How efficient is the human digestive system?

Factual Questions
1

As I was “taking care of business” this morning, a passing thought stuck in my caffeine-starved brain. Just how efficient, on average, is the human digestive system? Meaning: there must be some caloric value in our waste products. If I intake a bacon double cheeseburger, all 1200 calories don’t stay in my system. So, how many do? 1000? 900? It totally depends and we can’t say except for a range that doesn’t really answer my question? This isn’t something that was addressed in Biology 101.

Any answers?

2

Bump…cause I’m curious about this too.

3

Well, I do know that healthy folks absorb completely virtually all the fat they consume. If they don’t, this results in steatorrhea, a rather nasty condition.

I’m not sure about carbs and protein.

4

Sorry, silenus, but it does, in fact, totally depend on a number of factors, including what and how much you eat. Even with carbohydrates, which break down pretty simply, the glycemic index gives only a rough approximation as to how readily a product is digested, and doesn’t account for individual metabolic responses. With more digestively complex fats and proteins it is even more difficult to evalute efficiency; some fats and most proteins don’t generally provide energy at all, but are used as construction materials.

You can’t just measure the calorie intake versus what is excreted, either, because excrement has a significant caloric content, but not in a form that we will digest, and much of it is waste (like dead blood cells and GI bacteria) rather than undigested matter. We can say that the high protein level of the average American diet is far more than the body can effectively use unless the body in question is in a muscle weight training program. Vegetables vary in digestability, from simple starches like potatoes and most fruits, to domestic pulses and grains that break down fairly easily once hulls are removed or processed, to roughage-intensive plants and hard cereals like maize that are mostly indigestable. In addition, some dietary chemicals can enhance or inhibit absorption of cholesterals and other lipids, and the rate of absoption is also regualated by intestinal enzymes and bacteria in the large intestine that break down otherwise undigestable polysaccarides (large complex carbohydrate chains), and the amount of these enzymes and bacteria is dependent upon diet and overall health of the subject.

Here is a study from the Journal of Lipid Research (never heard of it before so I can’t vouch for its credibility) which indicates an average of 41% absoprtion of cholesterols in the experiment baseline but demonstrates a significant decrease in absoption when large amounts of cholesterol are added. So you can see that there is nothing intuitive about digestion, at least from a qualitative standpoint.

Regarding the hypothetical cheeseburger, I’m going to guessimate that you’ll digest:
[ul][li]virtually all of the bun (assuming it to be made of processed white flour) except for sesame seeds,[/li][li]most of the carbohydrates and lipids in condiments,[/li][li]a large portion of the cheese (if you’re not lactose intolerant),[/li][li]a majority portion of the bacon and beef fat,[/li][li]a fair portion of tomato,[/li][li]only a moderate amount of the bacon and beef protein,[/li][li]a marginal amount of onion (though you’ll absorb all the water which makes up most of its bulk), and[/li][li]vitually none of the lettace, except for absorbing the water.[/ul][/li]
Sorry for the lack of a simple answer, but once you start digging into the GI system–figuratively speaking, of course–you realize how incredibly complex it actually is. (This is unsurprising to gastroenterologists, who recognize that the neurological complexity of the lower GI tract is second only to that of the brain.) Here is an overview on the human digestive system.

Stranger

5

So, if I understand this correctly…if I have a Double Hamburger Protein-Style from In-N-Out (wrapped in lettuce instead of a bun) with onion, lettuce and ketchup, my total caloric usage will be minimal. Add a Diet Coke and we’re good to go. :smiley:

6

“Minimal” might be a misleading term; “less than a double bacon cheeseburger” would be more appropriate. The lack of carbohydrates will tend to inhibit digestion (or at least metabolizing the proteins in the hamberger patty), though the fats from the hamberger (15-30% by mass, but more calories by weight and also mostly the more reactive saturated fats). There are also a lot of sugars in the ketchup, so if you really want to minimize effective metabolization you might forego the ketchup as well. Still, lean protein by itself isn’t going to bust your calorie budget, even if you don’t account for the lack of metabolization by a lack of carbohydrate balance; with meat it’s the associated fats that cause high calorie loads.

Stranger

7

I don’t want to contradict Stranger’s typically excellent post, but two points should be emphasized.

One is that our digestive systems are the results of gazillions of generations of natural selection. Animals who cannot digest (i.e. break down into smallest pieces) and then absorb those nutrients of the great majority of the foodstuffs they eat are at a competitive disadvantage. Our digestions are excellent indeed at processing most of the foods that humanity would have approached in the wild. Our one weakness is the inability to digest cellulose, lignins, and other tough plant fibers, an indication, therefore, that we didn’t eat much of them.

The question becomes how well does our digestion stand up to the typical modern American diet? Presumably we have expanded enormously on what our African ancestors ate just by inventing agriculture and herding. There are signs that our bodies have changed to adapt, like the fact that at least 30% of humanity has the mutation that allows adults to continue producing the lactase enzyme that digests lactose, and so can have dairy products without symptoms as adults.

Our ancestors of 200,000 BCE wouldn’t recognize most of the ingredients in a bacon double cheeseburger on a bun. As Stranger notes, our systems will process the vast majority of the nutrients in one even so. (And the cheese is almost entirely protein with very little lactose, so even that is not a factor.)

The answer to how many of the calories in some random 1200 calorie sample of food we process is “most.” Almost always.

Pretty much by definition the food parts that turn up in the waste - mostly the fiber - doesn’t get counted in the calorie count in the first place. There isn’t much in waste that is useful even to a starving person, assuming normal digestion and that you chew your corns and peas enough to break the hulls so that they digest properly.

The other point is that the calories that are the result of the digestion of food (and 1200 food calories are 1200 normal kilocalories, which is a lot, especially if you have to try to burn them off) get used in a large variety of ways by the body. That’s the part in which I see trying to make a connection between intake and usage becomes so difficult, especially since the body spends all of its time converting food into other chemicals and then into body parts or muscle movement or radiated heat. If that’s the question you’re asking - and to be honest, you seemed to be going both ways in the OP - that’s the much harder one to answer.

8

Not necessarily true. Most ruminants have relatively low digestive efficacy, which accounts for the large amount food they eat and the high energy waste they excrete. Rabbits and hares redigest their own droppings. Most animals produce nutrient-rich waste. It’s not so much a matter of efficiency of digestion per se, but rather being able to digest enough material to thrive in that environment. Omnivores (like humans) tend to have a higher degree of digestive efficacy, but in part because they avoid low energy density foods and consume more foods with accessible carbohydrates and lipids. Obligate carnivores generally digest more completely, but they’re eating a high amount of proteins and fats with only a very moderate amount of carbohydrates found in muscle tissue and sweetmeats; they’re also not well suited to long endurance activities, generally stalking prey rather than cursorial hunting.

While you are correct that lactose sugar is not a major component in cheese, cheese (especially soft cheeses) have a signifcant amount of saturated fats, often 50% or more by weight. Only about 20% of ripened cheese is actually protein.

Stranger

9

I was wondering about this not long ago, and did a little poking around on the Internet. What I found suggested that our rate of conversion of the energy potential in food was not particularly high – 30% maybe? I’ll look around and see if I can find the cite.

10

How significant is proper chewing? Are there are enzymes in spit that helps digestion, and will you waste good nutrition if you don’t chew enough to help these do their work?

11

Was this the article you saw? : http://mb-soft.com/public2/humaneff.html

I believe those studies were based on the potential energy from foods that were really not terribly digestible: Cellulose, non-soluble fiber, etc. In other words, based on the calories one got out of the food from burning it in a bomb calorimeter, not based on calories that actually can be digested and absorbed in the human stomach.

12

Yes, and no.

The enzymes in saliva do help to start the gross breakdown of food into smaller pieces. They don’t do as much to digest those pieces into the final components of fatty acids, amino acids, and simple sugars. That happens mainly in the small intestine. Swallowing food whole has little to no impact on digestion or nutrition.

Stranger, I think we’re talking with different emphases. You could argue that an evolutionary strategy of putting food through the digestive track twice is merely one way of ensuring that as much food value is garnered as possible from particularly difficult to digest base products.

The crucial point is that humans do not have high nutrient waste. Both the diet and the digestion have converged to ensure that.

13

I was going to start a different thread then saw this one.

What about people who enter eating contests? I don’t see any way the body could absorb the nutrients in, say, 50 hot dogs.

14

Adding to Happy Wanderers question:

I caught a segment of TV show that was analysing the diet and eating habits of a really obese man. The claim was made that to deal with the vast amounts of food he was consuming on a daily basis, his intestines were basically lined with mucus, and much of the food basically fell through his digestive tract and out without delivering much (relative to the amount consumed) caloric value. Is this actually the case?

And on another show I watched last night, a man appeared who ate 60000 calories excess per week. Can his system process and absorb that much food?

UK TV is obsessed with eating shows - and I’m on a diet. I just have to watch (if only to assure myself that I am not actually that bad).

Si

15

This article would seem to imply most calories are absorbed. Of course, this also depends on whether things are going through at normal speed and assumes enzymes are not overwhelmed by fifty hot dogs eaten at the same time.

http://www.springerlink.com/content/02k315q878172358/

Refined foods lower in fiber and other indigestibles presumably have a higher efficiency rate of absorption.