My question is whether all the excess energy that is left every day always go to fat, or can it be wasted or disposed of somehow? (for example by not digesting everything to 100 percent?) Now, I am not asking for "ways" of consciously doing this, I understand that it can be difficult, but physiologically speaking, do any such disposing processes exist?

That is my main question, but just for some more clarity:

Let's call all the energy chemically contained in a day's food A. Let all the energy received by the body from digesting that food B. Is A = B? If B is less, is it always smaller than A by some sort-of constant factor, or does it depend on anything going on in the body?

Further, let C be all the energy used up by the body for "real work" like driving muscles, sending all the electrical signals in the neurosystem, running all the chemical reactions that require energy in the cells and building new cells.

Now, let's say that any given day C is less than B. So now, my main question explained in variables would be: any given day, does all the excess energy (B-C) all go to fat, or can a part of it be disposed of in some way? If so, how? (again, not "how do I do this?", more "what physiological processes are going on if this happens?".)

It sure seems that way because people seem to have very different metabolisms, some never get fat no matter how much they eat (not only can their metabolism burn a lot, but it is ADJUSTING to their diet) , and others gain weight very easily with just a little excess energy.

What are the physiological mechanisms behind this?

-- Update -- Thanks for your answers, I will try to clarify the point of the question.

Basically, the point is: "Physiologically, how do some people seem to keep their weight no matter how much they eat? (They keep the same weight if they eat a lot, BUT ALSO if they eat normal amounts). Because them doing so means that the rule "All excess energy that is digested but is not used up goes to fat" does not apply. For them the excess does not seem to go fat, so where does it go instead?" For example, see links in this answer to another question: No such thing as a hardgainer? {label "MAIN QUESTION"}

I see only 2 ways of this happening, and I will keep using my variables, even though they are not 100%-bulletproof of course, considering such things as body digesting its own cells, thus sort-of increasing B without increasing A; C being used up for digestion (increasing B), and so on. Think of variables as just a way of explaining the question and having a general mathematical picture of what is happening. Also note they only count the energy, not vitamins/proteins/etc.

Either 1) Any given day, ratio of A/B is sort-of the same, and depends only on a) generic personal metabolism (doesnt change much from day to day), b) that day's diet c) maybe some secondary random factors such as subject's stress level or so. This way, if B>C, there must be some mechanism in the body that can use up arbitrary amounts of energy for doing same thing, and it is using more energy if the people in question eat a lot, and is using less if they eat normally. What is this mechanism and how does it know how much energy to use? Any specific hormones doing this? Or is it building some chemicals instead of others?

2) Any given day, if there was a lot excess energy (B-C), the digestion process for the next day is regulated in such way that the next day (or "period of time", I don't think our body plans in days, but just used for clarity) the A/B ratio gets higher, thus the next day's B will get lower, and this way (B-C) of the next day will be lower or even negative, thus preventing the building of fat. Is this the case? Are there any specific known hormones/mechanisms that regulate the digestion this way? (dependent on the body's general energy vector (excess/deficiency)

Or I have some very big logic flaw somewhere. In that case please just answer the part labeled {label "MAIN QUESTION"}

  • One of the factors not mentioned is NEAT: Non-exercise activity thermogenesis. In other words, moving around, working, fidgeting. According to one review, "NEAT increases with overfeeding and decreases with underfeeding." So, that's one way to burn up excess calories.
    – Chelonian
    Jun 7, 2013 at 5:41

4 Answers 4


In an effort to support the very thorough answer given by @Berin Loritsch, here is a more explicit conceptual picture.

The mass food that is taken into the body (A) has 3 potential fates:

1) it will not be digested or absorbed by the digestive system (gut) and will pass out of the body via the anus. There are many factors that affect this but mainly it is due to the composition of the food and the enzymes found in the body. We cannot digest food w/o the appropriate enzymes.

Note that your B is the net amount of energy remaining after the indigestible material is expelled, so A does not equal B.

B can then have two remaining fates:

2) it will be broken up into nutrients (molecules your cells can use), absorbed by the digestive system into the blood and the chemical potential energy in the molecules will be used by your body for energy (C). Most of this mass will then leave the body as $latex CO_2$ when we respire although the non-carbon portion of it will leave as metabolic waste in urine.

3) it will be broken up into nutrients (molecules your cells can use), absorbed by the digestive system into the blood and then used for the process of biosynthesis. This is where the body builds its own molecules from those originally found in the food. This process makes new tissues so it can add mass to the body.

If B = C then there is no material remaining for biosynthesis (making new molecules and tissues) so this would be a deficient diet because case 3 could not occur. In this case the body must take molecules from existing tissues to fulfill more pressing biosynthesis needs.

If B < C the above is just more extreme because the body would need to take molecules from existing tissues for its energy needs as well.

If B > C then there is an excess amount of material and energy for cases 2 and 3. As @Berin Loritsch points out, there are a lot of factors that determine the fate of the excess molecules. The body generally stores excess energy as glycogen and fat but if for example one is working out and building muscle mass, then the additional material would go toward increasing the size of the muscle fibers. If one is pregnant, then the excess material would go toward producing the tissues of the new person, etc...

Edit: based on question edit

The answer to the main question is basically there are only 2 ways that the body can not gain mass following the consumption of food.

1) The food is not digested and absorbed by the gut and passes out of the body as fecal material. This loss is what is increased in some forms of bariatric surgery. In these forms of the surgery, the absorptive portion of the gut is shortened to make it less efficient in extracting nutrients from food. This is also how the weight-loss drug Alli works. It prevents the gut from absorbing fat in the diet.

2) The nutrients that are absorbed by the body are oxidized for energy and expelled from the body as carbon dioxide (most of the mass) and inorganic nutrients in the urine (a smaller amount).

Any other processes (fat production, muscle production, etc...) will add mass to the body. Since animals consume food in bulk we have mechanisms to store energy and nutrients in our tissues and make it available to the cells as they need them. The actual processes that control these mechanisms are much too extensive to even begin to get into in this type of answer - but, in short - yes, hormones are a major player in how the body regulates whether excess nutrients are converted to fat or meet another fate. Differences in these regulatory mechanisms control things such as the amount of nutrients that are oxidized to CO_2 and thus can give the appearance that people can eat as much or as little as they want and not change weight. However a long-term change in the amount of food that a person eats without a corresponding change in either their activity level or these regulatory mechanisms would result in a change in weight.


To answer that question it would require a long discussion about the pancreas, the colon, the kidneys, the liver and general digestion. I believe the thyroid also has a role to play.

The one organ that is most involved with regulating the fat stores is the pancreas. When your body has a surplus of blood sugar, it secretes insulin which stores the excess sugars as fat. When your body has a deficit of blood sugar, it secretes glucogon which converts fat to blood sugar.

The other organs help in various ways. Essentially the kidneys will dispose of the excess waste as urea (urine), and the colon disposes of the food that could not be digested as feces.

For most people, if you reduce the frequency and/or intensity of insulin spikes we can not only limit how much we store as fat, but how much fat we can burn throughout the day. Some people have a misbehaving thyroid, which requires a different approach. A bit of warning though, don't blindly cut all carbs without consulting an expert to make sure you have sufficient nutrients in other areas.

More info about the energy (measured in Calories or kcal). Understand first that not all calories are created equal. The body needs the following to function properly:

  • Vitamins and minerals
  • Protein
  • Calories

Any discussion about the digestive system needs to understand that the calories you take in are mixed with the vitamins, minerals, and protein. Your body can only absorb and use so much of all of these items and there is a certain balance it needs to maintain. You can introduce problems by having too many vitamins and minerals just as you can by having too few.

Also understand that your body is not a closed system. Trying to apply rules of the conservation of energy and such don't work because they defined as applying to a closed system. So let's look at your A, B, and C and give them new names that mean something.

Your A is dietary nutrients (i.e. what you consume). Your body cannot absorb all of the nutrients and calories stored in that food. Some things can never be processed (like dietary fiber), and simply pass through the system. Depending on the type of food you are eating, full processing can take anywhere between 6 and 12 hours. Processing food consumes calories, and the conversion rate is not perfect. Some types of food your body can absorb more readily, and others not so much. I don't have the space to talk about exactly which is which, nor do I have the background to say authoritatively.

Your B is processed nutrients (calories, vitamins, minerals, protein) that your body uses. The body can neither process everything nor can it use everything it processes. So your processed nutrients will always be lower than your dietary nutrients. How much less depends on how efficiently your body can absorb it. For example, pure glucose is already in a form that the body can use, so it can absorb and use almost all of it rather quickly. However, complex carbs such as black beans require much more processing and will take much longer to process and is a bit more inefficient. Protein is another substance that has easily absorbed sources and less easily absorbed sources. Natural sources tend to require more work to convert to something usable than engineered sources.

Your C is your daily nutritional requirements. This includes the Basal Metabolic Rate (BMR) which is the number of calories that your body needs just to perform its necessary functions (breathing, pumping blood, metabolism). In addition to this is the amount of work your body does in the course of a day whether by exercise or normal activity. So what happens when you have excess?

  • Excess sugar: pancreas secretes insulin to regulate the blood sugar levels, and all excess is stored as fat.
  • Excess protein: kidneys process the protein, and while they will process more protein than if you took in what your body needs the remainder will be converted to urine. People with improper renal function can cause renal failure by consuming too much protein.

What happens when there is a deficiency?

  • Deficient sugar/carbs: pancreas secretes glucogon to regulate the blood sugar levels, and converts fat to blood sugar. Kidneys increase the production of ketones which also assist in the breakdown of fat and protein into sugar. To protect yourself from ketonic acidosis you need sufficient protein which will be processed before your muscles/organs.
  • Deficient protein: your muscles will not be fed properly and they will shrink. In the case of ketosis (described in the bullet point above), your muscles will begin to be converted to calories so your brain can continue to function. This is a dangerous and unhealthy state.

D (new) is the natural byproduct of your body performing work. There will be waste products as your muscles are pushed to exhaustion, and as part of normal function. The waste is carried by the blood and filtered by the liver. The liver then converts the waste products into one of the components of urine. This includes lactic acid from exercising in an anaerobic state (which your body burns more sugar than fat).

  • I dont understand, does your comment about the kidneys and colon suggest that the body can in fact only digest a part of nutrients and dispose of the rest in form of unprocessed carbs/fats/whatnot? What does this depend on? How does it decide how much to digest and how much to leave? I know about the insulin and sugar, but what controls the initial sugar level? It can't be ONLY the diet, that's what I don't get.
    – Cray
    May 16, 2011 at 12:53
  • @Cray, there is only so much space to discuss these things. Did you read up on what all the organs do and how they function? The links I gave provide more information than fits here. The body will process everything it can, and what it can't it will dispose of either as urine or as feces. That is how the body is designed to work. Your body isn't going to delay processing food, but certain foods take longer to digest than others (such as your complex carbs vs. simple sugars). May 16, 2011 at 13:51
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    If you want something beyond just diet to burn fat, your muscles will do so as long as they are supplied with sufficient oxygen. In other words, an aerobic workout. However, do know that 80% of a weight loss journey is diet (i.e. what you eat). May 16, 2011 at 13:53
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    Thank you Berin for a big answer, however, I feel that you are a little off-topic. I am not trying to find out general facts about the digestion system, I am trying to answer the specific question. (And while other information may be required to learn before I can understand the answer, I don't see how this is the case here.) Please read the update in my question. Thanks for your response anyway. (Also, by my definition, B only stands for the energy (calories) - not vitamins, minerals, proteins or other things used up by any non-energetical mechanisms or cell construction.)
    – Cray
    May 17, 2011 at 11:41
  • @BerinLoritsch There is no such thing as glucogon, it is glucagon. Secondly fat can NOT be converted into sugar, only protein can be turned into sugar through gluconeogenesis. The only thing that comes somewhat close to sugar (functionally speaking) would be ketons. Glucagon is a hormon triggering the liver to let its stored glygon in form of glucose into the blood stream, nothing else. Jun 2, 2013 at 17:41

These are good questions and good answers. To a degree however one can get tangled in the details. This seems to be a problem in the medical community. My reply, since lacking the details of the others, may subject itself to being ignored. One assumption that comes into play when it comes to input and output of energy has to do with "acceptable outcome," or "desired outcome of energy placement." For example, when high amounts of sugar are assimilated without the addition of fiber there is usually the result of sugar spikes in blood sugar associated with diabetes. If the body has tremors as a result this uses energy derived from digestion having gone through ATP--in other words energy has been wasted--but only in the sense that is caused movement--or motion (technically can be considered work in the purely scientific sense) but certainly not the outcome desired. Though this is not the desired outcome it nevertheless falls onto the side of word-or consequence of the oxidation of food or fuel.

Next, another thing overlooked here is necessary microorganisms. If the flora (bacteria) in the intestines is not balanced (bacillus and candida) then a persons ability to properly break down food to derive necessary nutrition is greatly compromised. People who have a microbial imbalance are commonly too heavy or to thin. Too heavy is a more frequent outcome simply because one may eat and not derive proper nutrition simply because of a yeast infection or microbial imbalance. This person who has the correct balance of differing bacterium will be more likely to eat whatever is desired and still maintain a proper weight. The interesting thing about this imbalance is that it may occur to great varying of degree. This imbalance is often associated with people who gain weight not matter what they eat and until they correct this bacterial imbalance they will continue to gain weight and many will continue to have a problem satisfying their appetite.

Also it cannot be emphasized enough that any type of sugar that is eaten needs to be eaten with a proper amount of fiber either from the sugary food or from other foods containing fiber at the same meal. It is not merely about how much sugar, but about how well the body can both absorb and assimilate. An orange may be good for you, and orange juice may be detrimental--too much to absorb at once. But, if a person is a heavy runner, or exercises strenuously every day a glass of orange juice may be just what he (or she) needs, since the body is burning fuel fast then that extra amount of sugar can be both absorbed and assimilated.


Edit-- just go to this link. It's much better than my answer that follows


I'm a skinny dude, "hardgainer" type body, generally I am able to eat a lot and stay the same weight. Even if I'm depressed and lazy for a length of time, as a few weeks ago. (there were a few years where I gained a bunch of weight and got fat; since then I've returned to normal and that's been the only really major exception in my adult life that I can remember.)

I actually want to gain muscle weight now, and I've tooled around the web trying to understand this issue. The answers here as well as elsewhere are frankly inadequate and don't even pass a basic common sense test, as you suggest with the logic in your question. Obviously excess calories aren't stored as fat always. They can't possibly be. We have too much empirical evidence to the contrary, as you suggest in your question. Pretty much everybody knows someone or many someones who can eat whatever they want, not exercise, and still never gain weight.

I'm starting to think that scientists and others "educated" about nutrition actually haven't the foggiest idea about some of the most basic and elementary questions related to human digestive and metabolic function.

My guess is that a lot of calories simply get wasted in the case of overeating for many people. The body doesn't burn them as efficiently. It just does a half-assed job and your poop/urine has a different chemical composition. It's like half-burned wood or something instead of really fine ash. Probably examining stool more closely would lead to a better understanding. Any volunteers? :-)

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