A Primer on Glycemic Index

Interest in the glycemic index (GI) has continued to increase over the years, as has the number of claims regarding its effectiveness, yet research evaluating its potential role in the prevention or treatment of certain health conditions is still evolving. Consumers may wish to follow a low-GI diet or seek out a list of low-GI foods, but variations exist within food categories, among countries and within research. An individual’s response to foods also can differ, and factors such as how the food is prepared can additionally impact the GI.

The notion of a GI was first introduced in the early 1980s and is essentially a ranking of foods based on their effect on blood glucose over a two-hour time frame after a standard amount is consumed and compared to the same amount of a control food, which is usually glucose or white bread. While GI corresponds to individual foods, the glycemic load (GL) takes into account the GI of the food and the amount of carbohydrate consumed. Therefore, if a food with a low-GI was consumed in a large amount, the GL would reflect both the quality and quantity of the carbohydrate in terms of their combined impact on blood glucose.

A protocol for determining GI, along with thresholds for classifying foods as being high (>70), medium (56-69) or low (<55) was published in 2010 by the International Organization for Standardization, or ISO. This concerted effort served to establish consistency in how GI is measured in research and intended to aid in the regulation of food labeling claims for countries that are members of ISO. While the U.S. currently lacks a formal definition for voluntary claims regarding GI, its use in food labeling in other parts of the world ranges from not being permitted in Canada and the European Union to being regulated as a nutrient content claim or symbol in Australia and New Zealand, according to their joint Food Standards Code.

In 2021, researchers updated the International Tables of Glycemic Index and Glycemic Load, which were originally established in 1995. The current version includes more than 4,000 foods, approximately half of which are consistent with the ISO methodology. According to the authors, considerable differences exist in some food categories due to variations in manufacturing practices, and many foods are available in low-, medium- or high-GI versions.

These advances in measuring and categorizing foods based on the GI should aid in further research and with defining a low-GI/low-GL diet, which is often not described in the literature. According to the Dietary Approaches and Health Outcomes: An Evidence Analysis Center Scoping Review, a majority of the systematic reviews (published between 2017 and 2020) did not define a low GI/GL diet when comparing this type of eating pattern to any other diet and their effects on cardiometabolic disease and related risk factors.

A Cochrane review published in 2023, which included research from January 2006 through May 2022 and interventions that lasted a minimum of eight weeks, indicated daily amounts of 45 or less and 100 or less for a low-GI and low-GL diet, respectively, as criteria. According to this review, there was very little difference, if any, when comparing low-GI/GL diets to other diets, including higher GI/GL diets, in terms of changes in body weight or body mass index for individuals with overweight or obesity. This finding applied to all outcomes, (i.e., anthropometric measures, glycemic control and cardiovascular risk factors), only the certainty of evidence varied as being either low or moderate.

The American Diabetes Association's Standards of Medical Care in Diabetes — 2023 describes research involving GI and GL in people with diabetes as being "complex" and acknowledges that different definitions for high- and low-GI foods exist. In terms of their effect on fasting blood glucose levels and hemoglobin A1c, the results have been inconsistent thus far, according to this publication. The Position of the Academy of Nutrition and Dietetics: The Role of Medical Nutrition Therapy and Registered Dietitian Nutritionists in the Prevention and Treatment of Prediabetes and Type 2 Diabetes recognizes GI as one of the possible strategies for managing carbohydrate intake when providing education during a nutrition intervention.

Several dietary patterns may be considered to aid in weight loss for adults, including an eating plan which features low-GI foods, along with <30% of calories from fat, and higher amounts of dairy and dietary fiber, as indicated in Medical Nutrition Therapy Interventions Provided by Dietitians for Adult Overweight and Obesity Management: An Academy of Nutrition and Dietetics Evidence-Based Practice Guideline. Reducing glycemic load is one of several dietary approaches that may be considered in the treatment of pediatric patients with overweight and obesity, according to Treatment of Pediatric Overweight and Obesity: Position of the Academy of Nutrition and Dietetics Based on an Umbrella Review of Systematic Reviews.

The efficacy of a low-GI diet may come up when working with patients and clients who are pursuing weight loss or wanting to manage or prevent Type 2 diabetes. Registered dietitian nutritionists can play an integral role in translating the current evidence and how it relates to an individual patient or client, while taking into account their unique needs. Evaluating the advantages and disadvantages of any current or future eating plan, especially in terms of its nutritional adequacy, can help individuals make informed decisions, while honoring their food preferences and achieving their health goals.

References:

• ElSayed NA, Aleppo G, Aroda VR, et al., American Diabetes Association. 5. Facilitating positive health behaviors and well-being to improve health outcomes: Standards of Care in Diabetes—2023. Diabetes Care. 2023;46(Suppl. 1):S68–S96.
• Atkinson FS, Brand-Miller JC, Foster-Powell K, Buyken AE, Goletzke J. International tables of glycemic index and glycemic load values 2021: a systematic review. Am J Clin Nutr. 2021;114(5):1625-1632. doi:10.1093/ajcn/nqab233
• Barclay AW, Augustin LSA, Brighenti F, et al. Dietary Glycaemic Index Labelling: A Global Perspective. Nutrients. 2021;13(9):3244. Published 2021 Sep 17. doi:10.3390/nu13093244
• Handu D, Piemonte T. Dietary Approaches and Health Outcomes: An Evidence Analysis Center Scoping Review. J Acad Nutr Diet. 2022;122(7):1375-1393.e9. doi:10.1016/j.jand.2021.08.106
• Chekima K, Yan SW, Lee SWH, et al. Low glycaemic index or low glycaemic load diets for people with overweight or obesity. Cochrane Database Syst Rev. 2023;6:CD005105. Published 2023 Jun 22. doi:10.1002/14651858.CD005105.pub3
• Briggs Early K, Stanley K. Position of the Academy of Nutrition and Dietetics: The Role of Medical Nutrition Therapy and Registered Dietitian Nutritionists in the Prevention and Treatment of Prediabetes and Type 2 Diabetes. J Acad Nutr Diet. 2018 Feb;118(2):343-353. doi: 10.1016/j.jand.2017.11.021. PMID: 29389511.
• Morgan-Bathke M, Raynor HA, Baxter SD, Halliday TM, Lynch A, Malik N, Garay JL, Rozga M. Medical Nutrition Therapy Interventions Provided by Dietitians for Adult Overweight and Obesity Management: An Academy of Nutrition and Dietetics Evidence-Based Practice Guideline. J Acad Nutr Diet. 2023 Mar;123(3):520-545.e10. doi: 10.1016/j.jand.2022.11.014. Epub 2022 Dec 1. PMID: 36462613.
• Kirk S, Ogata B, Wichert E, Handu D, Rozga M. Treatment of Pediatric Overweight and Obesity: Position of the Academy of Nutrition and Dietetics Based on an Umbrella Review of Systematic Reviews. J Acad Nutr Diet. 2022;122(4):848-861. Doi:10.1016/j.jand.2022.01.008