My publisher wants a glycemic index (GI) chart for our upcoming book on smart glucose monitoring. I’m looking into GI and its sister, glycemic load (GL), but so many things can change GI and GL. How do you use these tools?
As Jacquie Craig, MS, RD, LD, CDE explained in an article on this site,
GI ranks carbohydrates from 0–100, based on how much and how fast they affect blood glucose levels. A higher number means the food has a larger impact on blood glucose levels. Glucose itself has a ranking of 100.
GI is important for people with Type 2 diabetes, because they often have a delayed insulin response. If glucose goes up fast, the body does not respond quickly enough, and glucose levels can get way too high after meals. So we want low-GI foods.
GI doesn’t tell you how much glucose will eventually get into your system, just how much of a blood glucose spike the food creates. To improve the GI, Dr. Walter Willett at Harvard helped develop the concept of glycemic load (GL). GL combines the GI with a measure of how much carbohydrate there is in a food.
So in theory, GL can tell you a given food’s total impact on your blood glucose levels, which should help in meal planning, insulin dosing, and food choices. But the reality of GI and GL is much more complicated.
How Are GI and GL Determined?
You can’t tell the GI or GL of a food by analyzing it in a lab. That’s because different people’s digestive systems handle different carbs differently. We also absorb and break down the same carbs into glucose differently at different times, depending on what other nutrients are being consumed.
For example, a plain pizza with tomato sauce and Parmesan cheese has very high GI, around 80. But a super-deluxe pizza with all the fixings has a low GI of 36. The GI is lower because the protein and fats in the toppings slow down the absorption of carbs and slows their turning into glucose.
Since you can’t tell GI or GL in a test tube, glycemic index values are determined by feeding 8 to 10 people a fixed portion of the food (after an overnight fast). Then samples of their blood are taken every 15–30 minutes and their glucose levels are measured. The GI values that the foods register in the participants are averaged to give a GI number.
The GL is calculated from the GI using the formula GL = (GI × Net Carbs) ÷ 100. (Net carbs are equal to the total carbohydrates minus dietary fiber.)
So if a plain pizza has a GI of 80, and 27 grams (about one ounce) of net carbs in 100 grams, its GL would be 80 multiplied by 27, divided by 100, for a total of 22. For GL, 20 and over is considered high, and 10 or below is considered low. Levels in the range of 11–19 are considered medium.
For GI, anything below 55 is low, and anything over 70 is considered high. Numbers from 55–69 are medium.
The same foods can have a very different GI and GL depending on how they are prepared. A boiled sweet potato has a low GI of 44 and a medium GL of 11. But if baked for 45 minutes, the same sweet potato has a GI of 94 and a GL of 42, both extremely high. Baking has essentially turned the sweet potato into candy.
White potatoes also have a higher GI and GL when baked. Microwaving often raises GI and GL. Have you ever noticed how sweet beets taste after baking or microwaving? That’s because much of their carbohydrate content has been converted into glucose.
Even the same cooking method can give different results. Spaghetti cooked al dente (boiled for 8 minutes) has a much lower GI and GL than soft (boiled for 20 minutes) spaghetti.
Different brands or varieties of the same food can have very different GI and GLs. Professor Jennie Brand-Miller of the University of Sydney, Australia, has created a list of thousands of foods based on her tests and the published results of others. Her list includes two types of rice milk, one with a GI of 92, another of 79. Sweet corn from New Zealand has a GI of roughly 37, while South African sweet corn gets about a 62.
Different People, Different GI/GL
Even if you could find the published GI or GL of a specific food, you couldn’t be sure of that food’s effect on your personal glucose numbers. People vary significantly in their response to foods. And in real life, foods are rarely consumed one at a time. We have drinks and other foods with them, which can affect their glucose response in the body.
It seems the only way to be sure about a particular food’s effect on you is to check your glucose after eating it. Preferably check before, too, so you can record how much change there has been.
Do you look at GI and GL numbers or think about them in meal planning or insulin dosing? What information sources do you use, and how do you use that information? Thanks in advance for any help you can give.