I’m also going to assume that you’re getting your nutrients through food and it’s going in through your mouth. Certainly nutrients can be given via infusion but this is usually done in a hospital setting (sometimes athletes will rehydrate and carb-load with IV fluids and glucose, mind you) and I’ll assume you’re not doing that.
Digestive Efficiency and Your Poop
Clearly anything you eat has to go through the process of chewing, swallowing and into the stomach for digestion. There a bunch of stuff happens where the nutrients are broken down to one degree or another. And either they get absorbed (moving into special cells to be released into the bloodstream, or lymphatic system in the case of dietary fats) or not. If you’re particularly interested in the digestion processes of the different macronutrients, I’d refer you to the specific articles:
Nutrients that aren’t absorbed in the stomach move further down the intestine where in some cases (for example, certain fibers), they are digested by special bacteria and re-enter the bloodstream as short-chain fatty acids. This is discussed in Fiber – It’s Nature’s Broom.
Nutrients that pass that stage eventually come out the other end in your poo and we needn’t talk about that much more. I’ll only note in this regards that digestive efficiency in humans is generally very high. Fats are absorbed with about 97% efficiency (e.g. if you eat 100 grams fat, you’ll absorb 97 grams of them), animal source proteins are about 90-95%, vegetable source proteins can be in the 80% range and carbohydrates vary drastically depending on their form, fiber content, etc. But for the most part, with the exception of high-fiber foods, you’re not losing a lot of calories in your poop.
I would note, having said more about poop than necessary at this point, that there appears to be slight differences (based on the gut bacteria present) in how efficiently individuals absorb calories from the diet but this only amounts to perhaps a 100 cal/day difference between the highest and lowest people. OF course, in cases of specific disease where there is nutrient malabsorption, all these comments go out the window but I won’t talk about that here. I’ll assume you have a normally functioning gut, etc.
Fates of Ingested Nutrients: Oxidation or Storage
So what happens after nutrients get through the stomach and intestines and into the body? Broadly speaking, there are two primary fates for nutrients at this point which are oxidation or storage. A third that I should at least mention is that, under certain conditions, nutrients will sort of ’sit’ in the bloodstream either causing problems there or eventually being excreted in the urine. Outside of various pathophysiologies (e.g. runaway diabetes where glucose is lost in the urine in large amounts), the urine excretion route is generally minimal approaching insignificant and I won’t focus on it further here.
Oxidation simply refers to the direct burning of fuels for energy. This can occur in the liver, skeletal muscle and a few others places and all 4 macronutrients can strictly speaking undergo oxidation after ingestion. So fatty acids from dietary fat ingestion can be used to produce energy, carbohydrate can be burned off, a little appreciated fact is that under normal circumstances as much as half of all dietary protein ingested gets metabolized in the liver via a process called deamination with some of it simply being burned off for energy.
Storage should be fairly clear and the nutrients (with the exception of alcohol) can be ’stored’ in the body for later use. Carbohydrates can be stored as liver or muscle glycogen, under rare circumstances they are converted to and stored as fat. Dietary fat is stored either in fat cells or can be stored within muscle as intra-muscular triglyceride (IMTG). Under certain pathological conditions, fat gets stored in places it’s not supposed to go, a situation called ectopic fat storage. In a very real sense there’s no true store of dietary protein although amino acids from protein digestion are used to make various proteins and hormones in the body. Skeletal muscle is, in essence, a ’store’ of protein in the body. There is no store of alcohol in the body.
Which is the segue into the only real point I have to make in this piece: as it turns out, the size of a nutrient’s store in the body is inversely related to the body’s propensity to oxidize it after ingestion. This is especially true in terms of the size of the store relative to the amount consumed on a daily basis.
Put a little more clearly, the better the body’s ability to store a given nutrient, the less it tends to alter/increase oxidize that nutrient after ingestion. And vice versa, the smaller the store in the body of a given nutrient relative to intake levels, the more likely the body is to oxidize that nutrient after ingestion. I’ve shown the implications of this in the table below and will make comments about specific nutrients below that.
|Nutrient||Size of store relative to daily intake||Oxidation increase due to intake|
Body fat stores are effectively unlimited as individuals reaching 1000 lbs (and 70-80% body fat) have demonstrated. Even a relatively lean male at 180 lbs and 12% body fat is carrying 21 pounds of fat. Each pound contains maybe 400 grams of actual stored fat and that means about 8500 grams of fat stored in the body. Contrast this to a relatively high daily intake of perhaps 100-150 grams per day and you can see that the body’s store of fat is much much higher than what you eat on a day. And most people aren’t 12% body fat.
But for the most part, ingested dietary fat has little impact on fat burning in the body; that is, when you eat dietary fat, your body doesn’t increase fat oxidation. One exception is if an absolutely massive amount of fat (like 80 g) is consumed all at once but even then the effect is fairly mild. Some specific fats, notably medium chain triglycerides, are somewhat of an exception to this; they are oxidized in the liver directly. Rather, the primary controller of dietary fat oxidation in the body is how many carbohydrates you’re eating, which I’ll explain momentarily.
For carbohydrate, the body’s stores are relatively close to the daily intake. A normal non-carb loaded person may store 300-400 grams of muscle glycogen, another 50 or so of liver glyogen and 10 or so in the bloodstream as free glucose. So let’s say 350-450 grams of carbohydrate as a rough average. On a relatively normal diet of 2700 calories, if a person eats the ‘recommended’ 60% carbs, that’s 400 grams. So about the amount that’s stored in the body already.
For this reason, the body is extremely good at modulating carbohydrate oxidation to carbohydrate intake. Eat more carbs and you burn more carbs (you also store more glycogen); eat less carbs and you burn less carbs (and glycogen levels drop). This occurs for a variety of reasons including changing insulin levels (fructose, for example, since it doesn’t raise insulin, doesn’t increase carbohydrate oxidation) and simple substrate availability. And, as it turns out, fat oxidation is basically inversely related to carbohydrate oxidation.
So when you eat more carbs, you burn more carbs and burn less fat; eat less carbs and you burn less carbs and burn more fat. And don’t jump to the immediate conclusion that lowcarb diets are therefore superior for fat loss because lowcarb diets are also higher in fat intake (generally speaking). You’re burning more fat, but you’re also eating more. But that’s a topic that I’ve not only addressed previously on the site but may look at in more detail in a future article with this piece as background.
The body’s total protein stores (and note again that this isn’t a true store in the sense of body fat and glycogen) is maybe 10-15kg or so when you add it all up. Which is pretty high compared to an average daily intake. The DRI for protein is only about 50-60 grams per day for the average person and even folks eating 200-300 grams per day are still eating far less protein than stored. Which is why protein oxidation rates can change with intake.
As I mentioned above, an under-appreciated fact is that about half of all ingested dietary protein is metabolized in the liver (details on this can be found in The Protein Book). Some of it is oxidized for energy while others are converted into other things (including glucose and ketones) for use elsewhere. But, protein oxidation rates do change in response to intake. So, when protein intake goes up, oxidation will increase; when protein intake goes down, oxidation rates decrease. This change isn’t immediate (as it more or less is for carbohydrates) and takes 3-9 days to occur but mis-understanding of this process has led to some goofy ideas such as protein cycling.
But it also explains one other issue of importance to protein which has to do with speed of digestion. Early studies, including the oft-cited study on whey and casein by Boirie find that fast proteins are burned off for energy to a greater degree than slower digesting proteins. Since the body doesn’t have anywhere to store the rapidly incoming amino acids, it simply burns off more for energy. This, along with differences in handling (e.g. the fact that fast proteins are absorbed by the gut as discussed in Casein Hydrolysate and Anabolic Hormones and Growth – Research Review) are a big part of why slower digesting proteins invariably lead to better overall protein retention in the body; not only does more make it into the bloodstream but less is burned for fuel.
And, finally, as noted above, there is absolutely no store of alcohol in the body. None whatsoever. Effectively, alcohol is seen as a sort of metabolic ‘toxin’ or ‘poison’ to the body. And this means that alcohol oxidation is 100% perfect, that is, the body will effectively do everything in its power to get rid of the alcohol increasing alcohol oxidation to maximum (which means decreasing the oxidation of other nutrients consumed with that alcohol) so that the alcohol can be gotten rid of.
I’m going to ask readers not to read anything into the above paragraph, don’t infer or try to draw conclusions about how alcohol might or mightn’t fit into the diet in terms of anything. As it turns out, alcohol is an oddity among nutrients with seemingly contradictory effects on things. I’m going to address that in detail in a forthcoming article and, for now, just take the above as some much needed background information.
And that’s that. After consumption and digestion, nutrients have a couple of primary fates in the body which are oxidation (burning) and storage (for use later). And, as it turns out, the propensity for the body to store or oxidize a given nutrient is related to the body’s built-in store relative to intake. In the case of dietary fat, where stored fat is much higher than daily intake, the body tends to store incoming fat and burn very little. Fat intake per se has very little impact on fat oxidation rates.
Rather, the rate of fat oxidation is related to carbohydrate intake as the body is able to precisely alter carbohdyrate oxidation to changing intake. Eat more carbs and burn more carbs (and less fat); eat less carbs and burn less carbs (and more fat). Protein is somewhere in the middle, oxidation can increase or decrease relative to intake but the effect takes time (3-9 days). Finally is alcohol, with no storehouse in the body, alcohol oxidation will take 100% precedence over everything else when it is consumed. I’ll discuss the implications of this in an article on alcohol (and it’s rather schizoid effects on body weight and body composition in a later article).