Modern Engineering Thermodynamics

17.6 Thermodynamics of Nutrition and Exercise 707
These energy content values were for dry or water-free foods. Most foods, especially carbohydrates, contain a
large amount of functional water (the mass of the human body, for example, is about 72% water). The energy
content of natural or wet food is lower that that of dry food due to the dilution effect of the energetically inert
water. The average natural state metabolizable specific energy content of the three basic food components is
Carbohydrate:
)
4:20 MJ/kg
Protein: 8:40 MJ/kg Metabolizable energy content natural state foods
Fat:
33:1 MJ/kg
Note the extraordinarily large specific energy content of natural state fat.
EXAMPLE 17.4
If the average meal consumed by an adult consists of about 45.0% carbohydrate, 15.0% protein, and 40.0% fat, determine
(a) the specific energy content of an average meal with natural state foods and (b) the total mass of an average meal needed
to provide 10.5 MJ per day.
Solution
a. The average energy content of natural (or wet) food components is
Carbohydrate:
Protein:
Fat:
Therefore, the specific energy content of the average meal is
4:20 MJ/kg
8:40 MJ/kg
33:1 MJ/kg
e avg: meal = 0:450ð4:20Þ+ 0:150ð8:40Þ+ 0:400ð33:1Þ = 16:4MJ/kg meal
b. A person who requires a daily food energy intake of 10.5 MJ must then consume
_m avg: meal = 10:5 = 0:640 kg of average meal/day = 1:4 lbm of average meal/day
16:4
Exercises
10. Suppose the person in Example 17.4 who needed 10.5 MJ of food energy per day goes on a diet that requires a food
energy intake of only 5.0 MJ per day. What total mass of average meal food should this person consume per day?
Answer: _m avg: meal = 0:300 kg avg: meal/d = 0:670:
11. If the amount of fat described in the average meal in Example 17.4 is reduced from 40.0% to 30.0% and the
carbohydrate and protein increase to 50.0% and 20.0%, respectively, determine the new specific energy content of
this meal and the total mass of this meal required to produce 10.5 MJ of food energy per day. Answer:
_m avg: meal = 0:770 kg avg: meal/d = 1:69:
12. People who live in very cold climates usually have diets that have a very high fat content. Suppose their average meal
consisted of 20% carbohydrate, 20% protein, and 60% fat. Determine the specific energy content of this meal and the
total mass of this meal required to produce 10.5 MJ of food energy per day. Answer: _m avg: meal = 0:470 kg avg: meal/d =
1:03 lbm avg: meal/d:
Overweight conditions place a greatly increased load on the heart and other organs. For example, each kilogram
of body tissue contains 0.885 km of tiny blood vessels. If an individual is 10. kg (22 lbm) overweight, the heart
must pump blood through an extra 8.9 km (5.5 miles) of small blood vessels.
EXAMPLE 17.5
People living in affluent societies generally know very little about starvation. Most feel that death is imminent if food is
withheld for only a week. However, we know that 1.00 kg of human body fat contains about 33.1 MJ of metabolizable
energy, and it would be useful to know:
a. The mass of body fat consumed per day if one uses 10.5 MJ of energy in normal activities.
b. How many days of total fasting are required to lose 10.0 kg of body fat.
Solution
a. A fasting person requiring 10.5 MJ of metabolizable energy per day consumes about
_m fat =
10:5MJ=d
33:1MJ=kg body fat ¼ 0:317 kg of body fat=d (Continued )