Can a human store energy within his or her body without noting an increase in mass?

Or: Is fat more like gasoline or electric batteries?
Or: Is a human more like a Mustang or Prius?
Or: Does 3500 Cal burned/inputted always equal one pound lost/gained?

I’m asking about the cost (in exercise) of cheat meals. If you put an excess 3500 calories into a human, will that human weigh one pound more, or will that excess energy be stored within the human’s existing fat cells without an increase in mass?

Loosely related, what is the weight of 3500 cals of food?. Ideally in bacon, so it looks more or less like what it will become in the body, but other foods are also ok (oreos, butter, etc)

A lead/acid battery holds about 144,000 joules per kg.
Fat (triglycerides) holds about 38,000,000 joules per kg.
Gasoline holds about 130,000,000 joules per kg.

Well, fat is about 9 calories per gram, so 3500 calories would be about 0.85 pounds of pure fat.

Nope - it is pretty much linear - an excess of 3500 calories will result in an increase of weight by about a pound.

However, it is often hard to see this directly - your weight fluctuates by more than 2 pounds over the course of a day. Exercise, fluid intake, sweating, urination and defecation (or not, as the case may be) all can affect weight. But if measured correctly and regularly, then an excess intake of 3500 calories will add a pound to your weight - and conversely, 3500 calories of exercise over your normal base energy expenditure will reduce your weight by a pound.

Its also pretty hard to eat that many calories - 12000 calories (as eaten by Michael Phelps) is a lot of food - large amounts of enhanced pasta. I have heard other sportsmen (Americas Cup grinders) complain that they get fed up with pasta, as it is the only thing they can eat to get that level of energy (6000+ calories per day).


Very cool little factoid. Do you mind if I ask your cite?

All of those and more can be found in this Wiki article. Figures differ slightly from those given by Squink; the energy density of a lead-acid battery is given as 108,000 J/kg rather than 144,000 as an example. Close enough for me.

Nice table. I just looked em up seperately, got concensus values, and converted Calories/Watt hours/whatever to Joules.

Wouldn’t fat do that, too? Like, couldn’t you eat, uh, lard shakes?

Don’t forget…not everything you eat gets absorbed. That’s the idea behind Olestra and other crap. Fiber, for one, goes nowhere. Sugar and water are relatively sucked up. Carbs and fat, somewhere in the middle.

I don’t understand what you mean by this. Carbs and fat are essentially 100% digested and absorbed by the body, the same as sugar. Slightly more than sugar if you include people who are lactose intolerant and so don’t digest lactose.

It’s more like gasoline in the sense that it’s in chemical form and the energy is potential. Anyway there is ‘energy’ in the physics sense of ‘the ability to do work’ and then there is energy in the sense of “not made of mass” - these are two different things.

But even in the second case, E=MC^2 so all energy has a mass equivalence. If you had enough energy in the form of light beams bouncing around inside you, you would weigh more on the scale.

And the pure substance mass of the food you are eating is so large in comparison to the increase of potential energy equivalent mass as to make it effectively negligible. You could have a very heavy food (lead) with very little potential energy and a very light food (sugar) with high potential energy. The problem is that even though the high energy extremely high amounts of energy are of very little mass equivelence, it’s attached to heavy molecules.

That said, I think it’s obvious that at least long term energy storage in humans involves saving up more molecules than you started with. This isn’t fundamentally so, though - theoretically, one could strip all of the potential energy from incoming molecules, add all that energy to the atoms that already exist in your body, and then get rid of then get rid of the consumed atoms. Effectively you would see no increase in mass. Technically there would be a small increase in mass equivelance to the tune of energy divided by the speed of light squared, so a tiny amount barely if at all measurable.

How much (chemical) potential energy a molecule holds is related not to how large it is, but rather to how close it is to filling a particular electron shell.

If you want to drink lard shakes…

There are limits to how well your digestion can cope with things like fat - you could possibly overload the ability of your gallbladder/liver to process the food.
Also, one of the primary goals of eating for these people is not just calories, they are aiming to generate glycogen - high availability muscle fuel. Most people have about 2000 calories of glycogen in their bodies - when that runs out, athletes hit the wall - about 20 miles into a marathon. Since easily digestible carbs (like pasta) can supply glycogen, the trick is to to load up on carbs to give peak glycogen ahead of time, and to be still digesting more to keep the supply going while using up the reserves. This attention to nutrition allows marathon runners to avoid hitting that glycogen barrier (but where Haile Gebrselassie keeps 2000 calories of glycogen is an absolute mystery). But glycogen is not as efficient (in terms of storage) an energy source as triglycerides (fat), and athletes also need protein to rebuild damaged muscles after use.

So between having something you can actually eat enough of without throwing up, trying to get enough energy to support your training/competing, and maintaining a healthy body, enhanced pasta seems to be it.


You could store a tiny bit of energy without mass increasing by warming up slightly, but the body would compensate by trying to increase heat transfer out.

I am sure I am missing a ton of details, but does that mean that I can eat .85 pounds of food and gain one pound?

Conversation of mass, my friend. If you were to eat .85 pounds of food and every calorie of that food were converted into adipose (fat tissue) then you’d gain exactly .85 pounds. As you burned that off you’d lose some of the energy as heat, primarily you would exhale most of it.

Most food however is not converted completely into, well, anything; that’s why we have toilets.

If you’re skeptical, approach it like this:

3500 calories of fat, at nine calories a gram, is 389 grams - or 0.85 pounds. The 3500 calories = one pound meme comes, I suspect, from our love for integers.

ETA: This may be a mild hijack and feel free to ignore me if it is - but wouldn’t it be possible to convert glucose/glycogen into adipose? If so, wouldn’t the corresponding loss of water from the condensation, having been excreted, leave you with the same calories with less mass and thus a higher energy density?

What did the boson say to the fermion? :wink:

Of course, that’s where all fat comes from. Plants can’t produce fat through photosynthesis. Instead plants and animals both convert carbohydrate to fat for long term storage.

I doubt it very much.
For starters the process of lipogenesis isn’t free. You lose quite a bit of energy in the conversion form one form to another. This is a large part of the reason why the body prefers to burn sugar and protein in favour of fat. It’s more sensible to store fat in its own form then to try to convert other substances to fat at an energy loss.

The second point to remember is that lipids are themselves much less dense than carbohydrate. So although lipids are far more energy dense than carbohydrate that is largely because they are far less physically dense. Think of it like this: 1kg of lipid contains far more energy than 1kg of glucose, but 10cm^3 of lipid doesn’t contain a lot more energy than 10cm^3 of glucose simply because the 10m^3 of glucose will weighs about 1.5 kg while the fat will weigh about 0.75 kg. The reason that’s important is because the body needs to store that fat somewhere. That means you will need mass for cell membranes, blood vessels and so forth.

Nope. While 3500 calories is .85 pounds of pure fat, in the body it is bound up in adipose tissue, containing water and cell walls and blood - giving a net gain of much closer to a pound.

The critical thing was that it is .85 pounds of pure fat - solid lard, no water (even butter has a fair amount of water content). And even then, digestive and energetic losses would mean that you probably would not gain the full pound, if you could actually eat it and keep it down in the first place.