Science Questions

Would my weight change if I ate a burger while standing on some scales?

Mon, 15th Jun 2015

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Abbas Dhanani asked:

If I stood on weighing scales while holding a big juicy burger would my total weight change if I ate the burger? I always thought I'd get lighter but I can't reason why.


Chris - I would argue that, actually, the burger is already mass in your hand on the scales and therefore, if you put it into your mouth, it just turns into A pair of cheeseburgersmass in your body.

There would then be some metabolism, obviously, because you’ve got to exert some energy to work the muscles of mastication to chew up the burger. And to produce some saliva; you’ve got to warm your body which means you're burning some energy doing that as well and you presume the burger is going to be a bit lower than body temperature probably by the time you’ve done your experiment.

So there's a little bit of a loss of energy there. So, that means you're going to lose a little bit of mass. Metabolism that’s going to carry on as you digest the burger as well, because you got to break it down into its component parts and absorb it. I reckon in the short term, no change. In the long term, there’d be a modest small reduction in mass...

Max - How accurate your bathroom scale is?

Chris - Well, that was going to be my bottom line, if you excuse the pun, because I was going to say that actually, you're talking about trivialities. Once the burger goes in, the burger becomes part of you. There will be some losses to the toilet though and so, a certain amount will be absorbed and a certain amount won't be absorbed. And so, you won't get all of the energy turning into mass. But let’s assume that none of the burger is wasted. It would all get converted either instantly into sugars that you would burn or fat that you would store, so your body mass would reflect the increase in weight gain owing to the burger...


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The scale would measure your weight as the same both before and after you ate the burger. The scale doesn't care whether the burger is in your stomach or in your hand; you weigh the same when added together either way. Supercryptid, Fri, 11th Mar 2011


Thanks for replying, but what about longer term?

Will I stay the same weight for the whole period of time between eating the burger and 'voiding' the burger?

i.e. does the chemical breakdown effect weight?

-Abb faxof, Mon, 14th Mar 2011

For dieting purposes, no, chemical breakdowns don't change the weight of the chemicals in question. A hamburger takes several forms after being digested: carbon dioxide gas which is exhaled from the lungs, fat that is stored in the body, liquid and solid waste, plus perhaps other things like sweat. Add the weights of all of these things together and you will get the same weight as the original burger (plus the oxygen used to "burn" the calories in it). Supercryptid, Tue, 15th Mar 2011

Abbas Dhanani  asked the Naked Scientists:

If I stood on weighing scales while holding a big juicy burger. Would my total weight change if I ate the burger? I always thought I'd get lighter but I can't reason why.

What do you think? Abbas Dhanani, Tue, 16th Jun 2015

I swear my stomach and thighs get bigger by every bite! Maizie, Wed, 24th Jun 2015

I don't understand this. I tried quoting some of the posts above and it gave me a message saying

When you eat, the matter breaks down in the process which means that chemical bonds are broken. During this process energy is converted to heat and that heat is radiated out of the body in various forms thus decreasing the mass of your body. However if we don't get that precise then in all chemical processes mass is conserved so that your bodies mass won't change unless you perspire or your body expels waste. PmbPhy, Thu, 25th Jun 2015

I think not. Digestion is not a nuclear reaction! You are converting the potential energy of e.g. sugar bonds, into kinetic energy. No nuclei are transformed by

3nO2 + 2(CH2)n → 2nCO2 + 2nH2O + heat

so the final mass is the same as the starting mass plus the mass of oxygen you inhaled to oxidise your food.

Were it not so, the universe would have disappeared into a burst of photons a long time ago. alancalverd, Thu, 25th Jun 2015

Ummm. I think so.

Obviously. But so what? You don't really think that E = mc2 only applies to nuclear reactions, did you?

That's right. And as I said above

I should have said that During this process the potential energy in chemical bonds is converted to kinetic energy which manifests itself as heat. etc

Again, so what? Why do you keep talking about nuclei? The reason that energy is released in nuclear transformations is because the potential energy in the nucleus from the strong force and the electric force is converted into the kinetic energy of the daughter nuclei and that's where nuclear energy comes from. In the case of digestion, if your equation is right (I don't know about the chemical processes in digestion) then there is potential energy in the chemical bonds of the sugars. That change in potential energy ends up being a change in the mass of the molecules.

Wrong. Again, you're confused. For some strange reason you don't know that mass changes in nuclear transformations happen for the exact same reason as mass changes in molecules. In the case of the former the potential energy is in the nuclear bonds between nuclei and the later the potential energy is in chemical bonds. The later is simply and insignificantly smaller than the former.

Not at all. The energy released in nuclear reactions is not just in the form of photons. It's also in the form of kinetic energy.

Again, I'm extremely surprised that you didn't know this. It's very small though. To see this go to:

See? There is energy released in chemical processes. It's six orders of magnitude smaller, but it's there.

I'm beginning to think that you didn't know that the energy released in chemical reactions!? That's not true. Tell me that's not true!!  PmbPhy, Thu, 25th Jun 2015

You are losing weight all the time (ignoring the mass of air being breathed in and out which causes fluctuations up and down if you include it), but if you eat the burger you will lose weight a bit faster as you munch and swallow it, even before you start digesting it. Some of the energy will be lost as sound and other vibrations (in addition to heat), but all of that lost energy starts as energy released from the rearrangement of bonds between atoms where a process similar to combustion is going on. David Cooper, Thu, 25th Jun 2015

PmbPhy, Thu, 25th Jun 2015

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