Carbohydrates Part 1: Classification and Digestion

On my Facebook group, someone mentioned that something about the GI of cooked versus raw carrots was moving through the fitness community (something like that) and that stimulated me to write this article series.  In it I want to ultimately look at the concepts of the Glycemic Index (GI) and Glycemic Load (GL) but today I need to give a quick primer on carbohydrates which I have done before so the next bits will make sense.

Classifying Carbohydrates

How to classify different carbohydrates in the human has been a topic for decades if not the better part of a century (I’m sure it goes back further than this but I’m not sugar historian) and to make this article make sense, I want to blather about the different types of carbohydrates.

Fiber

Let me start with fiber.  Composed of a variety of different compounds that I will not name but you can look at here if you really want fiber can be roughly divided into two categories.  The first are insoluble fibers.  These don’t mix in water, can’t be digested by humans and basically act as a broom in the gut.

The other are the soluble fibers, they mix in water (guar gum is a popular supplement, add it to water and wait and it will turn into a brick) and are digested in the body.  However, unlike other carbohydrates these are fermented in the gut by bacteria into short-chain fatty acids (SCFA).  These have a number of profound health benefits and actually provide 1.5-2 cal to the body (although it’s not considered carbohydrate grams of calories).

Digestible Carbohydrate

Digestible carbohydrates are all built from the same basic building blocks which are the monosaccharides (‘mono’ = one and ‘saccharide’ = sugar) which are glucose or blood sugar, fructose or fruit sugar, and galactose (not to be confused with Galactus, eater of worlds).   Quick note: all sugars end in -ose.  There are other monosaccharides, ribose (which was once sold as a sports supplement) is one but they aren’t that important in the diet.

These single sugars bond into two molecule compounds called disaccharides (‘di’ = two) and the primary ones are sucrose (table sugar) which is glucose+fructose, lactose (milk sugar) which is glucose + galactose, and maltose (used to make beer) which is glucose + glucose.

Quick note: high-fructose corn syrup or HFCS is a man-made sugar that I’ve written about before.  Despite it’s name, it’s almost identical to sucrose in composition and works the same in the body.  While blamed for all modern ills by some including obesity, diabetes, the economy and the dissolution of the nuclear family (Ok, not really), this is alarmist nonsense.  HFCS is found to a great degree in sugary sodas and, if there is a problem with it, it’s that most liquids don’t make people full and many are overconsuming a crap-load of CALORIES by drinking these drinks.

And while it’s popular to blame HFCS/sugary sodas for everything right now, let’s face facts: people who are drinking a Super Mega Giganto Big Gulp of coke have a diet and lifestyle that is total crap.  HFCS isn’t helping but it’s the least of the problems here: simply, the HFCS per se is irrelevant to the calorie load.  And yes, I’m aware of the fructose overfeeding studies but here’s the thing: HFCS is not pure fructose to begin with and most of these studies use impossible amounts of the stuff (one used 300 grams of pure fructose which would be 600 grams of HFCS and this is irrelevant to normal human consumption patterns).  Anyhow….

There are some intermediate chain lengths that aren’t that important in the diet overall and once you get past the mono- and di-saccharides you are generally looking at longer chains of straight glucose.  In the body these long chains of glucose are called glycogen and are found in both the liver and muscle.  In the diet they are called starch or simply complex carbohydrates.

There are some other weird carbohydrates that come up sometimes.  Maltodextrins are medium chains of glucose that are found in some specialty sports supplements as are amylopectins.  There are also what are called resistant starches, carbohydrates modified to be resistant to digestion that may have health and metabolic benefits.  These are being found in more and more foods and are good for dieting since you can eat the same volume of food but at least some of the starch won’t be digested or absorbed.

Digestion and Absorption of Carbohydrates

Focusing on the digestible carbohydrates, let me briefly talk about their digestion and absorption in the gut since this is important background for the actual stuff about Glycemic Index and Glycemic Load.   A little bit of carbohydrate digestion actually starts in the mouth, via an enzyme called alpha-amylase (note: all enzymes end in -ase and there are lipases which digest fat and proteases that digest protein).

You can test this if you want, put a small piece of bread in your mouth (or hell a big piece, I’m not here to judge you) and chew on it without swallowing.  You’ll start to get a sweet sensation in your mouth from the breakdown of the starch into individual molecules of glucose.

After what little digestion occurs in the mouth the food moves to the stomach where it is mechanically churned and broken down and acted on by more digestive enzymes.  And here’s the key to this: only single sugars can be absorbed across the gut.  You can’t absorb sucrose, you can’t absorb lactose, you can’t absorb starch directly.  You can only absorb glucose, fructose and galactose and that’s all that hits the portal vein on the way to the liver.

So sucrose is broken down to one glucose and one fructose, lactose is broken down into one glucose and one galactose and longer chains of glucose such as maltodextrins, amylopectins or starch are broken down bond by bond into single molecules of glucose.

Quickly: After absorption, sugars are metabolized differently.  Glucose generally moves straight through the liver out into the bloodstream where it is used for energy directly (most tissues can use glucose for energy and it is preferred when it is available) while fructose and galactose have to be metabolized in the liver first.  Fructose can be converted into glucose and either stored as liver glycogen or released into the bloodstream as glucose.  It can also be converted to lactate which is used by other tissues; in large amounts (more than about 50 grams per day, fructose can be converted into fatty acids in the liver).  Note that fructose never gets into the main circulation and I mention that as some fear tactic bs has been based on fructose INFUSION but this has no relevance to eating food.  Galactose is metabolized similarly to fructose but unless someone’s diet contains a shedload of milk, it’s really not worth worrying about.

Practically what this means is that regardless of what dietary carbohydrate is eaten, only glucose and fructose (and fine, galactose) make it out of the gut and only glucose ever makes it past the liver.

In one sense, this makes all dietary carbohydrates identical: one way or another they all end up as glucose in the bloodstream.  Whether you eat pure glucose (don’t do this, it’s disgusting), maltodextrins or starch, it all ends up as glucose in the bloodstream at the end of the day.  Even fructose eventually hits the bloodstream as glucose.

Which raises the question of what makes one dietary carbohydrate different from another.

And the short answer is their speed of digestion.

And I’ll pick up next time to talk about the longer answer and why it matters (or doesn’t).

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