A Primer on Dietary Carbohydrates – Part 2
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.
Oligosaccharides
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.
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| Name | Combination of | Where Found |
| Sucrose | Glucose + Fructose | Too many places to list |
| Lactose | Glucose + Galactose | Dairy products |
| Maltose | Glucose + Glucose | Malt Beverages (Beer!) |
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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).
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Polysaccharides
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).
Summary
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.
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| Common Name | What it Is | Where It’s Found | |
| MonoSaccharide | |||
| Glucose | Blood sugar | Bloodstream, various foods | |
| Fructose | Fruit sugar | Fruit | |
| Galactose | Milk sugar | Dairy | |
| Dextrose | D-glucose | Specialty nutrition products | |
| Oligosaccharides | |||
| 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 | |
| Polysaccharides | |||
| 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 |
Lyle, I think you got the relative sweetness of glucose and fructose wrong. Fructose is much sweeter than sucrose and glucose.
You may be correct, I remember thinking that when I typed it. Except that I remember doing OGTT and nearly throwing up, and I like sweet stuff. Powdered fructose doesn’t do that to me but might have been the concentration of the glucose solution. But you may still be correct.
thanks for the catch.
Thanks for the articles.
I’m confused about resistant starch though, specifically retrograded starch (when starch is cooked and then cooled off again, amylose and amylopectin rearrange themselves differently resulting in resistant starch):
Can you tell me if there’s any energy to be obtained from starch in food that has been cooked and cooled off again or is that starch 100% non digestible? I like to warm up leftovers (e.g. spaghetti) for a quick meal but because I have a fast metabolism, I need to ingest a lot of energy, so I don’t want to eat that much “fiber”.
Also I’ve read this applies to bread and cereals, would that mean no energy from starch in said products either? That wouldn’t conform to the nutritional values on the labels though.
Good stuff as always.
Hey Lyle!
I recently facebooked a request to all my friends to let me know what their favorite RSS feeds were. A friend of mine, whose opinion I respect, plugged your blog!
I really like the *detail* and focus of your blog. Keep up the great work!
Lyle, i would like to know what you think about this study on fructose: http://www.jci.org/articles/view/37385
This study is supposedly the only study on fructose with subjects having a fructose intake with only 25% of daily calories and being in energy balance. For a person at a caloric intake of 2000kcal, thats 500kcal or 125g of fructose. In my view, that amount is very easy to reach if you are eating sugary foods, fruit juces and soda. It only takes 250g worth of regular table sugar (since sugar is 50/50 glucose and fructose).
As the author of the studys says, dose-response studies are needed to determine what amounts of dietary fructose is needed to see ill effects. I think it’s interesting and fructose may be a bigger concern than what we might have thought.
For those in Australia, CSIRO released a breakfast cereal with a ton of resistant starch as well as being a good protein source (http://www.goodnesssuperfoods.com.au/)
Its available in pretty much all the big supermarkets and I gave it a try but ultimately I chose taste over potential heath benefits!
I don’t imagine people on RFL or similar diets are getting much resistant starch so perhaps it should be part of carb loads / refeeds for people worried about their bowel health? Any plans to write an article on it?
The relative sweetness of fructose is related to temperature. At room temperature it is about 30% sweeter than sucrose, but cold it is less sweet, and also when hot. At least so I have heard.
Chris: If cooking starch and cooling it made it impossible to digest, nobody would get fat eating cold pizza (which would be awesome). So no, that’s not what creates resistant starch.
Frameless: As I menioned in Part 1 of the article, a large body of work identified ~50 grams per day of fructose as about where problems start. So I’m not surprised that 125 grams/day caused problems.
But it’s worth doing a quick reality check on this. Given that sucrose is 50% fructose, someone getting 125 grams of fructose from sucrose is consuming 250 grams or 1000 calories of table sugar per day. That’s just slightly less than a large bag of jelly beans. I daresay that someone who’s diet included that every day is going to have bigger problems with their diet than simply over-consumption of fructose.
This especially holds for the biggest provider of HFCS in the diet which are sugared sodas. Liquids don’t blunt appetite the way solids do making overconsumption very easy.
But question: have you ever seen someone who drank 4-5 full sugar sodas per day have the rest of their diet in order?
Right. It’s more than just focusing on fructose or HFCS, it’s almost always the overall diet that is completely awful.
But people like simple villians and simple answers and, as discussed in the HFCS research review, fructose is currently it. Just don’t lose track of the overall picture.
Sam: No plan to write about it in any more detail than here. And high-fiber carb-loads tends to cause, shall we say, problems (of the exploding ass kind). A high-resistant starch carb-load would go poorly I fear.
Great articles! I thought the liver could not convert fructose to glucose and either stored it or released it as fatty acid. Hence the low GI rating. I guess that is not true? What if you ingested fruit after a meal containing carbohydrates? Wouldn’t your liver be full and then release the fructose into the blood as fatty acid?
Up to a point, fructose can be converted to glycogen and then released into the bloodstream as free glucose. When fructose is consumed in excess (~50 grams/day or so), that’s when you start to see conversion to triglycerides.
I am curious to read your take on the “carb blocker” products. Aside from what you mentioned about amylase blockers, have you discussed these products in any detail? If digestion of carbohydrates begin in the mouth, does that also mean that absorption begins in the mouth, before ingestion? Is it possible to block the absorption of carbohydrates before or after ingestion? I figure no, but is there any truth to these claims? Thanks for your blog!
They are crap. Most block salivary amylase if they do anything at all. And that’s a tiny tiny part of the overall carb digestion process.