Body Fat and Energy
Body Fat Energy
- Energy per unit mass of pure fat ≈ 9 kcalories / gram [1]
- Fat proportion of adipose tissue ≈ 80% 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
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 [2] 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 [3] 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
[1] http://www-istp.gsfc.nasa.gov/stargaze/Senergy.htm[2] How Many Calories Do You Burn?
[3] Calories Burned While Walking
2012 Colin Green.