You've been hearing a lot lately about the health benefits of so-called "low glycemic" foods. The theory is that these are foods which, though filled with carbohydrates, digest slowly and hence do not raised blood sugar. This, we are told, makes low glycemic foods ideal for everyone, especially people with blood sugar problems.
The truth is far different.
What the Glycemic Index Measures
To understand why the GI diet concept is flawed, you have to understand what it is that the glycemic index measures.
The way nutritionists create a tables of glycemic index values is this: They feed a measured dose of a single food to a group of completely normal people. Then they test their blood sugar two hours after they have eaten and come up with an average blood sugar value they compare to the blood sugar the same group experienced after eating some reference food, usually white bread or pure glucose.
You can read about the research that established the GI in this paper authored by GI inventor (and bestselling GI book author)Jenny Brand-Miller.International table of glycemic index and glycemic load values
Kaye Foster-Powell, Susanna HA Holt and Janette C Brand-Miller. American Journal of Clinical Nutrition
, Vol. 76, No. 1, 5-56, 2002
As you can see from that research, pure glucose--which goes right into the blood stream without any digestion at all digests as fast as any food can. When pure glucose is assigned a GI value of 100, the currently accepted table of GI values (which you can view HERE"
white bread is assigned a GI value of 71. Coca Cola made with high fructose corn syrup has a lower GI: its GI is 63. Fresh orange juice has a GI of 43. A banana is 46 and at the low end, kidney beans are reported to have a GI of 19-25. While Lender's bagels are listed as having a GI of 72 and rye pumpernickel bread is listed with a GI of 55. Spaghetti cooked al dente has a GI of 32.
But as you can also see from the Brand-Milller article, one huge problem with the GI index tables is that they are not reproducible. Test one group of people with the food and you will get one value. Test another group and you'll get another value. And that is just what happens to the averages. Test three individuals and you will get three different values. The authors of the theory really stretch to explain why this happens, because nonreproducible results usually mean death for a scientific theory.
But fortunately for them, nutritionists don't let nonreproducible results stop them when those results seem to confirm a strongly held belief, no matter how unscientific. Especially when they have been desperate for anything that could help them ignore the growing number of studies proving that low carb diets are not only safe but more effective than the low fat/high carb diets that they have been promoting for the past two generations.
So nutritionists have embraced the idea that Low GI foods are healthy with enthusiasm, because it allows them to continue to prescribe high carb diets. All that has changed is that they now prescribe low GI high carb diets. And they now attribute to them all the disproven health claims they used to make for the low fat diet.
What The Glycemic Index Misses
But for those of us who care about what is going on in our bodies, a little thought reveals what the problem is with the whole Low GI fad: The Glycemic Index tells you only what these foods do to the blood sugar of a normal person two hours after they eat the food. It does not tell you what they do to that blood sugar one hour after eating them, or--and this can be very important for pasta--four hours after eating them.
And even more importantly, the Glycemic Index does not tell you how much insulin the body had to secrete to process the glucose that resulted from the digestion of this food when it finally did digest.
Because no matter how slowly a carbohydrate-rich food digests, eventually it does digest. And when that food digests, every single gram of carbohydrate it contains turns into one of two substances: glucose or fructose.
If it turns into glucose--and most carbohydrate does--it goes into the bloodstream and raises the blood sugar until it encounters an insulin molecule. That insulin molecule can do one of two things. It can move that glucose into a hardworking muscle cell that burns it for energy or it can move the glucose to a fat cell where it will be stored for later use, in the form of fat. Most of the time insulin does the latter.
If, instead, dietary carbohydrate digests into fructose it won't show up in the bloodstream at all. Instead it will go right to the liver where it will be converted directly into body fat without ever having a chance to be burned off for energy.
Neither of these outcomes is good news if you are trying to avoid gaining weight.
But their tendency to turn into body fat is not the only problem with high carbohydrate low glycemic foods. The biggest problem with them is that while it may be true--for normal people--that foods that digest more slowly may avoid raising blood sugar, these foods still must stimulate insulin production. No matter how slowly these foods digest, they do digest, and when they do, insulin will be needed to process every gram of grams of carbohydrate contained in the food. So a low GI food that contains a lot of carbohydrate, like pasta, may give you a normal blood sugar but it will also give you a very high circulating insulin level, which is not good for your health.
What if you Don't Have Normal Blood Sugar?
The problems we just mentioned are the problems encountered by normal people eating low glycemic foods. People with prediabetes or actual diabetes have much more serious problems when they eat these low glycemic foods.
Normal people don't get damagingly high blood sugar spikes after eating high carbohydrate foods because they have healthy pancreases that secrete a big burst of insulin as soon as they start eating a meal.
This is called "first phase insulin release and is part of the normal blood sugar control process described here: How Blood Sugar Works--And Stops Working
If that burst of insulin doesn't do the trick, the healthy person's pancreas secretes a second phase insulin release which mops up any extra glucose that is left in the bloodstream. This second phase insulin release may go on for many hours after a meal and will keep blood sugars very flat though, as mentioned above, the insulin levels may be high for all those hours.
But people with prediabetes or diabetes usually don't have the ability to produce a first phase insulin release. Many have a damaged or absent second phase insulin release, too. So if you test someone with abnormal blood sugar at two hours after they have eaten a log GI food, you may see a very different reading than what you saw when you tested that food in a normal person.
Foods that work well for a normal person, like whole grain oatmeal, may cause a blood sugar spike just as high as white bread in a person with diabetes. That whole wheat bread raises blood sugars the same amount as does white bread when fed to people with diabetes was recently documented in a published study:Dietary Breads Myth or Reality
. Banu Mesci et. al, Diabetes Research and Clinical Practice
Volume 81, Issue 1, July 2008, Pages 68-71
The only reason for a person with diabetes to learning the Glycemic index values for various foods is that if you are injecting insulin, knowing the GI value can help you time your insulin so that it peaks in the bloodstream at the same time that the glucose from the foods you ate reaches it.
Except that even this is more complex than it sounds, because there is yet another problem with applying the glycemic index. The GI values are established by testing each food in isolation. But we don't eat these foods in isolation, we eat them as part of a meal.
And when you combine
foods in a meal, their GI values change dramatically. Any high carbohydrate food eaten with fat will digest more slowly than it does when eaten alone. Eating a lot of food at once will also slow digestion.
So while white bread might have a very high GI and be completely digested by two hours, A slice of pizza that contains the same amount of carbohydrate as three slices of white bread may not digest completely until 4 or 5 hours have passed, thanks to the very high fat content of the cheese and the sheer bulk of the food.
In fact, the table of GI values cited above gives a higher fat Pizza the GI value of 30--which is one of the lower values to be found on that table, though you never will hear nutritionists telling people to eat a diet rich in healthy high fat pizza--and you won't hear it from me either. The combination of all that carbohydrate and fat is probably the most unhealthy one you could come up with and though it takes it a while to digest when pizza does digest it can give people with diabetes some of the highest blood sugars they are ever likely to see.
All this should make us rethink just how "healthy" all those low GI foods really are. If we really want to eat foods that won't raise our blood sugar or stimulate insulin response, we will be a lot better off eating protein, fat, fresh greens, and colorful fresh low carbohydrate vegetables in place of any carbohydrate-rich foods.
And when we do eat foods with carbohydrate in them, no matter what their GI might be, we should consider the total amount of carbohydrate in that food, not just the speed with which it digests.
Why Do We Hear So Much About Low Glycemic Foods?
It is not coincidental that over the past five years the once obscure GI theory, which was first published in 1981 and completely ignored for the next 20 year
has suddenly risen to prominence.
A major reason is that the food industry reacted to the "low carb diet craze" that swept the U.S. around the year 2000 by funding a well funded and well orchestrated media campaign to promote this hitherto obscure and neglected theory. And they did a masterful job of it.
I base this conclusion on the press releases I have received from organizations like a trade group calling itself The Whole Grains Council
which as you can see if you follow the link above to the slick web site of this lobbying group cites a long list of such studies, many of which have been, not so coincidentally, funded with their members' money. These studies tout the healthfulness of their low GI foods. The site also contains a link filled with outdated misinformation intended to scare readers into believing low carb diets are dangerous.
The people who send me these press releases always ask me to spread the word about the supposed health benefits of their products to readers of my blog--though it is obvious they haven't ever visited the blog or they would not have wasted the electrons it took to mail me their spams.
There is a lot of money to be made selling cheap grains to a credulous public. Just look at the markup on a box of "whole grain" cereal! So there is also a lot of money available to promote health theories that would induce people to buy those overpriced grain products.
Just keep in mind when you review any study purporting to show the health advantages of a Low GI diet that they always compare a low glycemic or whole grain diet to one made up entirely of junk food.
I would be the last person to argue that a diet rich in whole grains is worse for you than one made up mostly of Sugar Frosted Flakes and french fries. But you will never see the Grain council comparing the health benefits of their foods with those of a diet low in carbohydrates of all kinds. That is because the evidence from non-industry funded studies that the fewer the carbohydrates eaten, the better the health outcome. You can review some of these studies here:Studies Proving the Safety and Efficacy of the Low Carb Diet