pie (one slice, 23 cm
diameter) 300
Pizza (large, cheese,
one slice) 350(^18) –
(^1) – 8