What is the glycemic index?

I’ve had enough questions about the glycemic index and the glycemic load that I’ve decided to take the time and explain what it all means.  If you know what the glycemic index and glycemic load are, then you might want to skip this post unless you’re just here for the scintillating writing.

If I were to bring you into my office while you were fasting and check your blood sugar, then check it again every 15 – 30 minutes over the next two hours, I would find that your blood sugar levels wouldn’t change much.  Your blood sugar would remain at about, say, 85 mg/dL over the entire two hours.  Now, suppose I bring you in fasting, measure your blood sugar, then give you a piece of cake.  You eat the cake and I measure your blood sugar over the next two hours.  Your blood sugar would rapidly rise, then fall slowly, and return (assuming you’re not diabetic or glucose intolerant) to around your normal 85 mg/dL.

Scientists have known for years that normal blood sugars follow this kind of rapid increase, slow return to normal curve.  At some point someone asked the question: do different foods cause a different curve?  In other words, if someone eats a piece of cake does that make a different blood sugar curve than if that person eats a bowl of ice cream?

Let’s say we decide to find out how different carbohydrates affect you.  I draw your blood while you’re fasting, then give you 50 grams of glucose to drink.  I collect blood samples every 15-30 minutes for the next two hours and use that data to construct a curve of your blood sugar response to 50 grams of glucose.  A couple of days later I again draw your blood while you’re fasting, then I have you eat a piece of potato containing exactly 50 grams of potato starch.  Again I check your blood sugars over the next two hours and create a curve with your blood sugar data.

Now I’ve got a curve representing what happens to your blood sugar when you consume 50 grams of pure glucose and a curve representing what your blood sugar does when you eat 50 grams of potato carbohydrate.  I can compare these curves to see if there is any difference between the two.

When researchers look at any kind of data curve, they calculate the area under the curve, which, in the case of your blood sugar curve, represents the total amount of rise in blood sugar over time.  Assuming you have a normal blood sugar response to a carbohydrate load I would find that the area under the blood sugar curve after you had eaten the potato is about 80% of the area under the curve that we found after you had eaten the pure glucose.  Since the potato causes only an 80% increase in total blood sugar rise over the two hours as compared to the rise driven by the pure glucose we can say that the glycemic index of the potato is 80% that of glucose, or 80.

If we tried the same experiment with refried beans (like those you would find in Mexican food), we would find that the area under the blood glucose curve caused by the 50 grams of carbohydrate from the refried beans would be only about 40% of that caused by the pure glucose.  We would say that the refried beans have a glycemic index of 40.

The glycemic index figures you see published are derived the same way as I explained above.  Young, healthy subjects consume a test dose of glucose (or sometimes 50 grams of carb from white bread, which is also a common standard) and have their blood sugar curves measured.  They then come in on subsequent days and consume the test foods (the potato, beans, tomatoes, whatever) and have their blood sugar checked over two hours to create the blood sugar curves driven by the carbs in these foods.  These curves are compared to the pure glucose curves and a glycemic index is derived.

As you might imagine, it requires an enormous number of subjects and a whole lot of testing to come up with the long lists of glycemic indices for all the various foods.  It required herculean efforts by a whole lot of data gatherers to come up with these lists, lists I believe have very little value.  Before we get into that, however, let’s look at why the researchers who have spent so much time collecting this data think it’s important.

It would seem to make sense that one would be better off eating carbohydrates that are low-glycemic carbohydrates and raise blood sugar less than carbs that are high-glycemic carbs.  Wouldn’t it be better to eat refried beans with a glycemic index of 40 than to eat a potato with a glycemic index of 80?  Of course it would.  And if one is comparing carbs based solely on their glycemic indices, I would always recommend the choice with the lowest glycemic index.  But, are a bunch of low-glycemic index carbs better than very few carbs at all?  I don’t think so.

I think the whole glycemic index idea is interesting, but flawed as a tool for those seeking weight loss and/or better health.  Here’s why.

Glycemic index figures are calculated using measured doses of certain foods compared to pure glucose.  We know that 50 grams of potato starch gives 80% of the blood sugar rise compared to glucose.  But what happens if we eat some butter along with the potato?  Will the glucose curve be larger or smaller than it is with the potato alone?  No one knows.  Could you imagine the complexities involved in just trying to figure that one out?  How about if we eat the potato with butter and drink a cup of coffee?

Again, no one has a clue.

It’s nice to know what the glycemic index is for a potato all by itself, but since most of the meals we eat are a mixture of foods, how do we calculate the glycemic index for the whole meal?  It’s impossible.

And glycemic index figures are calculated using data derived from young, healthy subjects.  What if I’m old and have a touch of the Metabolic Syndrome?  Will these figures be the same for me?  I don’t think so.  But who knows?  It’s never been tested.

If I take in 50 grams of carbohydrate as potato or sugar or anything else, it’s probably not going to affect me that much.  What if I take in 300 grams?  I might (and probably would) react much differently.  So what good is an index derived from multiple studies of only 50 grams of carb, when I will more than likely eat much more than that?

Another problem with using the glycemic index is that fructose has a low glycemic index.  50 grams of pure fructose gives a blood sugar curve that is about 22-24% of that produced by glucose, which means that fructose has a glycemic index of only 22-24, which is very low by anyone’s estimation.  Why is fructose low?  Because it doesn’t convert to blood sugar, it converts to fat in the liver instead.  And causes a lot of problems.  But has a low, low glycemic index.  So foods containing fructose then have a low-glycemic index.

So, we don’t have a clue as to what the glycemic index of a mixed meal is.  We don’t know if our own personal glycemic index is the same as that of the young, healthy people used to create the lists we all use.  We don’t’ know if the glycemic index is the same with amounts of carb much larger than the 50 gram test loads.  And we may be encouraged to eat foods containing a lot of fructose because fructose has a low-glycemic index.  See why I have a problem with the concept?

Another issue that I touched on earlier is the difference between a lot of low-glycemic index carbohydrates and a small amount of high-glycemic carbohydrates.   If the glycemic index of refried beans is 40, can I eat twice as much of those as I can potato with a glycemic index of 80?  Or can I eat as much low-glycemic index stuff as I want without consequence because, after all, it won’t raise my blood sugar by much?

People who think the glycemic index is a good thing decided to take it a step further and came up with the concept of the glycemic load.  The glycemic load is the glycemic index multiplied times the grams of carb in the food in question.

Let’s look at a few foods.  Pure glucose has a glycemic index of 100 so if you eat 20 grams of glucose you have a glycemic load of 20. ( 1 (glycemic load converted to a fraction) X 20 grams = 20)  If we eat 20 grams of carbohydrate as refried beans we have a glycemic load of 8 (0.4 X 20), and if we eat 20 grams of carb as a potato we would have a glycemic load of 16 (0.8 X 20).  Theoretically, we could eat twice as much refried beans as we could potatoes and still have the same blood sugar changes.  But, unless we eat the refried beans while we’re fasting, and eat them all by themselves, and make sure we don’t get more than 50 grams of them, we really don’t know.  If we eat the refried beans along with an enchilada all bets are off.

By my thinking, it’s a whole lot easier to just count carbs and not worry about the glycemic index.  And it’s really a whole lot easier to simply count the carbs than it is to go to the trouble to look up the glycemic index, convert to a fraction, then multiply times the grams of carb.

All in all, the whole idea of the glycemic index and glycemic load is that by limiting the diet to lower glycemic index carbohydrates one can eat more carbohydrates and not suffer the problems that more carbohydrates typically cause.  Which, were it true, would be a good thing since most everyone (yours truly included) likes to eat carbohydrates.

But the paper I wrote about yesterday that was presented at the ASBP meeting showed that a plain old low-carb diet provided better results (for diabetics, at least) than a low-glycemic load diet.

Make it easy on yourself, and just count the grams of carb and be done with it.