“Glycemic Index” and “Glycemic Load” have become buzzwords for the carbohydrate-conscious public, but the burning question remains: What exactly is Glycemic Index? Is it worth learning in an effort to control your blood sugar? How is it even different than counting carbs?
Counting carbohydrates will tell you just how many grams of carbs you are getting with each meal, but not all carbs break down in our body the same; some are made of longer-chain molecules or are higher in fiber and take longer for your body to break down. Simpler sugars tend to be digested and absorbed much more quickly, which leads to a more rapid blood sugar spike. Carbohydrate counting along does not provide that kind of information, but Glycemic Index does.
Glycemic Index (GI) measures how a 50 gram serving of a specific food will impact your blood sugar overall. It takes into account both how high your blood sugar will spike as well as how how long it takes for it to return to normal, and it summarizes this overall glycemic effect as a number on a scale from 0 to 100. Pure glucose has a GI score of 100, as a reference point, so lower scores mean a lower blood sugar response:
Low GI: 55 or less
Medium GI: 56-69
High GI: 70 or higher
The mortal flaw of GI, however, is that is is so incredibly variable that it is difficult to actually trust any specific number. Just take a look at the extensive table (over 55 pages long) found in the journal Diabetes Cares, which lists GI data for items based on a huge range of sources. Brown rice is listed multiple times with the GI ranging from 50 to 87— which obviously complicates the idea that we can quantify exactly how your blood sugar will react to 50 grams of brown rice. Tufts University recently studied glycemic response within individuals by measuring people’s unique blood sugar response to doses of glucose and white bread, and they found that a person’s blood sugar response varied by 20-25% at different times. This study is important in showing that Glycemic Index isn’t as precise as we’d like, though it provides a good ballpark for you.
Reference this abbreviated table from Harvard University which is based on an American Diabetes Association article that looked at GI as an average of multiple valid studies – this average is likely the most accurate you’ll find.
One major issue is that eating a large volume of a low-GI item can be worse than eating a little bit of a high-GI food. This is because GI measures the impact of 50 grams of an item, so you can only compare the GI of (for example) table sugar (GI 60) and watermelon (GI 72) if you assume you are eating 50 grams of each. Hopefully, you don’t actually eat the same amount of straight sugar and watermelon, so the GI comparison is deceitful. This is where Glycemic Load (GL) comes into play. The GL applies GI concepts to actual realistic portion sizes to help you gauge how your intake will really impact your blood sugar, so you can see that the glycemic load of ~2 teaspoons of sugar (approximately 6) is higher than that of a 4 ounce portion of watermelon (GL of approximately 4).
Again, all Glycemic Load data is based on Glycemic Index, so you need to keep some flexibility in mind when using this information for food choice.
Glycemic Index also varies based on a number of different food qualities; the GI will be higher in foods that are riper, cooked longer, or processed into a finer / easier to digest product (like quick oats as opposed to rolled oats). Even once you have a good estimate of an accurate GI score, the combination of foods in a meal will change how your body responds because fat, protein, and fiber all slow digestion and therefore blood sugar spikes.
So what’s the verdict?
There is mixed research about Glycemic Index (GI) and Glycemic Load (GL) in terms of how helpful it can be for actual blood sugar control. Some studies have found no link to improved blood sugar at all, but a Cochrane Review of glycemic index examined the literature and found that lower-GI diets are linked to better blood sugar control when you consider all of the data together.
My takeaway is that understanding the glycemic impact of food can be a helpful reference tool but is too variable to rely on entirely. It may help you understand how different types of foods might impact your blood sugar, and it’s great for comparing similar items and to see which types of foods lead to a better glycemic response. The result will likely be that you’ll see the benefit of foods that are higher in fiber, less processed, lower in simple sugars, and higher in protein – which is the same generally healthy carbohydrate-consistent diet I would recommend in the first place.
Here is a summary of some major Glycemic Index numbers to consider.
Remember, this is Glycemic Index, so it does not take portion size into account, and these numbers have a big error range so assume they can be about 10 points higher or lower. This information tells you how these items MIGHT impact your blood sugar ON AVERAGE if you consumed IDENTICAL PORTIONS of each. Use this more for reference between similar items with a fixed portion. Should you necessarily avoid high GI foods like watermelon and potatoes all together? No! Just monitor your portion sizes and eat them with higher protein / fiber / fat foods to help slow digestion.
- High GI: 70 or higher
- Corn flakes (~81)
- Puffed rice cereal (~82)
- Instant potatoes (~80)
- Instant oats (~79)
- Potato, boiled (~78)
- Watermelon (~76)
- White bread (~75)
- Cheerios (~74)
- White Rice (~73)
- Bagel (~72)
- Medium GI: 56-69
- Brown Rice (~68)
- Popcorn (~65)
- Table sugar (~65)
- Sweet potato (~63)
- Honey (~61)
- Pineapple (~59)
- Muesli (~57)
- Plantain (~55)
- Low GI: 55 or less
- Sourdough bread (~54)
- Rice noodles (~53)
- Stone ground or pumpernickel bread (<55 per the American Diabetes Association)
- Rolled oats (~55)
- Corn (~52)
- Banana (~51)
- Spaghetti (~49)
- Corn tortilla (~46)
- Canned peaches (~43)
- Oranges (~43)
- Grapes (~43)
- Fruited Yogurt (~51)
- Milk (~39)
- Apple (~36)
- Lentils (~32)
- Chickpeas (~29)
- Under 20: Eggplant, broccoli, raw carrots, cauliflower, mushrooms, lettuce, red peppers, onions, tomatoes
There are several Glycemic Index apps to help you keep this information in your pocket if you’re interested in learning more, though keep in mind that the numbers will always be estimates and you may find very different guidelines based on what the app references.
I also recommend the following resources and articles:
- The American Diabetes Association website
- The Glycemic Index Foundation website
- Harvard Medical School article
If you’d like to pursue more personalized counseling, contact us for information about in-person or virtual sessions!
Share your thoughts on Glycemic Index below – I’d love to hear your experience with this, whether you found it helpful, confusing, interesting, too restrictive, or otherwise!