Having previously done a fairly detailed Primer on Dietary Fats, I wanted to do something similar on the topic of carbohydrates (in the future I'll do
Having previously done a fairly detailed Primer on Dietary Fats, I wanted to do something similar on the topic of carbohydrates (in the future I’ll do one for protein as well). In this article, I’m not going to look at many of the debates surrounding the issues of carbohydrate intake (in terms of body weight, body fat, or health), you can read Carbohydrate and Fat Conteroversies Part 1 and Carbohdyrate and Fat Controversies Part 2 for somewhat of an examination of that. 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.Part I
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 A Primer on Nutrition Part 1). How many carbohydrates should be consumed in the human diet is a topic of endless debate and controversy, I’d refer readers to How Many Carbohydrates Do You Need? for a detailed look at the topic.
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.
To keep the article length under control, I’m only going ot discuss the monosaccharides today. I’ll discuss oligosaccharides and polysaccharides (along with an article summary on Tuesday of next week).
The term monosaccharide refers to a single ‘sugar’ molecule (‘mono’ = single; 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.
One 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 and glucose are a form of glucose (effectively they are molecular mirror images).
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 (in the liver) prior to release. That is, 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 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; amounts 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. I’d note, and this is discussed in some detail in the article (and especially comments section) of Straight Talk about High-Fructose Corn-Syrup: What it is and What it Aint that the fructose mainly becomes and issue when it’s being mainlined as part of sugary sodas.
It would take an absurd amount of say, fruit, to provide enough fructose to the diet to cause problems. 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). While that intake level might be easily achievable by someone consuming a lot of sugary soda, given taht an average piece of fruit is roughly 7% fructose (e.g. 7 grams fructose per 100 grams of fruit), well..that’s a lot of fruit (~7 average pieces per day). But, in my opinion, anyone consuming gallons of sugary soda per day has bigger issues in their diet than the HFCS/fructose intake. 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.
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); galactose intake will depend entirely on dairy intake or the lack thereof.
In a Primer on Dietary Carbohydrates – Part 1, I took a brief look at what carbohydrates are and listed the three primary categories of dietary carbohydrates which are monosaccharides, oligosaccharides and polysaccharides. As well, I looked in some detail at the monosaccharides (simple sugars) which are glucose (dextrose), fructose and galactose. Today, I want to examine the other two major categories of dietary carbohydrates: oligosaccharides and polysaccharides.
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’; ‘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.
|Name||Combination of||Where Found|
|Sucrose||Glucose + Fructose||Too many places to list|
|Lactose||Glucose + Galactose||Dairy products|
|Maltose||Glucose + Glucose||Malt Beverages (Beer!)|
While I’m hesitant to mention high-fructose corn-syrup (HFCS) in this article, I’m going to bring it up since it is also a combination of glucose and fructose (like sucrose), as discussed in the article Straight Talk about High-Fructose Corn-Syrup: What it is and What it Aint. 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; that’s all I’m going to say about it.
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, see A Quick Look at Food Intolerances and Allergies) 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’). 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. 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.
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, fiber would technically be discussed here as a polysaccharide but, as I mentioned in Primer on Dietary Carbohydrates – Part 1, I already detailed fiber in Fiber – It’s Nature’s Broom and would refer readers there for more detail so I won’t spend much more time on it here. 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).
And that’s that, a primer on dietary carbohydrates. Carbohydrates refers to a general class of compounds containing Carbon, Hydrogen and Oxygen (hence CHO) including monosaccharides (simple sugars), oligosaccharides (chains of 2-10 molecules) and polysaccharides (long chains of molecules including fiber). I’ve summarized the primary types of dietary carbohydrates in the chart below.
|Common Name||What it Is||Where It’s Found|
|Glucose||Blood sugar||Bloodstream, various foods|
|Dextrose||D-glucose||Specialty nutrition products|
|Sucrose||Table sugar||Glucose + Fructose||Just about everywhere|
|Lactose||Milk sugar||Glucose + Galactose||Dairy|
|Maltose||N/A||Glucose + Glucose||Malt Beverages/Beer|
|HFCS (sort-of)||Glucose + Fructose||Commercial foods such as soda|
|Starch||Starch||Amylose/Amylopectin||Starches (ha ha)|
|Resistant Starch||Resistant Starch||Resistant Starch||Small amounts in foods, specialty products|
|Fibers||Fiber||Cellulose, etc.||Vegetables, Grains, Fruits|
|Glycogen||Glycogen||Long chains of glucose||Skeletal muscle, liver|
About the Author Lyle McDonald
Lyle McDonald is the author of the Ketogenic Diet as well as the Rapid Fat Loss Handbook and the Guide to Flexible Dieting. He has been interested in all aspects of human performance physiology since becoming involved in competitive sports as a teenager. Pursuing a degree in Physiological Sciences from UCLA, he has devoted nearly 20 years of his life to studying human physiology and the science, art and practice of human performance, muscle gain, fat loss and body recomposition. Lyle has been involved, at various levels of success in competitive sports since his teens. Starting with triathlon, he spent altogether too many hours on his bike during college. Becoming involved with inline skating at the same time led him to compete for several years until he burned himself out with chronic overtraining. Many years passed until he decided to return to speed skating and move to the ice. He moved to Salt Lake City Utah to train full time at the Olympic oval, he is currently still there training with his coach Rex Albertson attempting to make the US National team or beyond.Lyle has written for the print magazines (Flex and the now defunct Peak Training Journal), too many online sites to mention (including Cyberpump, Mesomorphosis, MindandMuscle, ReadtheCore) and has published 5 books on various aspects of exercise and diet. Over the years, in addition to working with the general public, Lyle has worked primarily with endurance athletes, a few powerlifters, and some bodybuilders. Through his books, articles and his forum, he has helped thousands lose fat, gain muscle and get stronger or perform better.