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Someone once told me it is possible to have a net weight gain of more than what you eat. For example if you eat a quarter pounder burger you can potentially gain more than 1/4 pounds. Is this true?

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Evil plan: We could feed 1kg of lion meat to tigers, and get 2kg of tiger meat to feed to lions to get 4kg of meat, and... hey we just solved world hunger! –  Sklivvz Apr 30 '11 at 23:45
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You can, if you a) drink 2 liters of cola meanwhile or b) implement the opposite of nuclear fusion inside your body (see 'conservation of mass' @Purdy) E=mc². –  user unknown May 1 '11 at 0:21
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@user unknown: I was assuming for the sake of argument that nuclear fusion does not, generally speaking, occur within the human gastrointestinal system. ...Yet. –  Jon Purdy May 1 '11 at 0:42
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The other answers here are not considering the water that you drink. You are 65-90% water, but a cheeseburger is probably around 30% water. So, you have to count the water you drink with your cheeseburger. –  Neil G May 1 '11 at 7:12
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Sort of. Everyone else has answered from the conservaton-of-mass perspective. However, a 1/4 pound cheeseburger is made from 1/4 pound of meat, ergo the 1/4 lb. does not include the bun, cheese if you like, lettuce, tomato etc... Therefore, it is theoretically possible to gain more then a 1/4 lb. from eating what is commonly referred to as a "1/4 pound hamburger", but that is simply because the assembled hamburger itself weights more then 1/4 lb. –  Fake Name May 1 '11 at 8:33

3 Answers 3

The change in mass of ones body must be (mass ingested)-(mass excreted) by the principle of conservation of mass.

The only way to increase mass beyond what you eat and drink would be for mass to enter your body in another way such as injecting fluid.

For example according to Association of Ringside Physicians Releases Consensus Statement On Weight Management in Professional Combat Sports

Many cases of dehydrated athletes using intravenous fluids to re-hydrate after weigh-ins have been reported

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You're forgetting breathing. It would be theoretically possible to gain additional weight by combining oxygen from the air with the molecules in the food, though I don't know offhand if this actually happens. –  jamesqf Jul 21 at 0:10
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@jamesqf breathing results in net weight loss. See my more detailed answer here: chemistry.stackexchange.com/questions/26963/… –  DavePhD Jul 21 at 0:40
    
This seems like theoretical and logical answer to me, which I think is not accepted on this site. Truth is often counterintuitive and sometimes surprising and only a controlled experiment can give definitive answer. –  Suma Jul 22 at 11:14

Your friend mentioned only what you eat. In theory, a given mass of food could be converted into body tissue which contains a larger ratio of water to solids, causing, with the addition of sufficient water, a gain in body mass greater than the mass of the food.

But if this actually happens with any kind of food, it would likely be dry foods, not something juicy like a burger. This source says that while muscle tissue is 75% water, fat is only 10% water. So even in the unlikely event that lean ground beef were converted directly into an equal amount of fat as measured by the mass of its dry solids, the "live" mass of the fat would be significantly less than the mass of the beef because it contains far less water.

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Anyone care to explain the downvotes? –  Kevin Krumwiede Jul 22 at 18:27

Yes, it is possible. Burgers contain salt. Salt causes water retention.

http://www.worldactiononsalt.com/salthealth/factsheets/retention/index.html

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You still need to ingest the water, so the total is less or equal than what you eat. On the other hand, if we don't include ingested water in the total, the claim is trivially true even without counting water retention. –  Sklivvz Sep 4 '14 at 14:03
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Well if the term "weight" completely excludes and biological process necessary to preserve life, such as drinking water or respiration, or metabolism, then the correct answer obtained by correctly applying the law of conservation of mass, is that after any act of eating your new weight is exactly the sum of your weight before eating and the weight of the food. –  Steven P Sep 6 '14 at 14:51
    
If you include merely the biological process of respiration (breathing) then your weight after the meal can be either more or less than the sum of your previous weight and the meal because of the stochastic nature of oxygen exchange in the lung. If you want to wait long enough for metabolism to happen (as the popular, but incorrect answers presuppose) then you have to let that experimental subject drink too---unless you mean to kill him. –  Steven P Sep 6 '14 at 15:00
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You do not eat water. The question obviously is excluding any atoms added to the body thorough other methods like drinking and breathing, or skin absorption. –  Jonathon Wisnoski Sep 8 '14 at 16:08
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If you only count eating solid foods, then the claim is trivially false. There are liquid diets which can be fattening. Also, water retention is not permanent, so it's a bit of a stretch to say it's a "gain". –  Sklivvz Sep 8 '14 at 21:53

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