Thursday, April 17, 2008

The Glycemic Index and the Glycemic Load


What happens when we eat carbs?

Some foods make our blood sugars spike really quickly and others, not so much. The foods that have an effect on blood glucose contain carbohydrates. When you eat foods containing carbs they break down to their basic component, glucose or sugar, in the intestine and glucose gets into the bloodstream. As a result, your blood glucose goes up. In response, your pancreas secretes a hormone called insulin that helps sugar get out of the blood and into your body cells that need sugar. Glucose is actually the main fuel for your brain and muscles.

However, when lots of glucose comes in at the same time, your pancreas secretes a lot of insulin at once resulting in a quick drop of your blood glucose. When your blood glucose goes down that quickly, you feel hungry and, as a result, eat again, leading to potential weight gain. Another thing that happens when there’s too much glucose that comes in at once is that your muscle cells start to resist storing the extra glucose. This extra sugar gets stored as fat and also contributes to weight gain.

There are carbohydrates that break down really quickly into glucose- so eating too much of those types of carbohydrates quickly causes your blood glucose to spike quickly, causing all this havoc.

David Jenkins and colleagues from the University of Toronto created a system that ranks carbohydrate foods based on how quickly they break down to sugar. This is known as the Glycemic Index. They compared the impact of different foods on blood glucose to the impact of standard foods like pure sugar on blood glucose. Pure sugar was given a Glycemic Index (GI) of 100.

Foods that have a GI of less than 55 are considered to have a low GI or low impact on blood glucose, 55-69 is a moderate GI and a GI of greater than 70 is considered high.

For example, black beans have a GI of 30. This means that they boost blood sugar 30% as much as pure glucose.

Although GI offers some useful information, it’s not the whole picture. For example, carrots have a GI of over 70. Carrots do have sugar that’s highly absorbable but the total amount of sugar is very small and therefore has very little effect on blood glucose. Luckily, there’s something called the Glycemic Load.

The Glycemic Load (GL) was developed by Walter Willett and colleagues at the Harvard School of Public Health and considers the total amount of carbs in a food as well as the GI. A GL of less than 10 is low, 11-19 is medium and more than 20 is high.

The Glycemic Load of carrots is 5.

Although most health experts, including Marion Nestle and Walter Willett, call GL and GI useful tools, they caution to not build your diet around it. Marion Nestle says she ignores the long lists of foods and their GI and GL values (I included one of those lists below) because not only do the numbers vary depending on the source, but they also vary depending on preparation and cooking methods. It all gets very confusing.

Not only that but if you add butter and cheese on top of a potato, the Glycemic Index will be lower (fat and proteins have no GI because they have no carbs) but calories and saturated fats increase. So just because a food or a meal has a low GI or a low GL, doesn’t mean it’s necessarily a good choice!

What you should focus on is how much food you’re eating and choosing less processed starchy foods (that usually have high GI and GLs anyway).

Health Benefits of choosing low Glycemic Index and Glycemic Load foods

In both the large Nurse’s Health Study and Health Professionals Follow-up Study, it was found that people who ate low fibre cereals and high Glycemic Load foods more than doubled their risk of type 2 diabetes.

Other research has shown that the excess sugar and insulin that comes from eating high GI and GL foods contribute to other chronic conditions like heart disease, breast cancer, colon cancer and polycystic ovarian syndrome.

Now, I haven’t read any of these studies (yet) but just summarizing from Willett’s book. However, it seems to me that people who eat higher fibre and lower GI and GL foods (like whole grains, legumes, vegetables) are those that are most likely to have healthier lifestyles- eat healthier, exercise more, don’t smoke, not overweight etc. than those that are eating low fibre and high GI/GL foods. So is it just the higher intake of high GL/GI foods that are contributing to these health conditions or is it a less healthy lifestyle?

Mike- hope this answers some of your questions. There'll be more to follow.

Glycemic Index and Load of foods: http://www.ajcn.org/cgi/reprint/76/1/5

Sources:


Nestle, Marion. What to Eat. 2006

Willett, WC. Eat, Drink, and Be Healthy- The Harvard Medical School Guide to Healthy Eating. Free Press 2005.

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