# Do we lose weight through exercise according to E=mc^2?

1. Oct 22, 2006

### mtanti

Do we lose weight through exercise according to that equation? I mean fat is turned to heat during exercise right?

2. Oct 22, 2006

### Hootenanny

Staff Emeritus
No. That equation gives was is termed the rest energy of a particle of mass m. Indeed fat and carbohydrates (and protein to come extent) are metabolised to provide energy for respiration the rate of which increases during exercise. We lose weight not because we convert mass in to energy but when we excrete the metabolites. i.e when we break down carbohydrates we excrete the waste products. I am not a biologist so this is probably a rough and ready explanation but I can say without a doubt that when you exercise you do not convert mass into energy. In fact, immediately after exercise probably all of the weight lost is due to water loss (sweating, ventilation etc.)

For a moment imagine if we did convert our mass into energy though! Say we lost a pound through exercise that would mean that we would produce about 4x1016J of energy! That's the same energy output that a large nuclear power station would produce of nearly 13 years! All from one pound of mass.

Last edited: Oct 22, 2006
3. Oct 22, 2006

### mtanti

So where does the metabolised material go? All through roughage? I mean we are not losing weight because of lost water as sweat in the long run since water is something we consume a lot of during a diet...

4. Oct 22, 2006

### Hootenanny

Staff Emeritus
I believe most respiratory metabolites are excreted through urine. Look up respiration -Glycolysis, Krebs Cycle, Electron Transport chain to see what the metabolites are and perhaps you can see how they are excreted.
No, we are not. That is why I said immediately after exercise.

5. Oct 22, 2006

### Wally

What about the heat we give off after intense physical activity? (Or the heat we're constantly giving off) Where does it come from? Does it come into us as heat, or does it come in as food/drink? Is this "heat energy" even the same kind of "energy" referred to in e=mc2?

6. Oct 22, 2006

### Office_Shredder

Staff Emeritus
It's heat released from chemical reactions in our body. Which means yes, it starts as food and drink

7. Oct 22, 2006

### Wally

Thank you! Is it the same energy as referred to in e=mc2?

8. Oct 22, 2006

Staff Emeritus
No. To a very high degree, mass is conserved in biological processes and relativistic corrections are of the order of one part in billions. So the chemical energy given off is only from swapping electrons from higher energy states to lower ones.

9. Oct 22, 2006

### Bystander

Certainly. How much? Burn a couple thousand dietary calories; you're only 10% efficient, so you've only released 106 Joules as heat by combusting carbohydrates or fats, and producing water and CO2. 106 Joules is equivalent to 10-11kg mass loss.

No, fat is turned into water and carbon dioxide which you lose by respiration, perspiration, and excretion.

10. Oct 23, 2006

### mtanti

So losing weight is a question of how much water and carbon dioxide we lose?

Excluding solid excretion, what else leaves our body in order to lose weight?

Why does increasing your metabolism help?

11. Oct 23, 2006

### Hootenanny

Staff Emeritus
Increases your rate of respiration, more carbohyrates metabolised producing more CO2 and water.

12. Oct 23, 2006

### mtanti

So basically the more you breathe, the more weiight you lose?

13. Oct 23, 2006

### Hootenanny

Staff Emeritus
I'd prefer it if we said the greater the rate of respiration the more energy we use. If we use more energy than we take in, then we will lose weight.

14. Oct 23, 2006

### mtanti

Yes we all know that the weight we gain depends on the difference between energy input and output but the question is what happens to mass we lose... Does it turn to carbon dioxide, sweat and feaces only?

15. Oct 23, 2006

### DaveC426913

Well, there's also sloughing of material such as hair and skin cells, but this is not dependent on metabolism, it's just a constant.

16. Oct 23, 2006

### mtanti

Wow... So basically what you're after in a diet is to exhale a dense amount of CO2?

17. Oct 23, 2006

### Staff: Mentor

Intake less, output more...