# what happens to the food we eat? **URL:** \ **Category:** Factual Questions\ **Created:** [September 12, 2012, 12:57pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501 "2012-09-12T12:57:47Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [September 12, 2012, 12:57pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/1 "2012-09-12T12:57:47Z")
I know this is variable but give me a ballpark. What percent of what we eat do we poop out? What percent is actually is burned as energy? If I consume 3000 calories a day, do I actually burn that much (assuming I am not gaining weight)? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 12, 2012, 1:06pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/2 "2012-09-12T13:06:56Z")
The body is extremely efficient in processing food. It gets nearly everything of value out of all foods, unless there is a physical problem with the intestines. Food consists of proteins, carbohydrates, fats, vitamins and minerals, and fiber. The vitamins and minerals are in amounts too small to bother with and the fiber is excreted so it doesn’t count as calories. You digest the protein, carbohydrate and fat. Either you burn them off or they get stored for later use. The percent that gets excreted is hard to say because that will vary greatly depending on how much fiber there is in what you eat. You aren’t going to see a correlation in your feces because at least half of that is dead bacteria that naturally live in the colon. ---
**Author:** ![Don\_t\_Panic](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/don_t_panic/32/398_2.png) [@Don\_t\_Panic](https://boards.straightdope.com/u/Don_t_Panic)\ **Post date:** [September 12, 2012, 3:51pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/3 "2012-09-12T15:51:18Z")
> [@Exapno\_Mapcase](#): > > You digest the protein, carbohydrate and fat. Either you burn them off or they get stored for later use. Perhaps you could finally explain to me something that I’m ashamed to say I that still don’t really understand: If I consume more calories than I burn, I will gain weight in the form of fat. This seems to work independently of the actual proportion of carbs / fats / protein in the foot I eat. It’s all about calories in vs. calories out, right? Well, how does my body do this? As we all know, carbs are what really make you fat, not fat. Does my body actually convert non-fat food molecules into body fat? I have read explanations that involve blood glucose levels etc, but I don’t really get it. Please explain it to me like I’m four years old. 😉 ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [September 12, 2012, 3:59pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/4 "2012-09-12T15:59:17Z")
First off, if you consume too many carbs and only a little fat, the fat you ingest will be the first to be stored and the last to be burned. Secondly, yes - once carbohydrates are broken up into simple sugars, the liver can convert those into fatty acids and those will be incorporated into fat cells in the same way as fat you ingest. I think it’s possible that amino acids from protein could be converted into fat as well, but I’m less certain about that. If you’ve got a very carb-rich diet, then some of the excess calories will probably also be stored as glycogen in your liver and muscles, which can lead to some weight gain, but IIRC this won’t be as significant as the fat conversion. Does that help? ETA: It’s probably better to say that calories make you fat, not necessarily carbs. You can definitely have a calorie-rich diet by eating fatty foods and not that many carbs. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 12, 2012, 4:19pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/5 "2012-09-12T16:19:47Z")
Food is useless to your body. Bear with me, it gets worse. Fats, carbohydrates, and proteins are also useless to your body. Why? All those molecules are simply too big to be absorbed through the intestinal wall. They have to be broken down into component parts: fatty acids, simple sugars, and amino acids. If this doesn’t happen, as in lactose intolerance where the lactose sugar is not broken down into glucose and galactose, the otherwise useful nutrient is excreted, often with accompanying annoying symptoms. Digestion is literally the breaking up of large molecules to small molecules. These are then used by the body as building blocks to put together everything it needs. If there is a deficiency of building blocks, the body can develop a variety of medically interesting conditions, none of which you want ever to happen to you. If there is an abundance of building blocks, the body has several options. The one you’re concerned with is the conversion of building blocks, mainly fatty acids and glycerol, into a storage form known as triglyceride. Yep, that triglyceride. In addition to fat cells, triglycerides can attach to the walls of blood vessels and eventually block them. I won’t attempt to describe the active processes for all this because people on the Internet have already done it much more clearly and correctly. Try [How Fat Cells Work](http://science.howstuffworks.com/environmental/life/human-biology/fat-cell.htm) for a straightforward explanation. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [September 12, 2012, 6:00pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/6 "2012-09-12T18:00:35Z")
A lot of what we consume is actually breathed out. I’ve seen numbers that suggest that between 500 and 1000 lbs (depending on weight, diet, etc.) of CO2 is produced by a person each year. The O2 we breathe in, obviously, but the C comes from food. So, by weight, we have 12 (atomic weight of C)/44 (atomic weight of CO2) = 27% is food you consumed - between 125 and 250 lbs per year, say. Other byproducts from the breakdown of food (including H2O) are also exhaled, but are going to be a much smaller issue than carbon in terms of weight. ---
**Author:** ![Bass\_Chick](https://avatars.discourse-cdn.com/v4/letter/b/43a26b/32.png) [@Bass\_Chick](https://boards.straightdope.com/u/Bass_Chick)\ **Post date:** [September 12, 2012, 8:20pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/7 "2012-09-12T20:20:20Z")
\*\*Calories are not created equal. \*\* Proteins require your body to expend a higher percentage of your intake to digest them than either fats or carbohydrates. If you eat 100 calories of protein, it might take you as many of 30 of those to break it down. Protein is one of the best forms of nutrition for increasing your metabolism. The digestion of carbohydrates actually starts in the mouth and unless they’re very high in fiber it takes the body very little energy to digest them. An advantage to high fiber foods is they fill you up, help keep you “regular” and lower cholesterol. Fats are more calorically dense than proteins or carbohydrates, BUT healthy fats from olives, nuts, seeds and fish are good for you in reasonable amounts. If you’re trying to lose weight, remember that refined carbohydrates also have the unfortunate side effect of rapidly raising and crashing your blood sugar levels, which in turn increases your appetite and also facilitates the storage of body fat. Worst case scenario is eating a combination of refined carbohydrates and fat. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 12, 2012, 11:33pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/8 "2012-09-12T23:33:15Z")
> [@Bass\_Chick](#): > > Proteins require your body to expend a higher percentage of your intake to digest them than either fats or carbohydrates. If you eat 100 calories of protein, it might take you as many of 30 of those to break it down. Protein is one of the best forms of nutrition for increasing your metabolism. > > The digestion of carbohydrates actually starts in the mouth and unless they’re very high in fiber it takes the body very little energy to digest them. Since fiber is not digested, I’d imagine that virtually zero energy is expended on it. I don’t believe your assertion for proteins either. How about a cite? ---
**Author:** ![Chimera](https://avatars.discourse-cdn.com/v4/letter/c/8e8cbc/32.png) [@Chimera](https://boards.straightdope.com/u/Chimera)\ **Post date:** [September 12, 2012, 11:46pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/9 "2012-09-12T23:46:59Z")
Conservation of Mass. Everything that goes in, comes out in one form or another or is kept in the body. None of it is burned/destroyed/consumed with nothing left. Look at it this way. When you burn wood, it isn’t destroyed with nothing to show for it. All of that mass is going up into the air in another form or is left behind as ash. “Burning” is just a chemical reaction changing the component elements from one chemical form to another, usually in combination with other elements - oxygen in the case of a wood fire. [Carbohydrate Metabolism](http://en.wikipedia.org/wiki/Carbohydrate_metabolism) and [ATP](http://en.wikipedia.org/wiki/Adenosine_triphosphate). “A human will typically use up his or her body weight of ATP over the course of the day.[33] This means that each ATP molecule is recycled 500 to 750 times during a single day” ---
**Author:** ![Surreal](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Surreal](https://boards.straightdope.com/u/Surreal)\ **Post date:** [September 13, 2012, 3:41am UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/10 "2012-09-13T03:41:43Z")
> [@Exapno\_Mapcase](#): > > I don’t believe your assertion for proteins either. How about a cite? The large Thermic Effect of Food (TEF) for protein consumption is well documented: > **[Effect of different levels of carbohydrate, fat and protein intake on protein...](https://archive.unu.edu/unupress/food2/UID07E/UID07E0H.HTM)** > [@](#): > > The carbohydrate-induced thermogenic response is about 7% of the glucose energy administered (SCHUTZ et al., 1983), whereas the thermic effect of fat is approximately 3% (THIÉBAUD et al., 1983). By contrast, **the stimulation of energy expenditure following protein ingestion or amino acid infusion is close to 30% of the energy administered (FLATT, 1978)**, which is mainly due to the stimulation of protein synthesis, gluconeogenesis and ureogenesis. ---
**Author:** ![Bass\_Chick](https://avatars.discourse-cdn.com/v4/letter/b/43a26b/32.png) [@Bass\_Chick](https://boards.straightdope.com/u/Bass_Chick)\ **Post date:** [September 13, 2012, 1:10pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/11 "2012-09-13T13:10:56Z")
> [@Exapno\_Mapcase](#): > > Since fiber is not digested, I’d imagine that virtually zero energy is expended on it. > > I don’t believe your assertion for proteins either. How about a cite? I never said that the body digests fiber. I said that the presence of fiber in carbohydrates slows digestion. **Soluble** fiber, in particular, slows down the rate of nutrient and sugar absorption into the bloodstream. When it mixes with the water, it swells up into a gel. Metamucil is a bulk forming laxative made from the soluble fiber Psyllium. When I get a break today, I will be happy to provide citations. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [September 13, 2012, 2:00pm UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/12 "2012-09-13T14:00:12Z")
> [@Surreal](#): > > The large Thermic Effect of Food (TEF) for protein consumption is well documented: > > [http://archive.unu.edu/unupress/food2/UID07E/UID07E0H.HTM](http://archive.unu.edu/unupress/food2/UID07E/UID07E0H.HTM) I have a war going on with numbers taken out of context. That 30% figure is all over the Internet, but it is misleading when applied to meals. It absolutely does not mean that “If you eat 100 calories of protein, it might take you as many of 30 of those to break it down.” You don’t eat protein; you eat food. The only proper studies are the ones that are physiologic, a term meaning that the substance is not considered in isolation but as part of a normal meal intake. [The Effects of High Protein Diets on Thermogenesis, Satiety and Weight Loss: A Critical Review, by Thomas L. Halton and Frank B. Hu](http://www.jacn.org/content/23/5/373.full) puts together the results from 15 previous studies. They find that protein does of course have a thermic effect but that it is smaller than you might think for any given meal. > [@](#): > > Fifteen studies were identified and included in this review concerning thermogenesis [4–18] (Table 1). The thermic effect of food was measured in a variety of ways. Six of these investigations calculated the thermic effect of food as a percentage of ingested energy [4,6,12,15–17]. The difference in thermic effect between the higher protein and the lower protein diets ranged from 0.8% of energy to 22% of energy, with protein always exerting the higher effect. In an investigation by Luscombe [12] the higher protein preload exerted a significantly higher percentage of ingested energy at baseline compared to a high carbohydrate preload but did not differ after 16 weeks on an energy restricted or energy balanced diet. Another 3 studies represent the thermic effect in kilojoules [7,8,10]. In each of these investigations, the higher protein meal exerted a significantly higher thermic effect than the higher fat or higher carbohydrate meals. … > > This evidence suggests that diets higher in protein exert a larger effect on energy expenditure than diets lower in protein. A logical question is whether or not this difference is enough to affect body weight. In the Johnston investigation [9], it was estimated that the average difference between the high and low protein diet for their subjects was 126 kj a day. For Crovetti [7], this number was 168 kj after 7 hours, and for Robinson [15] it was 251 kj after 9 hours. In a recent review of high protein diets, Eisenstein and Roberts [19] estimated that for a 8,374 kj diet with 30% protein compared to 15% protein, there would be an increase of 96 kilojoules a day due to thermic effect. Although these numbers seem small, with chronic consumption, they could become clinically significant. If energy expenditure were increased by 168 kj a day, one would expect a weight loss of 1.9 kg a year (168 kj × 365 = 61,320 kj/32,238 kj = 1.9 kg). These numbers might be underestimated as most studies were conducted for a period of only 6 or 7 hours and many believe that the thermic effect of protein continues on for a longer amount of time. How significant is this effect? The question, [Is a calorie a calorie? by Andrea C Buchholz and Dale A Schoeller](http://ajcn.nutrition.org/content/79/5/899S.full), is to be answered in the affirmative. > [@](#): > > We conclude that a calorie is a calorie. From a purely thermodynamic point of view, this is clear because the human body or, indeed, any living organism cannot create or destroy energy but can only convert energy from one form to another. In comparing energy balance between dietary treatments, however, it must be remembered that the units of dietary energy are metabolizable energy and not gross energy. This is perhaps unfortunate because metabolizable energy is much more difficult to determine than is gross energy, because the Atwater factors used in calculating metabolizable energy are not exact. As such, our food tables are not perfect, and small errors are associated with their use. > > In addition, we concede that the substitution of one macronutrient for another has been shown in some studies to have a statistically significant effect on the expenditure half of the energy balance equation. This has been observed most often for high-protein diets. Evidence indicates, however, that the difference in energy expenditure is small and can potentially account for less than one-third of the differences in weight loss that have been reported between high-protein or low-carbohydrate diets and high-carbohydrate or low-fat diets. As such, a calorie is a calorie. Further research is needed to identify the mechanisms that result in greater weight loss with one diet than with another. There are no flat, simple, defined numbers that can be applied to human digestion. ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [September 15, 2012, 3:36am UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/13 "2012-09-15T03:36:31Z")
Got a bit sidetracked, didn’t we? Any answers to the original post? ---
**Author:** ![bardos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bardos/32/1069_2.png) [@bardos](https://boards.straightdope.com/u/bardos)\ **Post date:** [September 15, 2012, 3:40am UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/14 "2012-09-15T03:40:56Z")
What we call “food” is not really food. It _contains_ the food, (often microscopic) substances, we need to keep the organism alive. The end product of the carrier vehicle (“food”) is eventually called s-h-i-t. ---
**Author:** ![voltaire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voltaire/32/313_2.png) [@voltaire](https://boards.straightdope.com/u/voltaire)\ **Post date:** [September 15, 2012, 5:36am UTC](https://boards.straightdope.com/t/what-happens-to-the-food-we-eat/634501/15 "2012-09-15T05:36:00Z")
As you might expect, the truth of the points in dispute is usually somewhere in the middle. [This very relevant and recent article in Scientific American](http://blogs.scientificamerican.com/guest-blog/2012/08/27/the-hidden-truths-about-calories/) touches on pretty much everything that’s been mentioned here, and more. > [@](#): > > Carmody fed adult, male mice organic sweet potatoes (to, in essence, retest what was already known) or organic, lean beef. These foods were served up raw and whole, raw and pounded, cooked and whole, or cooked and pounded. The standard system of calories, the one used to put the numbers on the food you buy in the store, assumes (and hence also predicts) these have no effect on calorie content; but would they? The mice were allowed to eat as much as they wanted and how much they consumed was closely monitored (Carmody had to pick each and every bit of uneaten food up from inside the cage). > > The mice on the different diets got about the same amount of exercise. They all had a wheel to run on, and they did not differ one treatment to the next in terms of how inclined they were to take a jog. They did differ, however, in how much they weighed at the end of the study. As predicted, mice lost more than four grams of weight on raw sweet potatoes, but gained weight when given cooked sweet potatoes (whether or not they were pounded). But what about meat? Cooked meat was easier to digest. The mice lost 2 grams of body mass on raw meat but just 1 gram on cooked meat. In retrospect this does not seem surprising. Heat denatures proteins and makes them easier to digest. Heat also kills bacteria and might decrease the immune cost of eating meat by reducing the work the immune system has to do which allows the body to make, well, more body for a given number of calories. > **In general, it seems that the more processed foods are the more they actually give us the number of calories we see on the box, bag or other sort of label.** This applies not just to cooking and pounding but also to industrial processing.[A recent study found that individual humans who ate, as part of an experiment, 600 or 800 calorie portions of whole wheat bread (with nuts and seeds on it) and cheddar cheese actually expended twice as much energy, yes twice, in digesting that food as did individuals who consumed the same quantity of white bread and “processed cheese product](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2897733/).” As a consequence, the net number of calories the whole food eaters received was ten percent less than the number received by the processed food eaters (because they spent some of their calories during digestion). **(bolding mine)** That’s just a small teaser snippet of the whole article, and there’s a lot of other stuff covered, so it’s very much worth reading the whole thing.