Understanding the Glycemic Index

By Dr. Priyom Bose, Ph.D.Reviewed by Danielle Ellis, B.Sc.

The concept and acceptance of GI
How are foods ranked on the GI scale?
Limitations of GI
Clinical applications of GI
References
Further reading

The glycemic index (GI) is a nutritional tool that helps measure how quickly the carbohydrates in a particular food affect an individual’s blood sugar levels.¹ As per the GI values, foods are classified as low, medium, or high.

A low GI food induces a lower glucose response, in contrast to food with a high GI score. A diabetic individual can manage their blood sugar levels by minimizing high GI food consumption and increasing low GI food intake.²

The concept and acceptance of GI

In 1981, David Jenkins and his research team at the University of Toronto introduced and developed the concept of GI.3 This concept stemmed from the fiber hypothesis, which suggested that a higher fiber intake decreased the nutrient influx rate from the gut. GI is a ranking system of carbohydrate foods based on their ability to increase plasma glucose levels.  

Many countries, such as Sweden, France, and Australia, have incorporated GI into their dietary recommendations. The importance of GI is also supported by many international organizations, including the World Health Organization (WHO), the American Diabetes Association, the Food and Agriculture Organization of the United Nations (FAO), Diabetes UK, and the Canadian Diabetes Association.⁴

How are foods ranked on the GI scale?

GI provides a standardized comparison of carbohydrate levels two hours postprandial glucose response with that of white bread or glucose.⁵

The GI system ranks carbohydrates on a scale of 1 to 100 based on how much they raise blood sugar levels. A low GI food score is 55 or less (e.g., apples, chocolate, dates, and yogurt); a medium score is between 56 and 69 (e.g., brown rice, sweet potato, and muesli), and a high score is between 70 and 100.⁶

Processed foods (e.g., cake, cookies, candy, and bread) have a high GI, while whole foods that include non-starchy vegetables, unrefined grains, and fruits tend to have a lower GI. Proteins and fats are not included in GI based food list as they have minimal impact on blood glucose levels.

Low-GI foods are digested easily and are absorbed and metabolized more slowly than high-GI foods. Furthermore, low GI food causes a gradual and slow-paced increase in blood glucose levels.⁷

Limitations of GI

GI only indicates glucose response two hours after a meal and not beyond that. Many studies have shown that in individuals diagnosed with diabetes, blood glucose levels may persistently rise for up to four hours after a meal. Therefore, while planning meals based on GI, it is important to consider its limitations as well.⁸

The GI score only indicates the relative rise in plasma glucose levels after meals and does not consider insulin responses, which are measured via the insulin index. GI fails to consider the fact that the glycemic response to a particular food varies in accordance with an individual’s metabolism. A significant variation has also been observed at different periods of the day.

It must be noted that GI is affected by multiple factors, including the amount of fiber and fat content, type of sugar, cooking method, and food processing.⁹ Studies have shown that the extent of fruit maturity also affects GI.

For example, a fully ripe banana has a higher GI than an unripe banana. Generally, hyperglycemia, or high plasma glucose levels, occurs after the consumption of high-GI food, while hypoglycemia, or decreased blood glucose levels, occurs following a high-GI meal.

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Clinical applications of GI

The numerical classification of carbohydrate foods on consumption has been extremely useful for people with impaired glucose tolerance. Blood sugar management is important for many reasons.  For instance, a blood sugar crash makes one feel lethargic and hungry, which promotes increased eating, leading to obesity. An individual who experience regular blood sugar spikes and crashes are at a higher risk of chronic inflammation and metabolic illness (e.g., type 2 diabetes).¹⁰

Athletes benefit from eating low-GI meals before an event because it promotes a slower increase in insulin and glucose levels. This could help maintain carbohydrate availability during the last stages of exercise/game, which in turn improves exercise performance.

There are many short- and long-term benefits to maintaining a low GI diet. For instance, a study concerning younger adults observed that adherence to a low GI food for two weeks improved the overall glucose levels and reduced low-density lipid (LDL)-cholesterol and triacylglycerol concentrations. However, an increase in glucose concentration at 45 and 60 minutes has been attributed to the lower C-peptide response due to modified gut adaptive responses that led to a reduced incretin secretion.

When middle-aged individuals, particularly insulin-resistant women who experienced myocardial infarction, were subjected to low GI food, an overall improvement in insulin sensitivity was observed. These improvements were associated with higher synthesis of glycated proteins and reduced levels of plasminogen activator inhibitor 1.

Recent research highlighted the importance of GI in preventing cancer onset. Insulin-like growth factors and insulin resistance are associated with diet-related cancers. Consumption of high GI meals is directly correlated with colon and breast cancer risks. Furthermore, ovarian and prostate cancers are influenced by dietary GI.¹¹

The postprandial rise in glucose has also been associated with a decrease in antioxidant levels, particularly vitamin E and lycopene. Therefore, food rich in antioxidant vitamin E could improve glycemic control. A low GI diet reduces oxidative damage.

At present, researchers focus on understanding the precise relationship between GI and chronic diseases. This understanding will aid in developing dietary interventions and other therapeutics to prevent or alleviate many chronic diseases, such as diabetes and cancer.

References

1. Gao R, Chilibeck PD. Glycemic Index Meal Feeding and Lipid Profiling. The Molecular Nutrition of Fats. 2018; 135-149. https://doi.org/10.1016/B978-0-12-811297-7.00010-X
2. Yalçın T, Al A, Rakıcıoğlu N. The effects of meal glycemic load on blood glucose levels of adults with different body mass indexes. Indian J Endocrinol Metab. 2017;21(1):71-75. doi:10.4103/2230-8210.195995
3. Srichaikul K, Jenkins DJA. The Glycemic Index, Rate of Digestion of Carbohydrate Foods, and Their Potential Link with Cardiovascular Disease. J Nutr. 2022;152(4):920-921. doi:10.1093/jn/nxab427
4. Herforth A. et al. A Global Review of Food-Based Dietary Guidelines. Advances in Nutrition. 2019; 10(4), 590-605. https://doi.org/10.1093/advances/nmy130
5. Frost G. GLUCOSE | Glucose Tolerance and the Glycemic (Glycaemic) Index. Encyclopedia of Food Sciences and Nutrition (Second Edition). 2002; 2916-2922. https://doi.org/10.1016/B0-12-227055-X/00560-5
6. Scott S, Kempf P, Bally L, Stettler C. Carbohydrate Intake in the Context of Exercise in People with Type 1 Diabetes. Nutrients. 2019;11(12):3017. doi:10.3390/nu11123017
7. Imai S, Kajiyama S, Kitta K, et al. Eating Vegetables First Regardless of Eating Speed Has a Significant Reducing Effect on Postprandial Blood Glucose and Insulin in Young Healthy Women: Randomized Controlled Cross-Over Study. Nutrients. 2023;15(5):1174. doi:10.3390/nu15051174
8. Mandal, A. Glycemic Index Limitations. News-Medical. 2023; Accessed on June 30, 2024. https://www.news-medical.net/health/Glycemic-Index-Limitations.aspx.
9. Giuntini EB, Sardá FAH, de Menezes EW. The Effects of Soluble Dietary Fibers on Glycemic Response: An Overview and Futures Perspectives. Foods. 2022;11(23):3934.. doi:10.3390/foods11233934
10. Penaforte FR, Japur CC, Pigatto LP, Chiarello PG, Diez-Garcia RW. Short-term impact of sugar consumption on hunger and ad libitum food intake in young women. Nutr Res Pract. 2013;7(2):77-81. doi:10.4162/nrp.2013.7.2.77
11. Turati F, Galeone C, Augustin LSA, La Vecchia C. Glycemic Index, Glycemic Load and Cancer Risk: An Updated Meta-Analysis. Nutrients. 2019;11(10):2342. Published 2019 Oct 2. doi:10.3390/nu11102342

Further Reading

• All Nutrition Content
• The Role of Nutrition in Health
• What Are the Positive Health Effects of Eating Meat?
• What Are the Negative Health Effects of Eating Meat?
• Macrominerals and Trace Minerals in the Diet