Debunking the Glycemic Index — Registered Dietitian Columbia SC

Last week I listened to a recent episode of Maintenance Phase podcast where they debunked common sugar myths, and it reminded me that I’ve been meaning to do a blog post of my own debunking the glycemic index. As a nutrition student in a very traditional dietetics program, there was a lot that I learned that I’ve since had to unlearn (ahem, pretty much everything about ob*sity). But occasionally there’s something that’s part of mainstream nutrition discourse that even 15 years ago when I was in school we learned was totally bunk. Glycemic index is one of those topics! Even though it’s something that makes intuitive sense, per usual, nutrition and our individual biology is much more complex than some chart of what to eat and what to avoid.

What is the Glycemic Index?

The Glycemic Index (GI) was a tool that was originally created in the 1980s by a researcher named David Jenkins to provide guidance for choosing foods that have less of an effect on blood sugar levels. Jenkins developed the glycemic index by having research subjects consume 50 grams worth of carbohydrates from specific foods, measuring their blood sugar over the next two hours, and comparing the average results to the effect of pure glucose, which was used to rank the glycemic effect on a scale of 0-100. Foods less than 55 are considered low GI, moderate foods are 56-69 and high GI foods are above 70.

Since its development, the Glycemic Index has become a tools that is commonly used to provide guidance for people with diabetes. It’s also used as part of weight loss diets, or as a weight loss diet in of itself. While the Glycemic Index might seem to make sense at first glance – we do know that different foods can have more or less of an effect on blood sugar – there’s a TON of problems both with how the Glycemic Index was developed, and how it is applied. So let’s debunk the Glycemic Index!

Debunking the Glycemic Index

The Glycemic Index is based on a faulty study design.

The put it bluntly, the study design behind the Glycemic Index was a hot mess. For starters, there was an extremely small sample size, with each food tested on only 5-10 subjects. Because there is quite a bit of variability in glycemic effect among individuals, it’s easy for the GI of a food to be thrown off by someone who is either extremely reactive or non-reactive to a food.

Another problem with the study was that in order to separate the effect of the specific food versus the quantity of carbohydrate consumed, participants ate 50 grams worth of carbohydrate for each food. On the surface, this might sound like strong study design, but this meant that participants were consuming quantities of certain foods that were quite different from how one might eat it in real life, especially for vegetables. For example, 50 grams of carbohydrate from vegetables means 10 raw carrots, over 25 cups of cucumber slices, and 10 cups of steamed cauliflower. This may be why vegetables surprisingly had some of the of the highest glycemic effects.

Not only that, but to develop the glycemic index, foods were consumed alone versus in combination with other foods. Most people aren’t sitting down to a meal of plain boiled potatoes, a cup of canned black beans or dry cereal. We eat foods as part of meals and snacks, in combination with other foods that contain fat, protein, and fiber, which help lower the glycemic response. Conversely, other foods that were tested, like ice cream, yogurt, or potato chips, are often consumed on their own as a snack or dessert. These are foods that already contain some amount of fat and/or protein, which helps lower the glycemic response. It’s not to say these foods are “bad” or “good,” only that evaluating them on their own doesn’t necessarily give us a good picture of the real world impact on blood sugar.

The glycemic impact of a food is dependent on many different factors.

A potato is a potato, right? Not when it comes to glycemic index. Is the potato waxy or starchy? Has it been boiled, steamed, roasted, microwaved or fried? Are you eating the potato freshly cooked, or was it cooked and cooled (like in a potato salad), or was it cooked, cooled and reheated? Was the potato mashed, sliced or left whole? How long did the potato sit on the shelf at the grocery store?

Factors like preparation method, ripeness, and processing all impact the glycemic effect of a food. That’s because these processes can break down or transform the starches, fibers, and sugars present in a food. For example, a green-ish, less ripe banana has a glycemic effect that’s about half as much as a fully ripened yellow banana. As a banana ripens, the starches break down into sugars, hence the sweeter flavor of a ripe banana. Another example of how the glycemic index can be altered happens when starchy foods are cooked and cooled, for example boiled noodles that have been refrigerated to make pasta salad. Cooling the noodles transforms some of the starches to resistant starch, which doesn’t break down into sugar during digestion, lowering the glycemic effect.

While understanding how these processes impact blood sugar could certainly be helpful for someone with difficult to manage diabetes, it’s not helpful for the average human trying to feed themselves, especially when their body is already doing a pretty stand up job of metabolizing sugars. It’s also helpful in understanding why the Glycemic Index is an extremely imperfect tool. When I think of all the different factors that can impact the GI of a food, I can’t help but think how overwhelming it would be to keep all this information in mind and make a simple decision about what to eat for dinner!

The Glycemic Index ignores individual variability in how foods impact blood sugar levels.

Even in the original GI study, there was quite a bit of variability among participants in how individual foods impacted their blood sugar. Since then, additional research has shown that different individuals may be more or less sensitive to different foods. I often reference a fascinating study what was done in 2016, looking at the glycemic effect of white bread across a group of 63 individuals. They found that while the average GI for white bread was 62 (placing it in the medium GI category), it could range by 15 points either way across individuals. That meant that for some individuals, white bread was a low GI food and in others it was a high GI food. Even within the same individual, the glycemic effect varied by up to 60 points between trials! This study is a fantastic reminder of how individualized nutrition can be, but also a reminder that other factors outside of food can impact blood sugar levels, like stress levels, lack of sleep, or hydration.

What Can I Do to Manage Blood Sugar?

So what to do if you are concerned about your blood sugar? While the Glycemic Index may not be a very helpful tool, there are other things you can focus on that are thankfully much less stressful than memorizing the GI of thousands of foods! Here’s a few of the things we work on with clients who have diabetes, PCOS, or other conditions that impact blood sugar levels:

• Eat regular meals and snacks throughout the day. Going long periods of time without eating then consuming a larger meal can lead to big fluctuations in blood sugar, while eating regularly helps keep it steady.

• Pair carbohydrate foods with other foods that contain fat and protein. Aim to include the gang at your meals!

• Focus on high fiber carbohydrate sources. While you can see from this post that there’s not a ton of consistency in how foods impact blood sugar levels, we do know that as a general rule of thumb, carbohydrate foods that are higher in fiber tend to be better for blood sugar. Examples include whole grains, like brown rice, whole grain pasta, and and whole grain bread, starchy vegetables, like potatoes, sweetpotatoes, and winter squash, and fruit.

When we work with clients on managing blood sugar levels, we take an individualized approach. No one-size-fits-all advice here! We also try to take the stress out of eating by simplifying nutrition guidance, because eating doesn’t have to be complicated. If you’re interested in getting support with food, read more about our practice philosophy and reach out if you’d like more information about services.

Resources:

• Wikipedia

• Maintenance Phase: The Trouble with Sugar

• Jenkins, D. J., Wolever, T. M., Taylor, R. H., Barker, H., Fielden, H., Baldwin, J. M., Bowling, A. C., Newman, H. C., Jenkins, A. L., & Goff, D. V. (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange. The American journal of clinical nutrition, 34(3), 362–366. https://doi.org/10.1093/ajcn/34.3.362

• Matthan, N.R., Ausman, L.M, Meng, H., Tighiouart, H., and Lichtenstein, A.H., “Estimating the reliability of glycemic index values and potential sources of methodological and biological variability.” American Journal of Clinical Nutrition, 2016. doi:10.3945/ajcn.116.137208