what happens to the food we eat?

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)?

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.

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. :wink:

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.

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 for a straightforward explanation.

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.

**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.

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?

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 and ATP. “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”

The large Thermic Effect of Food (TEF) for protein consumption is well documented:

http://archive.unu.edu/unupress/food2/UID07E/UID07E0H.HTM

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.

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 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.

How significant is this effect? The question, Is a calorie a calorie? by Andrea C Buchholz and Dale A Schoeller, is to be answered in the affirmative.

There are no flat, simple, defined numbers that can be applied to human digestion.

Got a bit sidetracked, didn’t we?
Any answers to the original post?

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.

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 touches on pretty much everything that’s been mentioned here, and more.

(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.