How much weight will I gain?

August 3, 2005 10:38 PM

Can your weight increase by a greater amount than the weight of the food you eat? It seems to me that if I eat 500g of cake, then it couldn't possibly increase my body weight by more than 500g. Does this make sense? All my searching regarding this stuff deals in calories rather than mass/weight.
posted by bunglin jones to Health & Fitness (38 answers total)

It can and does. It's because when you put on fat there's a corresponding retention of water in the cells.
posted by abcde at 10:51 PM on August 3, 2005

If it's water that you drank, (as opposed to what? absorbing it from the air?) then it still comes under the heading of gaining the weight you eat.

Of course, if you stand on a scale holding a 500g piece of cake, then eat it, the reading on the scale won't change. You just gained exactly 500g, by putting it inside you. Of course almost all of that will come out the other end later...

The reason all this is done in calories instead of mass is that long term, you never gain as much weight as the weight of the food you eat. Rough estimate -- I guess I eat 2 kilograms of food in a day, but if I gained all that weight, I would be too big to fit through doors. Long term weight gain is dictated by the parts of the food (fats or proteins, depending on if you're getting fat or fit) that your body actually absorbs.
posted by rossmik at 11:25 PM on August 3, 2005

abcde : "It's because when you put on fat there's a corresponding retention of water in the cells."

But the water has to be included in that 500g or has to come later. Mass is a (scalar) conservative field (at classical scales, atleast).
posted by Gyan at 11:28 PM on August 3, 2005

Does this make sense?

Perfect sense. Conservation of mass, aka you can't get something for nothing. I challenge any crazies to argue otherwise. If the human body was somehow able to generate additional mass from a certain food intake, you could possibly build a scary human-engine that could take us to the stars and back.
posted by Civil_Disobedient at 11:42 PM on August 3, 2005

Not an answer: <sup>I think abcde's point was pretty clearly that increased water intake linked directly to the cake was being overlooked in the equation, rather than suggesting water isn't consumed. As described, the question was a little ambigious - the weight of the cake was the only factor being considered, not the water retention later. I would think that abcde's interpretation of the question as a metabolic one, (putting aside whether the answer is correct), is a more useful interpretation to a weight-watcher than interpreting it as a physics question about conservation of mass.

(I'm not in any position to know, but I would be surprised if a cake could cause enough water retention to match it's original weight. That would be scary :-)</sup>
posted by -harlequin- at 12:07 AM on August 4, 2005

But what about the Breatharians?

More to the point: No. Why? Because we don't gain anything through osmosis. The idea that you're gaining more weight from the water retention is a canard, because you'd have to, you know, consume that water in order for it to be retained.
Further, you tend to excrete a good deal of your food. I know I do.
posted by klangklangston at 12:10 AM on August 4, 2005

I think what you're looking for is the rough estimate that 3500 calories is stored as a pound of fat in the body (using water and anything else needed for the pyhsical arguments above). This is the estimation used in the Hacker's Diet.
posted by deaddodo at 12:55 AM on August 4, 2005

Response by poster: Thanks all - I did know the calories-to-kilos-of-fat conversions, but for some reason I thought I'd seen something stating that the conservation of mass doesn't work ...er... properly when it comes to converting food mass to body mass. Now I know better. Cheers.
posted by bunglin jones at 1:12 AM on August 4, 2005

The so-called "best answer" ignored both water and respiration. You are intaking more than just the food you eat, bunglin jones, and there are chemical reactions taking place inside your body which may affect the mass you ultimately retain. Your question is far more complicated than you would like it to be (I think) but I'd suggest that at least you consider your water intake if you are going to be measuring your weight. Also, if you are going to value the physicist's pure laws point of view, then please don't use mass/weight as a combined/interchangeable concept in the future :)

Any woman knows that your weight will fluctuate several pounds due to the amount of water you retain. I can't comment at length on how much of the air you breathe may ultimately increase your mass due to the chemistry of your digestive system, but your problem likely does not lie in misconstruing the law of conservation of mass - it lies in trying to draw an equals sign between the FDA label on your food and the number you see on your scale (which is hardly an equation sanctioned by tried and true physics, though thanks for talking us back from the brink of insanity, Civil_Disobedient).
posted by scarabic at 1:20 AM on August 4, 2005

Basically then, changes in your weight will be everything that goes in through your mouth minus whatever you excrete (plus sweat). Sweat can be significant in hotter climates but you will tend to make up the loss by retaining more water next time you drink rather than flushing it straight through your system.
Remember not to fall into the trap of thinking that exercise = weight loss. Apart from sweat you won't lose weight until your body metabolises your energy stores and you take a dump.
posted by biffa at 2:44 AM on August 4, 2005

What I'm confused on is how klangklangston is excreting a good deal of bunglin jones's food.
posted by Alt F4 at 4:08 AM on August 4, 2005

Fat is the most calorie-dense nutrient. A gram of fat contains about 9 calories. (I think the real number is 8.7 calories or something, but everyone rounds to 9.)

As others have mentioned, there are roughly 3500 calories to a pound. By this I mean that if you consume 3500 calories more than you expend, you'll gain a pound. If you burn 3500 pounds more than you take in, you'll lose a pound.

If you ate one pound (454 grams) of pure fat, you wouuld consume approximately 4000 calories. This is, of course, more than the 3500 calories needed to gain a pound of weight. In theory, a person eating one pound of pure fat would be consuming 1.14 pounds worth of calories.

However — and here's where I could be wrong — I believe that the body might do one of several things with 4000 calories from pure fat consumed all at once:

1. In order to process the calories, it may require some amount of water, probably at least 0.14 pounds of the stuff.
2. The body may be unable to actually process 4000 calories worth of fat at one time. The unprocessed fat may be shed — likely in unpleasant ways — and the body only use some portion of those calories.
3. Or, least likely, maybe the body really can convert 4000 calories from one pound of fat into 1.14 pounds of you. If it can, however, I think that it's a metabolic state that cannot be sustained.

I am not a dietician. I am, however, a dieter.
posted by jdroth at 5:56 AM on August 4, 2005

Stop it, jdroth. I dont want to eat a pound of fat to test out your theory.
posted by Plutor at 6:18 AM on August 4, 2005

Conservation of mass, aka you can't get something for nothing.

I'm no scientist, but this doesn't sound right in terms of ingesting things that are chemicals (food is a conglomeration of various chemical compounds) that cause metabolic reactions in the body. You are not simply taking mass and putting it in a closet inside the body. If your rule was true, steroids would do nothing (without excercise, I believe they will cause water weight gain with no concern towards the proportion of steroid weight to gained body weight.) My thiniking is that food causes metabolic reactions that cause the body to generate byproducts, either fat, muscle, water or waste products (some that are stored, some that are eliminated?) so we're not just talking about conservation of mass here. I'm more than willing to admit I'm wrong.
posted by spicynuts at 6:22 AM on August 4, 2005

To add to that: isn't the rule conservation of energy not conservation of mass? In that case, that's where calories come in. If it takes x amount of energy to break down food, then x amount of energy is a) converted to heat or b) used to fuel metabolic processes whose byproducts may be things that are heavy (water, etc). In this case, energy is conserved, but mass can be created.
posted by spicynuts at 6:29 AM on August 4, 2005

I believe jdroth is essentially correct though his conversions are funny. Calories are not a measure of mass, they are a measure of heat/energy. Literally, they are a measure of the effort required to break various chemical bonds. Lots of energy can be stored in little mass because such bonds are damn good at what they do (see uranium). So saying one pound of pure fat is 1.14 pounds "of" calories assumes an implicit digestive agent--in this case humans.
posted by nixerman at 6:29 AM on August 4, 2005

>Does this make sense?
Yes, if you never eat or excrete anything again. Then Civil_Disobedient's response has got you covered. Open and shut case.

Practically, you have to think about how the food you consume is going to be processed and stored and how it will affect your metabolism in the future. This is so complicated that no-one really has the full story. I don't, anyway.

But here's a way to maybe ask a little more practical question with the same gist: suppose A and A' are absolutely identical dieters. A has 500g of cake today; A' does not. Otherwise, they eat an identical diet. Is it possible for A to weigh more than 500g more than A' three days later?

Or a little more dramatically: suppose A eats 250g of cake every day for a week, which A' does not. Can it happen at the end of the week that A weighs more than 7*250 = 1750g more than A' does? In this more dramatic case I would say yes, due to water retension issues; and even over that amount of time metabolic changes could contribute, too. And I think that's more relevant to a dieter than the conservation-of-mass kind of argument.
posted by Wolfdog at 6:41 AM on August 4, 2005

I believe jdroth is essentially correct though his conversions are funny.

As my wife is fond of noting, I am not a real scientist. (By which she means: "I have a chemistry degree and work as a forensic scientist, while you, dear husband, have a psychology degree, and write." sigh)

You're correct that calories are a measure of energy. Yet somehow — magic? — these calories get converted to mass in my big gut!
posted by jdroth at 7:26 AM on August 4, 2005

A gram of fat contains about 9 calories

No, I think it contains about 9 K calories (or 9,000 calories). I can't tell if that 3,500 calories in a pound of fat is expressed as actual calories or as K calories. Does anybody know? If it's actual calories, then that would explain why you seem to gain less mass from eating a pound of fat than the chemistry would seem to indicate.
posted by willnot at 7:39 AM on August 4, 2005

willnot, when speaking about food and calories, it's always kilocalories that are under discussion. It's a given. At least in non-scientific circles.
posted by jdroth at 8:19 AM on August 4, 2005

Really, it all works - you just have to take a lot of things into consideration.


Like I said, I'm fully willing to admit I'm wrong. This is an interesting discussion. Thanks for the feedback.
posted by spicynuts at 8:22 AM on August 4, 2005

The so-called "best answer" ignored both water and respiration. You are intaking more than just the food you eat.

Not according to the question. Can you gain more than a pound of weight if you eat a half-pound of cake, and then drink two pounds of water? Sure. Because your intake is 2.5 lbs. No surprise there. But you cannot gain more mass than you take in. If you drink additional water because of something you've eaten, the rule doesn't change.

This whole discussion reminds me of a section of "Surely You're Joking, Mr. Feynman!" entitled "Mixing Paints":

The guy seemed to know what he was doing, and I was sitting there, hanging on his words, when he said, "And you also have to know about colors--how to get different colors when you mix the paint. For example, what colors would you mix to get yellow?"

I didn't know how to get yellow by mixing paints. If it's light, you mix green and red, but I knew he was talking paints. So I said, "I don't know how you get yellow without using yellow."

"Well," he said, "if you mix red and white, you'll get yellow."
"Are you sure you don't mean pink?"
"No, he said, "you'll get yellow"--and I believed that he got yellow, because he was a professional painter, and I always admired guys like that. But I still wondered how he did it.

I got an idea. "It must be some kind of chemical change. Were you using some special kind of pigments that make a chemical change?"

"No," he said, "any old pigments will work. You go down to the five-and-ten and get some paint--just a regular can of red paint and a regular can of white paint--and I'll mix 'em, and I'll show how you get yellow."

At this juncture I was thinking, "Something is crazy. I know enough about paints to know you won't get yellow, but he must know that you do get yellow, and therefore something interesting happens. I've got to see what it is!"

[...]
So I went to the five-and-ten and got the paint, and brought it back to the restaurant. The painter came down from upstairs, and the restaurant owner was there too. I put the cans of paint on an old chair, and the painter began to mix the paint. He put a little more red, he put a little more white--it still looked pink to me--and he mixed some more. Then he mumbled something like, "I used to have a little tube of yellow here, to sharpen it up a bit--then this'll be yellow."

"Oh!" I said. "Of course! You add yellow, and you can get yellow, but you couldn't do it without the yellow."

The painter went back upstairs to paint.

posted by Civil_Disobedient at 8:39 AM on August 4, 2005

Conservation of mass and conservation of energy are the same thing: e=mc^2.

However, the amount of energy in a given change in mass is so enormous that for practical purposes we don't normally notice or care about e=mc^2. The two conservation laws are treated independently most of the time. To make this idea practical, if that is the right word for it, the mass equivalent of the kinetic energy of a cyclist traveling at 20km/h is around one one hundred billionth of a gram.

So, due to conservation of mass:
start mass + input mass - output mass = final mass

In a complex system like the body, with so many different intakes and outputs, that calculation is very difficult to perform. Things that seem small, like the amount of oxygen absorbed during respiration, can add up over time.

My first guess at the intakes and outputs that need to be considered would be this (approximately in order of mass per day, I guess):

Intake

1. drink (includes plain water)
2. solid food
3. inhaled breath
4. material absorbed through the skin (must be a very small amount)

output

1. urine
2. sweat
3. feces
4. exhaled breath
5. shedding of dead skin
6. shedding of hair
7. removal of hair and finger nail material

Imagine if the mass of all the hair you have ever grown stayed with you. By thirty that must be in the kilograms!
posted by Chuckles at 8:44 AM on August 4, 2005

harlequin is right, I do realize that water counts as intake, but he did say "food" and I assume he was wondering the more common question of, why is it that I can have a half-pound of chocolate in excess of my normal diet and seemingly gain more than that weight.
posted by abcde at 8:55 AM on August 4, 2005

>This whole discussion reminds me of a section of "Surely You're Joking, Mr. Feynman!" entitled "Mixing Paints"

Hm. Parts of this discussion remind me of any number of jokes whose punchline is "...completely correct, and totally useless."
posted by Wolfdog at 9:18 AM on August 4, 2005

if you stand on a scale holding a 500g piece of cake, then eat it, the reading on the scale won't change.

I thought there was a famous experiment which demonstrated that the scale ultimately showed less -- I recall an engraving of a man wearing Victorian garb, dining at a table on a platform suspended from an enormous scale. Somehow the result of chemical reactions and the release of gases was a lower reading on the scale.
posted by Rash at 11:49 AM on August 4, 2005

The body may be unable to actually process 4000 calories worth of fat at one time. The unprocessed fat may be shed — likely in unpleasant ways — and the body only use some portion of those calories.

As far as I understand it, this is one of the reasons that Atkins works from some people. They simply can't absorb all that bacon grease. And, yes, it can be shed in unpleasant ways.
posted by MrMoonPie at 11:54 AM on August 4, 2005

Lots of energy can be stored in little mass because such bonds are damn good at what they do (see uranium).

Yes, except for the example. The energy generated from uranium in nuclear reactors and nuclear weapons is a result of energy stored in the nuclei of those atoms. This is a different beast from the energy stored in chemical bonds, which is primarily a result of the arrangement of electrons in chemical compounds. Atomic nuclei don't change in chemical reactions; they do in nuclear reactions. But yes, see any number of conventional explosives for an example of the energy that can be stored in chemical bonds.
posted by DevilsAdvocate at 12:18 PM on August 4, 2005

Interesting discussion; thought I would throw a little more fuel on the fire. Some people have mentioned respiration; this definitely needs to be taken into account. Some of the oxygen you breathe in is used to oxidize compunds that then remain in the body, thus increasing your wieght by more than the original ingested compund weighed. This is more than compensated for, however by the fact that most of the oxygen you breathe and some of the carbon in the food you eat is used for fuel via the Krebs cycle and electron transport chain, leaving your body as CO2. This is where exercise/metabolism comes into play and why a given number of calories does not produce the same weight gain in everyone.

Plants on the other hand, truly are breatharians, in that they synthesize sugars and then other compounds from atmospheric CO2 and sunlight, thus growing more than their intake of water and soil nutrients would suggest.
posted by TedW at 1:45 PM on August 4, 2005

Oh, so to answer your question: No, once you metabolize your food and excrete the remains, your weight will increase by less than the weight of the food ingested. Exactly how much your weight will increase for a given number of calories will vary according to how many calories you expend. Lance Armstrong will gain far less weight from a peice of cake than TedW will.
posted by TedW at 1:50 PM on August 4, 2005

Body uses energy (=mass) in digesting food, too. It's entirely possible to eat massive quantities of "something" and *lose* weight doing it, if the digestive process consumes more energy than it gets from the good.

IIRC, this is why you shouldn't eat ice or snow when dehydrated: the amount of water used in the chemical processes that produce the energy (body heat) to melt it, is greater than the amount of water taken in: ie. a net loss of water.
posted by five fresh fish at 1:51 PM on August 4, 2005

Any woman knows that your weight will fluctuate several pounds due to the amount of water you retain. I can't comment at length on how much of the air you breathe may ultimately increase your mass

Obviously you can't, since you don't know anything about the process. Every breath you take, you lose weight. You breath in O2, and you breath out CO2, which weighs more.

In fact, 'breathing' is the only way that you actually lose body mass. That's what happens to all the fat that goes away when you 'lose weight'. Water enters and leaves your body, along with 'solids' that never get digested, but the only way you actually lose fat is by breathing out the results of the chemical reactions (for the most part).

To add to that: isn't the rule conservation of energy not conservation of mass? In that case, that's where calories come in. If it takes x amount of energy to break down food, then x amount of energy is a) converted to heat or b) used to fuel metabolic processes whose byproducts may be things that are heavy (water, etc). In this case, energy is conserved, but mass can be created.

Actually, energy and mass are interchangeable, you can convert mass into energy and vise versa, however, that requires a nuclear reaction. The vast majority of weight changes in a person are related to gaining and losing mass by taking in or putting out chemicals.

I think there may be a slight amount of mass lost when chemical bonds break, but only a very slight amount. Not relevant to a dieter.


No, I think it contains about 9 K calories (or 9,000 calories). I can't tell if that 3,500 calories in a pound of fat is expressed as actual calories or as K calories. Does anybody know? If it's actual calories, then that would explain why you seem to gain less mass from eating a pound of fat than the chemistry would seem to indicate.

It's kilocalories, of course. All "food" calories are kilocalories, not 4.5whatever joules.
posted by delmoi at 2:23 PM on August 4, 2005

Response by poster: Maybe my saying that "now I know better" was premature. Truth is, now I'm a bit confused.... but even after all this discussion, I'm still standing by my original best answer
posted by bunglin jones at 6:06 PM on August 4, 2005

Every breath you take, you lose weight. You breath in O2, and you breath out CO2, which weighs more.

It's not quite that simple; you don't usually breathe out one molecule of CO2 for each molecule of O2 you breathe in. The ratio is called the respiratory quotient and is discussed in detail here. Of relevance to this thread: one of the variables affecting the respiratory quotient is the relative amount of carbohydrates in the diet
posted by TedW at 7:01 PM on August 4, 2005

delmoi, I still don't buy it. For example, my nutrionist has often told me 30g of peanuts is the equivalent of 700g broccoli (or the somesuch, I don't really listen to her). Intuitively, this suggests to me that you don't gain 700g when you eat 700g of broccoli. In terms of actual long term weight gain, what matters is not the 'mass' density of the food but rather the 'caloric' density. As somebody pointed out earlier, somebody eating 500g of cake each day over a sufficient period of time won't be X*500g at the end of the experiment. (I'm not a biologist and never liked biology since it's so messy but I do know quite a bit about physics and I suspect the conservation of mass is just too simplistic a model to accurately describe human metabolism.)
posted by nixerman at 8:16 PM on August 4, 2005

Nixerman, yes. The amount of weight you gain is relative to the amount of calories that you eat. But no food is going to have you gaining more weight then the weight of the food.

In other words, if you ate a pound of pure fat, you would not gain one pound. If you ate a pound of something less calorie dense then pure fat, then you would gain even less weight.

As somebody pointed out earlier, somebody eating 500g of cake each day over a sufficient period of time won't be X*500g

Yes, they will weigh less then the weight of the cake. (+ their origional weight + anything else they ate) The theoretical maximum amount of weight you can gain with the most calorie dense food you can find, is the weight of the food that you eat. Which would mean you shat out nothing.
posted by delmoi at 10:24 PM on August 4, 2005

It's not quite that simple; you don't usually breathe out one molecule of CO2 for each molecule of O2 you breathe in. The ratio is called the respiratory quotient and is discussed in detail here. Of relevance to this thread: one of the variables affecting the respiratory quotient is the relative amount of carbohydrates in the diet

Hmm. So your body produces water when it burns fat, along with CO2 (just like a car). Still, I think the weight of the CO2 would be more then the weight of the H2O, which also breath out as well.
posted by delmoi at 5:11 PM on August 5, 2005

Just to further muddy the waters: cake is pretty high in carbs, both simple and complex. That means you're going to secrete a bunch of insulin and other hormones in the process of digesting and metabolizing it.

Now, the Atkins folks and their cronies claim that this begins a metabolic cascade that lasts much longer than it takes for you to digest the cake. (Don't ask me what a cascade is, I don't know.) But their point is that the piece of cake may affect how much weight you gain from everything you eat the next day, because all that insulin and whatever else has put you into an anabolic (weight gaining) state.

So in that sense, the cake may have effects that outlast its 500 gram weight. (Jesus, you ate a pound of cake at one sitting?!)

I don't know if I believe this or not, but lots of Atkins dieters do.
posted by ikkyu2 at 11:29 AM on August 6, 2005