Having previously done a fairly detailed guide to dietary fats along with a complete guide to dietary protein I wanted to do a similar primer on dietary carbohydrates. I won’t be discussing the various controversies that surround carbohydrates and fats since they are discussed elsewhere on the site. Rather, I just want to focus on some basic definitions and concepts since there tends to be a lot of general confusion over the topic of carbohydrates.
What is a Carbohydrate?
The term carbohydrate is sort of an overall classification referring to a number of different organic compounds, which I’m going to detail below. You may often see the abbreviation CHO (for Carbon, Hydrogen and Oxygen) to refer to carbohydrate. Although fiber is a carbohydrate, I’m not going to discuss fiber in detail in this article. Rather I’d refer you to Fiber – It’s Natures Broom for a detailed look.
As I have discussed in many articles on the site, the primary role of carbohydrate in the body is energetic, that is it is broken down in cells to provide energy through a variety of pathways. At the same time, strictly speaking, carbohydrate is not an essential dietary component; that is, you can survive without eating it at all (an explanation of essential vs. inessential nutrients can be found in basic guide to nutrition). How many carbohydrates individuals need is another topic of endless debate and controversy.
Now, another term that is sometimes used to describe carbohydrates is saccharides and, with that trivial note made, there are three primary classes of carbohydrates which I’m going to first list and then describe in more detail below in terms of what they are, what they do in the body and where they are found in the food supply.
I’ll discuss each in this article.
The term monosaccharide refers to a single “sugar” molecule (“mono” = single and saccharide = “sugar”) and are often referred to as simple sugars. The monosaccharides are glucose (blood sugar), fructose (fruit sugar) and galactose (milk sugar). I should mention that there are other monosaccharides but the above three are the ones primarily found in the diet.
Now, free glucose is found in some foods (fruit has some and many types of candy will contain glucose) although it’s primarily associated with blood sugar (when diabetics are measuring their “blood sugar levels” they are measuring blood glucose levels specifically). This is because glucose is the type of sugar found almost exclusively in the bloodstream of humans. Quite in fact, almost all other dietary carbohydrates will either be converted to or simply appear in the bloodstream as free glucose.
This is certainly true of dietary fructose, found primarily in fruits (hence the name) which must first be converted to glucose/glycogen (in the liver) prior to release into the bloodstream. Contrary to some claims being made, free fructose is almost never found in the bloodstream in large quantities unless it was put there through infusion.
Rather, dietary fructose will either be stored in the liver as glycogen (see below) before being converted to glucose and released into the bloodstream, or simply converted to glucose and released after consumption. Again, free fructose is rarely found in the bloodstream in appreciable quantities even when it’s consumed in the diet.
I should probably mention that fructose got a lot of press one point as either a superior sweetener or a sugar that was ideal for diabetics. In terms of the latter, due to it’s slow digestion and general lack of effect on blood sugar or insulin response, dietary fructose was thought to be superior to sucrose. However, it’s turning out to be much more complicated than that. Excess dietary fructose (and please note my use of the word “excess”) can cause problems in terms of raising blood triglycerides (TG, a type of fat) and having other negative effects.
So, while the fear and scare-mongering of the anti-fructose brigade (who are often looking at insanely non-physiological amounts of fructose that simply aren’t achievable in a normal human diet) tends to be a lot of nonsense, there’s little doubt that too much fructose can be a bad thing. For this reason, fructose is being wrongly blamed for the impact of high-fructose corn syrup in the body. And HFCS is being blamed wrongly to begin with.
Let me put this into perspective. The liver can generally handle approximately 50 grams of fructose or so before you start to see conversion to triglycerides or other negatives (when you divide the studies up into those that find problems versus those that don’t, 50 g/day is about where the cutoff occurs).
The average piece of fruit is perhaps 7% fructose, that is it contains 7 grams of fructose per 100 grams of fruit. To get 50 grams of fructose per day would require about 7 average sized piece of fruit. That’s a lot of fruit. Can it be done? Sure. Will most do it? I consider that unlikely.
Now that intake might be easily achievable by someone consuming a lot of sugary soda although it would take 100 grams of HFCS or so to provide 50 grams of fructose (HFCS is just about 50% fructose, just like table sugar). Of course, the people consuming lots of sugary soda almost invariably have other, much larger issues with their eating and lifestyle patterns. Blaming HFCS is missing the forest for the trees, it’s usually just a symptom of a much bigger problem. But I digress.
Finally is galactose, or milk sugar, found, as you might imagine in milk and dairy products. I don’t have much to say about this one here but will come back to it below. I’d only note that galactose tends to be metabolized similarly to fructose in the body; that is it’s dealt with in the liver. Since galactose tends to make up a fairly small amount of the overall diet (unless massive amounts of dairy are being consumed), I don’t usually consider this worth worrying about.
As a final note, there are a host of other types of monosaccharides that can either occur in small amounts in the diet or be made. Ribose is one simple sugar, for example, that was one promoted to improve performance. Some of the sugar alcohols (e.g. xylitol) also fall into the category of monosaccharide but since they are modified carbohydates (simple sugars with an alcohol molecule tacked on), I’m not going to discuss them here.
One monosaccharide that does come up on fitness forums (due to its use in many sports products) is dextrose which is simply d-glucose. The “d” refers to the chemical structure (normal glucose would be more accurately described l-glucose and you can technically have both d- and l-fructose) and I’ll leave it to the organic chemistry nerds to worry about it beyond that. Simply recognize that both dextrose (d-glucose) and glucose are a form of glucose. They are just chemically mirror images of one another.
Simply while there are far more monosaccharides that can be found in the diet (or at least obtained), glucose and fructose are going to be the major players (and glucose more than fructose for the most part). Galactose intake will depend entirely on dairy intake or the lack thereof.
The term oligosaccharide is used to refer to any carbohydrate chain between 2-10 molecules long (“oligo” = ‘several’ or as I like to call it “a buncha” and “saccharide” = sugar). Chemically, that is, an oligosaccharide, is a buncha monosaccharides that are chemically bonded together but there are only 2-10 of them in the chain (this will make more sense when I discuss polysaccharides).
And while some of the longer chains may be found in small amounts in the diet or in specialty food products (e.g. some maltodextrins which are a combination of maltose and dextrose may be about this length) by and large the primary oligosaccharides are the disaccharides, two sugar molecules bound together. I’ve listed the primary dietary disaccharides in the table below including what two sugars they are made up of along with where they are generally found in the diet.
While technically an oligosaccharide, I’ve left high-fructose corn syrup (HFCS) off the list since I’ve discussed it in detail elsewhere. I’d only ask that you take any comments about HFCS to that article instead of this one since most of what needs to be said is there and needn’t be repeated here. Simply recognize that, nutritionally, HFCS and sucrose are essentially identical in that both are made up of roughly 1/2 glucose an 1/2 fructose. And despite blathering to the contrary, they act basically identically in the body.
Sucrose is arguably what most think of when they think of ‘sugar’. Table sugar is pure sucrose and sucrose has traditionally been used as a sweetener for, well, forever in various forms (including cane sugar, refined sucrose and many many others). As a sweetener, sucrose is used in various candies, as a sweetener in some sodas and is also found occurring in many foods whether naturally occurring (even fruit contains some sucrose) or man-made.
Lactose, as mentioned is a mixture of 1/2 glucose and 1/2 galactose and is found in dairy products. Many people are probably familiar with lactose due to issues of lactose intolerance. Lactose intolerance occurs due to an inability to digest lactose in the stomach due to either the complete lack (or more usually an inadequate amount) of the enzyme lactase in the stomach. Lactose intolerance typically develops shortly after weaning (if it happens at all) and is more common in ethnic groups who did not evolve consuming milk past that point. As a very gross generalization, the darker someones skin, the more likely there is for lactose intolerance to be present.
The symptoms of lactose intolerance generally include gas, diarrhea and stomach upset. This is due to the undigested lactose hitting the colon where it ferments. Individuals with lactose intolerance (not to be confused with a true milk allergy) must either avoid dairy products, consume special lactose removed products (e.g. Lactaid milk) or consume lactase supplements with dairy products. Lactose intolerance and how to deal with it (for example, some with more minor lactose intolerance can consume dairy with meals) is discussed in more detail in The Protein Book.
Maltose, as noted above, is used primarily to brew beer and make malt drinks, it is also produced during the digestion of the polysaccharides (discussed next). As well, maltodextrins are often found/used in specialty food products. But for the most part sucrose and lactose are the primary oligo/di -saccharides found in the “normal” diet. Even if we include HFCS as a disaccharide, it’s generally made for commercial products and doesn’t occur naturally in the diet (to my knowledge).
And finally are the polysaccharides, a term that refers to chains of sugar molecules which can range from several hundred to many thousands long (“poly” = “many” and “saccharides” means “sugar”). In terms of the human diet, polysaccharides almost universally refers to starch which is simply a long, long (long) chain of glucose molecules strung together.
Even there there are slight distinctions with recent research finding differences between what are termed amylose and amylopectin, both of which are found in dietary starches. The difference has to do with the molecular structure: amylose is simply a straight chain of glucose molecules while amylopectin has a branching structure.
Both types of carbohydrates are found in dietary starches although amylose is usually more prevalent. Both high amylopectin and high amylose starches are available (e.g. waxy corn is 98% amylopectin) for specific food purposes. Foods such as grains (refined or otherwise), potatoes, etc. are all food examples of starches that are eaten in various proportions in the human diet.
Polysaccharides actually start digestion in the mouth due to an enzyme called alpha-amylase. You can test this by putting a piece of bread or something in your mouth and chewing without swallowing. After some time you’ll get a very sweet taste in your mouth due to the breakdown of starch to free glucose. Yummmmy.
The old “carbohydrate blocker supplements” usually derived from white kidney bean) were actually alpha-amylase blockers, they prevented digestion of carbohydrates in the mouth. Unfortunately, they didn’t do anything for the next step in digestion which occurs mainly in the stomach. There, the long chains of starch molecules are broken down into smaller and smaller chains (producing some maltose for example) until the free glucose is available for absorption.
Although this isn’t related to the diet specifically, in the body (specifically muscle and liver) long chains of starch are called glycogen. Again, these are simply long chains of glucose that are bonded together (in the liver, fructose is converted to glucose before being stored as liver glycogen) for breakdown at some later date.
Some recent work even suggests that there may be small stores of glycogen in the brain or fat cells but the majority will be found in muscle and liver cells. On average, the liver may hold about 50 grams of carbohydrates and the skeletal muscle of an average sized person about 300-400 grams. These values can be doubled with carbohydrate loading.
Trivia for the Day: A question that I have seen enough times to think it worth addressing is why meat doesn’t provide carbohydrate to the diet due to the presence of glycogen in muscle (animal meat is just muscle). And the major part of the answer is that, after death, the glycogen will be broken down as part of the process of the animal going into rigor. If you ate it fresh off the kill, depending on whether or not the animal stored glycogen or not (not all animals do), there might very well be glycogen still present. But I’m talking fresh off the kill, like Ted Nugent you just shot it in the head with an arrow and you dig in with a knife right then fresh off the kill. If you don’t do that, the glycogen will be gone.
I should also mention another type of starch which is “resistant starch”, this is actually a type of starch that is resistant to digestion, hence the name. That is, it passes through the stomach and intestines without digestion (and may have certain health or weight loss benefits because of it). Resistant starch is found naturally in small amounts in some foods but most of the focus seems to be on developing commercial foods higher in resistant starch.
Finally would be fiber, which I already detailed elsewhere on the site. Just realize that the various fibers are long chains of, generally indigestible, carbohydrate molecules (I say “generally” as some fibers are metabolized in the colon to short-chain fatty acids as discussed in the linked article).
A Summary of the Dietary Carbohydrates
So let me sum up all of the information from above in a handy dandy chart.