Glycemic Index is Only Part of the Story Christina Badaracco, MPH, RD
Updated: Nov 14, 2019
In the health centers at the hospital where I work, I have met many patients with prediabetes or diabetes who are in search of dietary advice. They have been referred to a dietitian by their primary care doctor to improve their blood sugar control through a healthy diet and lifestyle. My role involves introducing them to the health benefits of following a diet based on foods with a low glycemic index (GI). What is the glycemic index? The glycemic index was first developed by Dr. David Jenkins at the University of Toronto in the 1980s to quantify, by means of a ranked score, the effect of eating carbohydrates on blood sugar. Foods’ indices range from 1 to 100, with pure glucose earning a perfect score of 100. The following figure shows a ranking of the GIs of many commonly eaten foods.
To determine each food's index, a sample of healthy people consumed a serving of the food and researchers measured the change in blood glucose via a measurement known as the area under the curve (AUC). The researchers charted the change, showing the rise in glucose over time. The figure below shows two curves: the blue line shows low-glycemic foods and the red line charts high-glycemic foods. The University of Sydney hosts a searchable database of the glycemic index and glycemic load (GL) of many different foods.
Simple carbohydrates, starchy vegetables and most sweet fruits top the list of foods with high glycemic indices. After eating these foods, the concentration of glucose in your blood increases quickly and reaches a higher peak because these foods are the simplest to break down from complex networks of simple sugars into pure glucose.
When eaten regularly, these high-glycemic foods can cause you to have chronically high blood glucose. This leads to insulin resistance and inflammation. In addition to type 2 diabetes, these conditions increase the risk for myriad other chronic conditions like atherosclerosis (i.e., clogging of the arteries), or cancer, when inflammatory molecules create free radicals that damage cells’ DNA and promote tumor formation. Beans, nuts, yogurt and green vegetables dominate the low-glycemic foods at the bottom of the list. These foods have fiber and fat in addition to the starch that is broken down into glucose. With the addition of fiber and fat, glucose is released more slowly into the blood following digestion. Your pancreas can produce enough insulin to keep up with this release, keeping your blood sugar low and your metabolism running smoothly. What factors determine glycemic load and index? Unlike the glycemic index, the glycemic load also accounts for differences in portion size. Consuming an entire bowl of white rice will raise your blood glucose much more than just a spoonful. Calculating a GL involves multiplying the glycemic index of a certain type of food by the grams of carbohydrate eaten.
Glycemic Load = (Glycemic Index X Grams of Carbohydrate Per Serving) 100
Thus, the glycemic load of a three-cup serving of pasta is much higher than that of a more modest half-cup serving. A score above 20 is considered high and a score below 10 is considered low. An example of a carbohydrate portion with a low glycemic load would be two cups of popcorn, which has an index of 55 but a load of just 5.5 since the serving size includes so much air and is not a dense source of carbohydrates (with just 10 grams). The glycemic load equation for popcorn will look like this: GL= 55 X 10/100 = 5.5
As you might expect, the glycemic load (and index) rise as food becomes more processed. So, white rice has a higher glycemic index and load than does whole grain brown rice. Flour has a higher load than whole grain wheat kernels. Riper fruits have higher loads than less-ripe fruits and grains that are cooked for less time have a lower load. Starches that are cooked and then cooled, such as pasta left over for the next day, also have a lower load because they develop into what is called “resistant starch.” In these foods, the molecules comprising the starch have reconfigured and become less readily digestible.
How useful is this ranking? Scientific evidence regarding the benefits of a diet with low GI/GL foods or the risks of a diet with high GI/GL foods are mixed. Recent evidence suggests that, for the general population, meal planning shouldn’t focus too much on these criteria. A 2015 study in Cell by Dr. David Zeevi and colleagues found that a high glycemic index food doesn’t predictably raise people’s blood glucose after eating. The response appears to vary widely by individual. Variability in factors such as time since last sleep and activity level also contributed to subjects’ responses. The authors, therefore, concluded that it is not appropriate to form general dietary recommendations based on these criteria. Also, the inconsistency in food quality and other meal contents introduce a great amount of variability that can’t be controlled in a research study. Additionally, the method of cooking affects a food’s glycemic load; for example, a baked sweet potato has a much higher glycemic load than does a boiled sweet potato because the latter has developed less simple sugar in the cooking process.
Interestingly, the authors also found that an individual’s microbiome (i.e., gut bacteria) had a significant impact on the individual blood sugar response to food. See this SOULFUL Insight, “The Trillions of Mouths You Feed Each Day”by Dr. Erica Sonnenburg, for more information about the role of the microbiome in health outcomes.
So how does this affect what I eat? For people with normal glucose control and low diabetes risk, recent evidence suggests that monitoring the glycemic load of meals is not a top concern for nutrition. Of course, it is still beneficial for everyone to:
Choose whole grains over refined grains,
Avoid processed foods and refined sugar, and
Include many fiber-rich fruits and vegetables at mealtime.
Patients with diabetes or at high risk of diabetes (due to family history, obesity, etc.) may benefit from focusing on meals with a lower glycemic load by:
Eating foods that are less refined with a low GI;
Cooking pasta al dente;
Choosing whole grains, beans, green and root vegetables, and temperate fruits (like berries and apples) rather than tropical fruits (like bananas and pineapple); and
Adding vinegar or lemon juice to your carbohydrates, since the presence of acid slows gastric emptying—thereby reducing your glycemic response.
Many registered dietitians (RDs), as well as the American Diabetes Association, suggest this additional monitoring is not as useful as carbohydrate counting and consuming small portions of carbohydrates throughout the day to keep blood sugar under control. This means eating snacks with a balance of protein and carbohydrate (such as an apple with peanut butter) and having a balanced plate with protein, fat, and fruits and vegetables at mealtime. Harvard’s School of Public Health has produced a Healthy Eating Plate (seen above) as a guide and you can speak with a dietitian for more guidance.
Carbohydrates and Blood Sugar. The Nutrition Source. Harvard Chan School of Public Health. Accessed March 15, 2018. hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/.GI Foods Advanced Search. The University of Sydney. Accessed March 31, 2018. glycemicindex.com/foodSearch.php.Glycemic Index and Diabetes. American Diabetes Association. Accessed March 15, 2018. diabetes.org/food-and-fitness/food/what-can-i-eat/understanding-carbohydrates/glycemic-index-and-diabetes.html.Glycemic Index and Glycemic Load. Linus Pauling Institute. Oregon State University. Accessed March 15, 2018. http://bit.ly/2BtOuqK.Kreiger, Ellie. "Some popular diets are based on this carb-rating scale. Here’s why it could be misleading." 8 March 2016. The Washington Post. Accessed November 24, 2018. wapo.st/2Aje2oGZeevi, David et al. "Personalized Nutrition by Prediction of Glycemic Responses." Cell. 2015; 163 (5), 1079—1094. cell.com/cell/fulltext/S0092-8674(15)01481-6
Christina Badaracco, MPH, RD
Christina is a registered dietitian and author who aims to improve access to healthy and sustainable food and educate Americans about the connections between food and health. She loves to experiment with healthy recipes in the kitchen and share her creations to inspire others to cook. Christina completed her dietetic internship at Massachusetts General Hospital and earned her Master of Public Health degree from the University of California, Berkeley. Previously, she graduated with a degree in Ecology and Evolutionary Biology from Princeton University, after conducting her thesis on sustainable agriculture and energy in Kenya. She has done clinical nutrition research at the National Institutes of Health, menu planning and nutrition education at the Oakland Unified School District and communications at the Environmental Protection Agency’s Office of Water. She has also enjoyed contributing to children’s gardens, farmers’ markets and a number of organic farms.
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