Metabolism
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Starving to slim
EW is 26 years old. She weights 90 kg and is 1.52 m tall, and is about to undergo elective surgery.
What is her body mass index?
Body mass index (BMI) = weight in kg / (height in metres squared).
= 90 / (1.52 x 1.52) = 38.9
You would classify her as being obese
A desirable range of BMI is 20 - 25. If she is to achieve a BMI of 25, what should she weigh?
If BMI = weight / height-squared then weight = BMI x height-squared = 25 x (1.52 x 1.52) = 57 kg
If her target weight is 57 kg then she has to lose 33 kg.
The energy yield of adipose tissue can be calculated from the fact that it contains 80% triacylglycerol (at 37 kJ /gram) and 5% protein (at 17 kJ /gram). Therefore 100 g of adipose tissue yields:
from triacylglycerol: 80 x 37 = 2960 kJ
from protein: 5 x 17 = 85 kJ
total = 3045 kJ = 3.045 MJ / 100 g adipose tissue = 30 MJ / kg adipose tissue (rounding off)
Assuming that she has a total energy expenditure of 10 MJ /day, that all her weight loss will be adipose tissue, and that she starves totally (apart from drinking water), how long would it take her to lose 33 kg?
33 kg adipose tissue yields 33 x 30 MJ = 990 MJ
she requires 10 MJ / day, so it will take her 990 / 10 = 99 days to
lose 33 kg.
On the basis of these calculations she can lose 33 / 99 = 0.33 kg /day, or a maximum theoretically possible 2.3 kg / week by starving completely.
During the first week of her fast she actually lost 5 kg.