Modern Engineering Thermodynamics

708 CHAPTER 17: Thermodynamics of Biological Systems
EXAMPLE 17.5 (Continued )
b. The number of fasting days required to lose (consume) 10.0 kg of body fat is
t = m fat 10:0 kg of body fat
=
_m fat 0:317 kg of body fat=d ¼ 31:3d
Exercises
13. If the person in Example 17.5 requires only 8.40 MJ per day instead of 10.5 MJ per day, how much fat would be lost
(consumed) by fasting just one day? Answer: m fat = 0.250 kg fat.
14. You are stranded in the wilderness without any food (but you have plenty of water to drink). If you have 15.0 kg of
excess body fat, how long can you wait to be rescued before your body fat is consumed by your metabolism of
9.00 MJ/d? Answer: t = 55.2 d.
15. Suppose you have absolutely no excess body fat, and you are stranded without food. Your body then begins to consume
your muscle protein, which has a metabolizable value of 8.4 MJ/kg. How much protein would you lose (consume) after
being stranded for 30.0 days with a metabolism of 10.5 MJ/da? Answer: m protein = 12.5 kg protein.
Example 17.5 shows that, if you have 10. kg (220 lbm) of excess body fat, you can theoretically fast for
31.3 days just living on that body fat alone. This also gives you some idea why weight loss by dieting is such
a slow process. Fasting for long periods is not a medically sound method of weight loss since the body soon
begins to consume its own protein, and this can seriously affect the functioning of the body’s organsystems
(especially the heart). No one should ever willingly attempt a total fasting diet without consulting a qualified
physician.
Whereas most adult humans can survive long periods of fasting, they cannot withstand long periods without
water intake. Since the body continually loses water through the skin and lungs, it must be replaced or the body
soon becomes dehydrated and death quickly follows. Healthy adults have been known to fast for over 100 days,
but no human can survive for more than 10 to 20 days without water.
Tables 17.4 and 17.5 present the metabolizable energy content values for various common foods and the average
energy expenditure requirements for various human exercises. Common nutritional tables today have food
energy content and exercise energy expenditure levels listed in Calories. The capitalization of the word Calorie
indicates what nutritionists call a large calorie, that is, a kilocalorie: 1 Calorie = 1000 calories = 1 kilocalorie. This
is confusing notation, since only the capital C tells you that it is not the normal calorie energy unit, a subtle
point often overlooked by the publishers of nutrition tables.
Table 17.4 Approximate Energy Content of Some Common Foods
Metabolizable Energy Content
Food
Calories MJ Btu
Fast foods (average values)
Hamburger 275 1.15 1090
Cheeseburger 325 1.36 1490
Quarter pound hamburger 450 1.88 1790
With cheese 550 2.30 2180
With cheese and bacon 650 2.72 2580
Fish sandwich 450 1.88 1790
With cheese 500 2.09 1980
Hot dog 300 1.26 1190
With chili or cheese 350 1.47 1390
Regular fries 250 1.05 992
Regular onion rings 350 1.47 1390
Baked potato 250 1.05 992
With sour cream and chives 450 1.88 1790
With chili and cheese 500 2.09 1980
With broccoli and cheese 500 2.09 1980
With bacon and cheese 550 2.30 2180
With cheese 550 2.30 2180