Nutrition 101: Carbohydrates

[Nutrition 101: Carbohydrates mp3]

I need a co-host!! As you can no doubt tell I don’t have the dulcet baritone voice I might want on the podcast. I also don’t have special ribbon or condenser mics normally used for radio personalities. I can do some EQ-ing in post to try and maximize the bass that’s picked up, but I usually don’t do that. It’s not just my voice, though; it’s also about how strange it feels to talk into a mic alone. In any case, if you know me personally and would like to contribute let me know. I could use a different perspective and dual dynamic.

So with that out of the way today’s podcast is about carbohydrates. I don’t know how many of you know what they truly are. I certainly didn’t a few years ago; I just knew that there was a lot of ‘em in pasta, bread, potatoes, and sugary stuff. Of course that didn’t stop me from telling people to stay away from them because they were “bad for you” and made you fat, even though I had essentially zero knowledge of nutrition or food science. I bet I was really annoying back then.

I will abbreviate carbohydrate sometimes as “CHO” for typing ease and because it refers to Carbon, Hydrogen, and Oxygen which are the constitutive elements of carbohydrates. Also if you take any higher education course that might deal with CHO the instructor(s) will likely abbreviate them this way as well.

As I mentioned in the podcast there are primarily three types of CHO: monosaccharides, disaccharides, and polysaccharides.¹  Below are examples of the first two.

 

The main monosaccharides are glucose, fructose, and galactose. These are not the only monosaccharides that exist in nature; others include ribose, xylose, deoxyribose, arabinose, mannose, glucosamine, and on and on. However, glucose, fructose, and galactose are going to be the most nutritionally relevant because you will be eating far more of them than any other monosaccharides. Below is a diagram comparing the three sugars and, as you can see, they are quite similar. In fact, their chemical formulas are all exactly the same (C₆H₁₂O₆), but each of their structures differs slightly.

There is nothing fancy about disaccharides either except that they are two monosaccharides bound together that will eventually be pulled apart by the enzymes in your gut. There are also a few more disaccharides in existence than pictured², but they are not commonly found in the diet.

Lastly we have polysaccharides which are classified as either starch or dietary fiber.³ I also mentioned glycogen in the podcast. Glycogen is physiologically significant and plays an important role in muscle tissue, but as far as nutrition goes it’s pretty inconsequential. That said, here’s a pretty picture of glycogen that I took from Wikipedia.

 

Your muscles will just break off some carbon whenever it needs a little energy.⁴

Regarding starch, there are two main types: amylose and amylopectin. I have provided another picture for you. You are welcome.

 

As you can see they are both composed of long glucose chains and have a fairly similar structure, although the amylopectin is highly-branched. I have been looking through the literature to see if there is any meaningful difference between the two in terms of health benefits. The evidence is mixed, but it seems that amylose is considered by more researchers to confer health benefits over amylopectin because it is more slowly digested, thus giving amylose a lower glycemic index. Again, the evidence is not so clear-cut so avoiding amylopectin like the plague is probably unnecessary.

And lastly we have dietary fiber which also comes in two major forms: soluble and insoluble. Soluble fiber has a wide range of benefits that include lowering total cholesterol, regulating blood glucose, delaying gastric emptying which increases satiety, and providing tasty goodies for the bacteria in your gut. That last part is actually a relatively new field in nutrition science, and one I happen to be intimately involved in at the FHCRC⁵ at the moment. The bacteria can take soluble fiber or other compounds that you might not otherwise digest and use them for energy. As a by-product of this fermentation in your gut you can receive short-chain fatty acids or other bioactive compounds. This is not a bad thing. In fact these fatty acids and other bioactive compounds can be quite beneficial to you. As for the insoluble fiber, well… it helps you poop. And for this it deserves its own special place in heaven.

Why can’t we digest fiber and harness the energy contained it its chemical bonds like we can with other carbohydrates? Like I mentioned in the podcast dietary fibers such as cellulose are achingly similar to other CHOs like starch, but the reason we can’t mess with it all boils down to chemistry and the physical bonds that link them. Take a look at this simple picture:

 

We just don’t possess the enzymes capable of breaking the bonds on cellulose.

What about digestion and absorption? Here are a couple diagrams on how starch molecules are broken down to monosaccharides.

 

And here’s a very brief and simplistic overview of how those monosaccharides are absorbed.

 

(click to embiggen)

I did not get into carbohydrate metabolism or blood-glucose homeostasis because I wanted to start with the very basics. CHO metabolism is certainly an important topic and one I’m sure I will cover in the future but not today. However, if you’re interested here is a nice overview of how the body maintains blood-glucose levels in the fed and fasted state.

1. You can make the argument that there are also “oligosaccharides,” but the difference between an oligosaccharide and a polysaccharide is relative – much like the difference between a “group” and a “crowd.” In this instance I am lumping oligosaccharides in with polysaccharides.
2. like trehalose or cellobiose
3. At least as far as the diet of primates like you and me are concerned
4. Much like a Kit-Kat bar. Well…. Not really.
5. Fred Hutchinson Cancer Research Center

• amylopectin
• amylose
• carbohydrate digestion
• carbohydrates
• CHO
• fructose
• galactose
• glucose
• nutrition 101
• sugar