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.
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|