Using the Glycemic Index & Glycemic Load

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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14 Responses to Using the Glycemic Index & Glycemic Load

  1. Sarah Gilbert says:

    Would it be helpful to have this information on nutritional labels? I think it should be. We often don’t even count the rice or pasta at dinner because giving my son insulin to cover it will cause a low within the hour. We end up doing a correction at night to handle it. Sometimes is good to know what foods are digested slowly to avoid lows.

  2. Cristina (Spain) says:

    This was very useful information. Thankk you very much for explaining it using simple language and give us some examples.

  3. David Wheeler says:

    Onc e of the best ecplanation of Gi & Gl. I still need to visually work with the foods and their ratios.

  4. Steve Zamcheck says:

    Most helpful & easier for insulin pump users…. since we can program the insulin delivery to cover thecarbs, time & amounts as needed.

  5. Cristina (Spain) says:

    Hello again,
    I’m not sure if this is the place to ask this question but I’m really interested in understanding how cooking time can affect the GI. For example, when I boil 60 g brown pasta (I always go for brown pasta and rice), sometimes I get 120g of boiled pasta, sometimes I get 150-160 g, depending on the time the pasta was boiling. Does it affect the GI? If so, what is the reason if the amount of CH is the same in the raw food?. I have a 14 year old diabetic son.

  6. You’re right, what you’re describing is the amount of water in the finished pasta. Cooking time can have an impact on the GI of a food. Nora discusses this in her blog posting http://blog.joslin.org/2011/06/using-the-glycemic-index-for-better-blood-glucose-control/

    Hope this helps.

  7. Cristina (Spain) says:

    Thank you very much. I understood everything. Nora explains it all perfectly. When I understand why things are like they are, I feel more confident and can help my son much better. I know this is not so easy in diabetes but it’s really worth trying to do our best!

  8. nora says:

    Dear Christina:
    The longer starch cooks the more it swells with water and the easier time the gastrointestinal enzymes have to digest it.

  9. Howard says:

    The largest online database of glycemic index and glycemic load of foods: http://dietgrail.com/gid/ . It’s free and easy to use.

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  13. Great explanation here Joslin! We should never underestimate the importance of GI & Gl in our food, especially those who have diabetes. Thanks for the article!

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