3 Myths Surrounding the Glycemic Index
By Susan M. Kleiner, Ph.D., R.D.
Science moves quickly, but urban legends move with the speed of light. Sometimes the most well-meaning scientists and educators get caught up in the lighting strike (including this author). The glycemic index (GI) is the science, but the following three myths surrounding it are the urban legends that have been caught up in the lightning strike surrounding the science.
Myth #1: The glycemic index is a measure of how fast the carbohydrates from a food, drink or supplement are digested and absorbed after consuming a specific amount.
True or False?
FALSE. The GI is not a measure of how fast a carbohydrate source gets into the blood. Rather, it’s a measure of the total rise in blood sugar over time.
The difference is that nearly everyone (including myself, at one time) made the assumption that a total rise in blood sugar represents speed of entry into the bloodstream. However, making assumptions in science, unless it’s part of your study design, is not a good thing. We should never assume that because we know fact “A,” it automatically translates to fact “B.”
Research conducted by two different laboratories has shown the rate of glucose entry into the bloodstream, called glucose kinetics, for two high-GI carbohydrates can be remarkably different. In fact, a low-GI carbohydrate may possibly enter the bloodstream more rapidly than a high-GI carbohydrate-rich food (Eelderink et al, 2012a; Eelderink et al, 2012b; Schenk et al, 2003).
Which brings us to the next myth…
Myth #2: Simple, refined carbohydrates are fast, and more complex carbohydrates are slow.
True or False?
FALSE. A 2003 study demonstrated that the interpreted concept of what the GI represents is substantially incorrect. In that study, scientists compared the glucose kinetics of a low-GI breakfast cereal (bran cereal) and a high-GI breakfast cereal (corn flakes) that each contained 50 grams of available carbohydrate. The GI of corn flakes was more than twice as high as the GI for bran cereal, despite the fact that there was no significant difference in the rate of appearance of glucose into the bloodstream between the two foods during the 180-minute test. In other words, the rate of entry of glucose into the bloodstream was the same for corn flakes and bran cereal.
However, the appearance of insulin into the bloodstream after the consumption of bran cereal was significantly faster and higher compared to the corn flakes. Insulin release is not part of the GI testing, and it is the job of insulin to remove extra glucose from the bloodstream and transport it to muscle and liver cells. Twenty minutes after consuming bran cereal, participants’ insulin concentration was 76 percent greater, leading to a 31 percent greater rate of disappearance of the glucose from the bloodstream compared to corn flakes. The lower GI of bran cereal is not due to the slower appearance of glucose into the bloodstream, however. On the contrary, the rate of appearance of glucose into the bloodstream between the two foods is the same. The difference is that the glucose from the bran cereal was being removed from the bloodstream more rapidly, and therefore the total rise in glucose concentration was lower compared to the corn flakes.
This is so mind-blowing that I’m going to repeat it with a more visual example. Think of your bloodstream as a bathtub with a faucet pouring water (blood sugar) into the tub and a drain where the water drains out of the tub (insulin concentration). Now imagine two tubs, side by side. The water is pouring out of the faucet into the tub at the identical rate for both tubs. In the first tub, the drain is mostly closed, but in the second tub the drain is wide open. The total rise of the water level in the first tub keeps going up and up as long as the faucet is pouring water in and only a small amount trickles out the drain. In the second tub, the water is draining out rapidly from the bottom at the same time that the water is pouring in from the faucet, and the water level never goes as high. By measuring only the total rise of the water level, you never see that the rate of water entry into the tub is identical, only that the total rise is much higher for the first tub versus the second.
The total rise in the water level (blood glucose) is what the GI actually measures. In the case of this study, the high-fiber bran cereal stimulated a more profound insulin response, draining glucose from the bloodstream more rapidly, creating a lower total blood sugar rise; thus the low GI designation. This has nothing to do with the speed of digestion, absorption or glucose entry into the blood. Glucose from the more refined and simple sugar-sweetened corn flakes entered the bloodstream at the same rate as glucose from the more complex bran cereal.
We cannot translate this data from these studies to all foods, unless we conduct glucose kinetics studies on all foods. However, the examples from these studies completely disprove the assumption that GI represents speed of entry into the bloodstream. Moreover, it is absolutely an untruth that simple carbohydrate ingredients are automatically high-GI foods that are more rapidly absorbed into the bloodstream compared to more complex carbohydrate, low-GI foods.
Which undeniably leads us to…
Myth #3: Choose all your foods and supplements based on GI rating.
True or False?
FALSE. You might think that because a low-GI food was shown in research to stimulate a better insulin response and faster movement of glucose out of the bloodstream and into the cells, that using GI is a smart way to design a healthy diet. Unfortunately, that is jumping the gun yet again. Knowing this data about one food, or even several foods, does not give us license to assume (there’s that word again) that this is what happens with all foods. Additionally, we cannot assign a health benefit to an observed function until it is tested and proven to result in a healthier outcome. When it comes to GI, many tests have actually been done, yet there is no consensus on whether GI truly makes a difference in long-term health outcomes.
The OmniCarb Randomized Clinical Trial is one of the most recent large-scale trials to test the effects of high versus low GI on cardiovascular risk factors and insulin sensitivity (Sacks et al, 2014). Subjects followed either a low- or high-GI DASH-type diet, which is a plant-rich diet plan that has been shown to be very healthy in multiple international studies, for five weeks. After five weeks, they had a “washout” period and then crossed over to the opposite GI plan. There were no significant differences or improvements in insulin sensitivity, lipid levels or systolic blood pressure levels between the subjects on the different GI diets. In this study, following a low-GI diet did not result in a healthier cardiovascular risk or insulin sensitivity clinical outcome.
There are many claims that the GI is a factor in successful weight loss. While many diet approaches have been shown to be helpful in facilitating weight loss, there are very few dietary plans proven to support weight-loss maintenance. In a multicenter trial, researchers tested the effects of dietary composition on energy expenditure during weight-loss maintenance (Ebbeling et al., 2014). Three diet styles were tested—low-carbohydrate, low-GI and low-fat—in a four-week crossover design with a washout period in between each diet.
The low-carbohydrate diet created the highest resting energy expenditure (REE) and total energy expenditure (TEE), but also resulted in the greatest release of the stress-related hormones and markers, cortisol and C-reactive protein (c-RP). The low-fat diet led to the lowest REE and TEE output, but also the lowest cortisol release with a moderate c-RP measurement. In this study, the low-GI diet may be the better choice for a maintenance plan, resulting in only a moderate drop in REE and TEE, and a moderate response of cortisol and c-RP.
When it comes to athletic performance, there is absolutely no data to support that any specific GI translates to enhanced performance. Any claim regarding the effects of certain foods, diets, beverages or supplements on athletic performance should be backed up by scientific research data—not simply the GI of the food or product. And remember: A muscle biopsy study is required to show that carbohydrate is entering the muscle cell for fueling and recovery. Therefore, only when solid, well-designed research has demonstrated an association between a food or product and enhanced performance can a proven claim be made about the food or product.
With what you now understand about the variability of foods, their transport in the body, and their GI, the only way you can know if something enhances performances is to see data that it actually enhances performance. If manufacturers are talking about speed of digestion and absorption, and transport to the muscle cell, you now know that you need to see that data from the testing of that product, not the GI of the product.
Fast Facts About the Glycemic Index
- The GI is not a measure of speed. It takes a glucose kinetics study to know the speed of the entry of carbohydrate into the bloodstream.
- It takes a muscle biopsy study to show that carbohydrate is entering the muscle cell for fueling and recovery.
- Absorption of simple, processed carbohydrates versus complex carbohydrates may be similar.
- Using only the GI to design a health- or performance-based food plan is not an evidence-based strategy.
- Put the focus on food, not an abstract set of numbers surrounded by misunderstanding and mythology.
If Not the Glycemic Index, Then What?
The GI by itself is not a bad thing. It was designed as an experiment to see if the data could actually provide dietary guidance and, in some cases, it may. The unveiling of these myths doesn’t take away from what the GI actually represents, but it illuminates the fact that we are giving the relatively abstract number of the GI more dietary credence than it deserves.
Moreover, the packaged food industry has presented the GI as a badge of goodness or healthiness for foods that are highly processed and engineered. While processing and engineering of foods is not inherently bad or unhealthy, a diet abundant in these foods is generally higher in salt, sugar and disease-promoting fats, and lower in micronutrients, fibers, phytochemicals and other food factors that are widely known to promote health. The general public’s misunderstanding of the three GI myths outlined here, combined with the labeling of packaged foods with a GI rating, has led to a misplaced notion of the contents of a healthy diet. Because fresh foods like produce, meats, breads and dairy do not typically have GI ratings attached to them at the store, the consumer may place a higher health value on packaged foods because the GI rating is clearly labeled. The intent of the labeling is that “low GI is healthy,” regardless of the ingredients in the package.
My recommendation is to teach your clients about food, not the GI, which has not held up to the assumptions first mae about it. It is an abstract set of numbers that do not necessarily relate to the factors that we had hoped it would. Instead, put the focus on food—as fresh as possible, including a variety of foods from all the food groups, with the purpose of fueling your body and feeding your mind.
And don’t forget to always look for the data.
Ebbeling, C.B. et al. (2012). Effects of dietary composition on energy expenditure during weight-loss maintenance. Journal of the American Medical Association, 307, 24, 2627-2634.
Eelderink, C. et al. (2012a). The glycemic response does not reflect the in vivo starch digestibility of fiber-rich wheat products in healthy men. Journal of Nutrition, 142, 2, 258-263.
Eelderink, C. et al. (2012b). Slowly and rapidly digestible starch foods can elicit a similar glycemic response because of differential tissue glucose uptake in healthy men. American Journal of Clinical Nutrition, 96, 5, 1017-1024.
Sacks, F.M. et al. (2014). Effects of high vs. low glycemic index of dietary carbohydrate on cardiovascular disease risk factors and insulin sensitivity: The OmniCarb Randomized Clinical Trial. Journal of the American Medical Association, 312, 23, 2531-2541.
Schenk, S. et al. (2003). Different glycemic indexes of breakfast cereals are not due to glucose entry into blood but to glucose removal by tissue. American Journal of Clinical Nutrition, 78, 4, 742-748.
Dr. Susan M. Kleiner is a renowned authority on eating for strength, endurance, power and speed, and the co-founder and co-CEO of Vynna®, LLC, an evidence-based, female-centric performance nutrition brand. She is an international columnist and speaker, and has consulted with professional teams, Olympians and elite athletes in all sports. Dr. Kleiner has been a faculty member at several esteemed universities, authored numerous academic chapters, articles and scientific manuscripts, and penned seven books, including the bestselling POWER EATING, 4th Edition (Human Kinetics, 2014).