You may know about the glycemic index (GI), particularly if you have a disease or condition that affects your blood sugar, like diabetes or hypoglycemia. Or you might have heard the glycemic index diet is good way to lose weight. But is this true? Experts say there's more to the story.
The glycemic index is a food rating system created in 1981 to give people a better idea of which foods contain the types of carbohydrates that cause blood sugar to spike.
"It is an indicator of how quickly the carbohydrates in an individual food get digested and absorbed. The higher the number, the more quickly and easily the carbs are broken down, turned into glucose, and absorbed into the blood stream," says Daphne Olivier, a dietitian with My Food Coach, via email.
It's ideal to maintain stable blood sugar because frequent spikes can put a person at risk for serious problems, like obesity, heart disease and Type 2 diabetes, among others. "
In a 2008 paper, University of Sydney researchers assigned reliable GI scores to more than 2,480 foods, available online. "Dairy products, legumes, and fruits were found to have a low GI. Breads, breakfast cereals, and rice, including whole grain, were available in both high and low GI versions," the researchers explain in the study.
"We were wanting to give the research community a 'one-stop shop' for all the GI values published in the scientific literature," says University of Sydney nutrition professor Jennie Brand-Miller, in an email interview. "By doing this, we hoped that studies would have greater separation of the high and low GI diets."
So how did they figure out the numbers? Brand-Miller says the researchers fed a 50-gram glucose solution to their study subjects and measured their blood glucose response several times. Then they measured a 50-gram carbohydrate portion of the test food in the same way. They compared the glucose response to the test food with the response to the reference food and expressed the result as a percentage. So, a potato with a GI of 82 means that it would raise a person's blood sugar level 82 percent of what a gram of pure glucose would.
Glycemic Index vs. Glycemic Load
However GI doesn't tell us everything. Sometimes the portion size of a food with 50 grams of carbohydrates is much more than a typical person would eat.
"For instance, it takes four cups of carrots to equal 50 grams of carbs, which is not a typical portion," explains dietitian Olivier. "Given the portion conundrum with the glycemic index, the glycemic load was developed [in 1997] ... It is calculated by multiplying the glycemic index value by the number of grams of carbohydrates in a regular, human portion, then dividing by 100."
Olivier notes that carrots have a GI of 49, which puts it in the middle of the scale (100 is the max), but a GL of only 2, which puts it at the low end. This is atypical — most foods with low GIs have low GLs — but it illustrates one reason glycemic load (GL) is important. Another food in this category is watermelon, which has a GI of 72 but a GL of 4.
"The sugars in a watermelon get absorbed quickly, so the GI is high, but watermelon contains a lot of water so the actual amount of sugar consumed when eating watermelon is relatively low, even if you ate a lot of it," emails nutritionist Hartje Andresen. On the other hand, "there are also some foods with a low GI that could be eaten in very large amounts, like pasta," says Brand-Miller.
Low-GI foods clock in at 55 or below. Moderate GI foods fall in the range of 56 and 69. High GI foods, like white bread, are between 70 and 100, causing rapid blood sugar fluctuations. A GL of 10 or less is low, a GL of 11 to 19 is medium, and a GL of 20 or more is high.
What Really Matters
Although both GI and GL are tools to guide people down a blood sugar-friendly path, some experts say not to obsess too much over the numbers. "The GI of a product changes whether it is raw, cooked, has been frozen, very fresh or stored for a while. The GL is also not 100 percent reliable since the actual effect on a person's blood sugar may also be influenced by the individual's metabolism, insulin response and the amount of fat and protein consumed in the same meal," says nutritionist Andresen.
She tells her clients wanting to lose weight to select a healthy variety of vegetables and fruits, "pick sensible portion sizes, choose whole grains over simple sugars, have an adequate amount of fiber in your diet and avoid overly processed foods."
"Studies show that the total amount of carbohydrate in food is a stronger predictor of blood glucose response than the GI," says registered dietician Rene Ficek of Seattle Sutton's Healthy Eating via email. "For most people with diabetes, the first tool for managing blood glucose is some type of carbohydrate counting. Balancing total carbohydrate intake with physical activity and diabetes pills or insulin (if needed) is key to managing blood glucose levels."