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?
46Evil 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!– SklivvzApr 30, 2011 at 23:45
6You 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 unknownMay 1, 2011 at 0:21
8@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 PurdyMay 1, 2011 at 0:42
6The 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 GMay 1, 2011 at 7:12
19Sort 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 NameMay 1, 2011 at 8:33
This is because of something called ecological efficiency (or trophic level transfer efficiency). This is the efficiency with which energy or biomass is transferred from one trophic level to the one above it. This is virtually always less than 100%.
This is because biological organisms are not perfect converters (so the thermal efficiency is not 100%) and some energy is lost as heat, as well as the fact that not all energy can be used by the organism (it isn't in a form that can be digested completely or not fully bioavailable).
As a very rough average, only about 10% of the energy is kept as you go up to the next trophic level but this varies a bit depending on circumstances. This is e. g. why you need substantially more than 1 kg of grain to get 1 kg of beef.
So not only is it pretty much out of the question that you can gain more mass than you eat, it is also virtually impossible to gain as much as you eat, because some of that energy is lost as e. g. heat or used in basal metabolism.
Sure, you can invent fanciful situations with a high-tech, futuristic substance that has an enormous energy density and bioavailability while having a tiny mass, but for all practical intents and purposes, the answer is a resounding no.