In response to last week’s blog entry about fats and carbs ("Carbs vs. Fat—Who’s to Blame?"), Burbot wrote: “And don’t forget the glycemic index! …The ADA doesn’t recognize the glycemic index as important … however, practically every other major source of information on diabetes (Joslin, the WHO) does.”
So what is the glycemic index (GI)?
The GI is a measure of how fast a particular food raises your blood glucose. It’s hard for people with diabetes or prediabetes to respond quickly to having a lot of glucose dumped into their systems, so it’s better to eat foods that release glucose slowly (have a “low glycemic index.”) A low-GI food will cause a smaller rise in blood glucose, while a high-GI food will trigger a dramatic spike.
A list of carbohydrates with their glycemic values can be found here. A GI of 70 or more is high; a GI of 56 to 69 inclusive is medium; and a GI of 55 or less is low. You may notice some surprises on this list. It’s often not the sweetest foods that have the highest GI. It’s the foods made with refined, or “white” flours—things like white bread, pancakes, or donuts. That’s because the type of starch present in these foods is easy to digest. They may not taste sweet, but they break down quickly into glucose in your digestive system. In preparing my book Diabetes: Sugar-coated Crisis, I interviewed many people who told me, “I don’t have a problem with sweets. But I can’t keep my hands off that bread.” I came to think of this food preference as a major risk factor for Type 2 diabetes.
But GI is not the whole story. A newer and more informative number is the “glycemic load,” or GL. David Mendosa of the diabetes site www.mendosa.com says, “A GI value tells you how rapidly a particular carbohydrate turns into sugar. It doesn’t tell you how much of that carbohydrate is in a serving of a particular food. You need to know both things to understand a food’s effect on blood sugar. That is where GL comes in. GL equals GI x the amount of carbohydrate in a particular food. The carbohydrate in watermelon, for example, has a high GI. But there isn’t a lot of it, so watermelon’s glycemic load is relatively low.” A GL of 20 or more is high; a GL of 11 to 19 inclusive is medium; and a GL of 10 or less is low.
High-GI foods tend to be unhealthy, especially if you have diabetes or prediabetes. They force the body to pump out a lot of insulin quickly to handle the glucose spike. Then blood sugar drops again. If you don’t make enough insulin or have too much insulin resistance, you will not be able to process all the glucose, and your blood glucose level will stay up. Or it may bounce up and down as the liver and pancreas try to find the right balance.
But GL is just as significant, because a high-GL food will keep blood glucose up for a long time—especially if you eat a lot of it, because quantity is just as important or more important than quality of carbohydrate. The Linus Pauling Institute at Oregon State University says, “After a high-glycemic load meal, blood glucose levels rise more rapidly and insulin demand is greater than after a low-glycemic load meal. High blood glucose levels and excessive insulin secretion are thought to contribute to the loss of the insulin-secreting function of the pancreatic beta-cells that leads to diabetes. You can read more of what the Insititute has to say about GI and GL here.
What about proteins, fats, and fiber? All of these nutrients slow down the rate at which glucose gets into the blood. In general, this is a good thing, because your body can handle gradual changes better than sudden ones.
In diabetes, a low-GI, low-GL diet is best for most people. But as Joan commented on the same blog entry: “There is no one best method that fits every person with diabetes!” The best way to find out what works for you is by checking your blood glucose and noticing how you feel after a eating particular food, to see how it affects you personally.
For more information about GI and GL, you can also check out dietitian Amy Campbell’s blog entry on the topic, “Glycemic Index and Glycemic Load”.