﻿ Body Fat and Energy

## Body Fat and Energy

#### Body Fat Energy

• Energy per unit mass of pure fat ≈ 9 kcalories / gram 

• Fat proportion of adipose tissue80% by weight

• Therefore energy storage per unit mass of adipose tissue ≈ 7.2 kcal/g

#### Common units for mass and energy...

##### Mass
• 1 Stone = 6.35029318 kilograms
• 1 Stone = 14 pounds
• 1 Pound = 0.45359237 kg
##### Energy
• 1 kcalorie = 4.184 kjoules
• 1 Kilowatt hour (kWh) = 3.6 megajoules
• 1 Kilowatt hour (kWh) ≈ 860.42 kcalories

Note. a calorie is the amount of energy needed to increase the temperature of 1 gram of water by 1°C, therefore a kcalorie (or kilocalorie) is the amount of energy needed to increase the temperature of 1kg of water by 1°C.

#### Body fat energy per various units of mass

kjoules kcalories kWh
gram 30.1248 7.2 8.37 x 10-3
kilogram 30,124.8 7,200 8.37
pound 13,664.4 3,265.87 3.8
stone 191,301.31 45,722.11 53.14

#### Energy Expenditure

• A typical human expends approximately 2000 kcalories per day = 8368 kjoules ≈ 83 kcalories/hour ≈ 96.8 Watts (joules/sec). The human brain typically accounts for 20% of that energy, so a human brain runs on about 19 watts.

• Running expends about 720 kcal/hour  depending on weight, speed and fitness level. 720 kcal/hour = 0.2 kcal/sec = 836.8 Watts (so approaching the energy of a 1 bar/kilowatt electric fire, or about 9x the average energy expenditure rate). One stone of body fat is equivalent to about 63 hours of running or about 380 miles! You would of course not be able to run that far in one session due to tissue damage caused by exercise, requiring protein and rest to repair (and of course the need for sleep).

• Walking at a moderate pace of about 3 mph expends about 250-300 kcal/hour  depending on weight, speed and fitness level. One stone of body fat is equivalent to about 166 hours of walking or about 500 miles!

#### References

 http://www-istp.gsfc.nasa.gov/stargaze/Senergy.htm
 How Many Calories Do You Burn?
 Calories Burned While Walking 2012 Colin Green.
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