By Nora Saul M.S, R.D., L.D.N., C.D.E.
Manager of Nutritional Education at the Joslin Diabetes Center
Everybody has an opinion on the Glycemic Index as a method for glycemic control—it’s either the greatest thing since sliced bread or a trivial addition to carb counting.
At the Joslin, we still emphasize the quantity of carbohydrate (white rice and black beans just aren’t equivalent) when making food choices. But the value of the glycemic index is no longer underrated.
So what is the glycemic index (GI), and why should you pay attention to it if you have diabetes?
The glycemic index is a ranking system for carbohydrates, based on how fast a particular carbohydrate will make blood glucose rise in comparison to an equal quantity of pure glucose.
The basis for the comparison was established through clinical research. First, volunteers were given 50 grams of pure glucose, and their blood glucose was measured every 15 minutes for a two-hour period. Then (using a calculus formula) they determine the rate at which the volunteer’s blood glucose level rose.
The same procedure is then repeated with 50 grams of different carbohydrate test foods.
Then, to come up with the rankings for the test foods, the score for each test food is divided by the score for glucose and multiplied by 100.
The GI has three major rankings:
- Low, which is a score below 55
- Moderate, where the score is between 56-70
- High, for foods with a score above 71.
The low group raises blood glucose slowly, while the high GI foods tend to spike blood glucose very quickly.
Take the white rice (high GI) and black bean (low GI) example. If you eat a ½ cup of each separately and measure your blood glucose over a two-hour period, you generally will find that white rice raises your blood sugar sooner and may push it higher.
Why This Is Important
For both type 1 and type 2 diabetes, the GI can be a valuable tool in anticipating and matching the rise in blood glucose that results from eating carbohydrate to your body’s uptake of insulin.
If you have type 2 diabetes you have lost your ability to adequately secrete insulin when you start chewing a food containing carbohydrate (what is termed the first phase of insulin response). As a result, your blood glucose can start to rise before the insulin has been released.
If you have type 1, you know that the rapid acting insulins don’t start working for 15 minutes. So when you eat high GI foods there is often a mismatch between when your blood glucose rises (soon) and your insulin starts working (later).
So How Does Carb Counting Fit Into This?
The GI ranking is a fixed number. No matter how much of a food you eat, the GI stays the same.
For example, white rice (according to the entry for white rice boiled type NS on glycemicindex.com) has a GI of 72—whether it’s a ½ cup or 2 cups.
But we know the amount of carbohydrate is important. If you monitor your blood glucose, you probably know this from experience. You can manipulate the impact (the GI) of the carbohydrate foods you eat by limiting their quantity. If you eat less of some foods, your blood glucose level doesn’t rise as fast.
There is a way to predict how you might react to the GI of foods, without having to run experiments on your body, by determining the Glycemic Load (GL) of foods. Knowing the GL of foods can also be a big help when it comes to portion decisions and overall meal planning.
Like the glycemic index, GL is expressed in three levels:
- Under 10 is low
- From 11 to 19 is moderate
- Over 20 is high
The GL score for foods is calculated using a simple formula: (GI x total carbs)/100. That is, you find the product of the GI multiplied by the carb content in the amount you’re eating, and then divide that product by 100.
So, for example, here’s how we can calculate the GL for white rice.
For ½ cup of white rice, it goes like this: Take 72 for the GI for white rice and multiply that by the 22 mg of carbs in ½ cup. The result is 1,584. Divide 1,584 by 100. The result is 16. (Actually, it’s 15.84 but we round it up). 16 is the GL score. This puts a ½ cup of white rice in the moderate range.
For two cups of white rice, follow the same arithmetic. Multiply 72 (the GI for white rice) by 90 (the number of carbs in a two cup serving) and you get 6,480. Divide that by 100 and round up. The result is a GL score of 65.
Because white rice is a high GI food, both the ½ cup and the two cups of rice will raise your blood sugar quickly. But based on the GL, we can predict that two cups will raise it higher.
In our next blog on the glycemic index, we’ll talk about what makes foods low and high in GI, and how to use the GI/GL every day to help control your diabetes.
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- On nutrition classes at the Joslin Diabetes Center
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