Diabetes and the Glycemic Index

The glycemic index (GI) is often explained as a measure of “how much” or “how fast” or “how high” ingesting a certain food effects blood sugar compared to the effect of ingesting glucose. Since glucose has the most immediate and pronounced effect on blood sugar (i.e. blood glucose) its value is set as a benchmark value of 100, and other carbohydrate-containing foods fall into the high (>70), medium (55- 70) or low (<55) GI range. Illustrations of the blood sugar effect of low GI foods often show little or no impact whatsoever.

It would seem logical, therefore, that eating for effective diabetes management (i.e. blood sugar control) and the glycemic index rating of foods would go hand in hand, and to some extent this is true. But it is important to know more about how diabetes and the glycemic index are related in order to effectively balance the two.

The standard explanation of this observable variation of food and blood glucose focuses on how fast our digestive processes can convert the carbohydrate content of a particular food into glucose, which is then absorbed into our bloodstream. Foods that convert carbohydrate rapidly into glucose (or glucose itself) show a sudden and prolonged effect on blood sugar, and foods that digest very slowly show a slow and subdued effect. But the “fine print” of the GI explains that the observed blood sugar response to certain foods involves not only the time frame of glucose absorption into the bloodstream, but also the insulin response of individuals with “normal” glucose metabolism.

That is to say that whether the blood sugar response is extreme, as with high GI foods, or subtle, as with low GI foods, a critical element of the story is that the blood sugar response (going up and coming down) is mediated by the natural and normal process of pancreatic beta cells releasing insulin. When glucose is absorbed slowly from low GI foods the insulin response is often sufficient to show little or no impact on blood sugar levels…..in individuals with normal glucose metabolism.

But, those of us with diabetes do not have normal glucose metabolism. We have “issues” with insulin, and this complicates the picture considerably. Insulin’s “job” is to collect the extra glucose in our blood that makes blood sugar “too high” after eating carbohydrate foods and see that it’s stored away in muscle and liver cells returning blood sugar levels to normal. When we have no natural insulin available (type 1 diabetes) or do not respond normally to our natural insulin (type 2 diabetes), our response to carbohydrate foods of any kind is not “normal”. Moreover, the potential range of our responses is nearly infinite.

Without injecting insulin, for instance, my blood glucose as someone with type 1 diabetes may rise steadily to dangerous levels regardless of the GI value of the food I ingest because I would have no insulin available to counteract the appearance of glucose, slowly or rapidly, in my blood. The response of individuals with type 2 diabetes could vary based upon so many factors, including medication, that space won’t allow a discussion. The point is that the glycemic index values and the associated “glycemic load” (which combines GI with the carbohydrate grams) are established based upon observations of “normal” carbohydrate metabolism, and diabetes is actually defined by “abnormal” carbohydrate metabolism.

So, are the GI or glycemic load useless tools for diabetes management? Absolutely not. One could make a convincing case that a diet focused on low GI foods is beneficial to diabetes management by allowing time for insulin, natural or injected, to do its job. Many healthy foods like whole grains, fruits and vegetables are lower on the GI scale, and a healthy diet is an essential part of effective diabetes management. But effective diabetes management balances total carbohydrate intake with medication and individual health status. And even though the glycemic load takes total carbohydrate grams into account the values are still based upon normal carbohydrate metabolism. So, while the glycemic index and glycemic load can serve as tools, we are still better served as individuals if we “count carbs” and monitor blood glucose levels after meals to identify patterns we can act to change or to replicate. That is how we can know.

By Toby Smithson, RD, CDE


National Spokesperson for the Academy of Nutrition & Dietetics

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