Understanding the Glycemic Index

The concept and acceptance of GI
How are foods ranked on the GI scale?
Limitations of GI
Clinical applications of GI
References
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The glycemic index (GI) is a nutritional indicator that measures how quickly the carbohydrates in a particular food affect a person’s blood sugar levels.1 According to the GI values, foods are classified as low, medium or high.

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A low GI food induces a lower glucose response, unlike a high GI food. A diabetic can control his blood sugar levels by minimizing the consumption of high GI foods and increasing the intake of low GI foods.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 arose from the fiber hypothesis, which suggested that higher fiber intake reduced the influx of nutrients from the intestines. GI is a classification system for carbohydrate-rich foods, based on their ability to raise plasma glucose levels.

Many countries, such as Sweden, France and Australia, have included GI in 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 after a meal with the glucose response to 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 yoghurt); an average score is between 56 and 69 (e.g. brown rice, sweet potato and granola), and a high score is between 70 and 100.6

Processed foods (e.g. cakes, biscuits, sweets and bread) have a high GI, while whole foods containing non-starchy vegetables, unrefined grains and fruits tend to have a lower GI. Protein and fat are not included in the GI-based food list, as they have minimal impact on blood sugar levels.

Low GI foods are easily digested and are absorbed and metabolized more slowly than high GI foods. In addition, low GI foods cause a gradual and slow rise in blood sugar levels.7

Limitations of GI

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

The GI score only indicates the relative rise in plasma glucose levels after a meal and does not take into account the insulin response, which is measured by the insulin index. GI does not take into account the fact that the glycemic response to a given food varies according to an individual’s metabolism. Significant variation has also been observed at different times of the day.

Keep in mind that the GI is affected by several factors, including the amount of fiber and fat, the type of sugar, the cooking method, and the food processing.9 Studies have shown that the degree of ripeness of the fruit also affects the GI.

For example, a fully ripe banana has a higher GI than an unripe banana. In general, hyperglycemia, or high plasma glucose levels, occurs after consuming high-GI foods, while hypoglycemia, or low blood glucose levels, occurs after a high-GI meal.

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Clinical applications of GI

The numerical classification of carbohydrate foods when consumed has proven extremely useful for people with impaired glucose tolerance. Blood sugar management is important for many reasons. For example, a blood sugar crash makes one feel lethargic and hungry, which leads to eating more, which leads to obesity. A person who experiences frequent blood sugar spikes and crashes is at higher risk for chronic inflammation and metabolic diseases (e.g., type 2 diabetes).10

Athletes benefit from eating low GI meals before an event as this promotes a slower rise in insulin and glucose levels. This could help maintain carbohydrate availability during the latter stages of training/play, which in turn improves training performance.

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There are many short- and long-term benefits to maintaining a low-GI diet. For example, a study in younger adults observed that following a low-GI diet for two weeks improved overall glucose levels and reduced LDL cholesterol and triacylglycerol concentrations. However, increases in glucose concentrations at 45 and 60 minutes have been attributed to a lower C-peptide response due to adaptive gut responses that resulted in reduced incretin secretion.

When middle-aged subjects, especially insulin-resistant women who had suffered a myocardial infarction, were given a low-GI diet, an overall improvement in insulin sensitivity was observed. These improvements were accompanied by increased synthesis of glycated proteins and decreased levels of plasminogen activator inhibitor 1.

Recent research highlighted the importance of GI in preventing cancer. Insulin-like growth factors and insulin resistance are associated with diet-related cancers. The consumption of high GI meals is directly correlated with the risk of colon and breast cancer. Additionally, ovarian and prostate cancer are affected by dietary GI.11

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

Currently, researchers are focused on understanding the precise relationship between GI and chronic disease. This understanding will help develop dietary interventions and other therapies to prevent or alleviate many chronic diseases, such as diabetes and cancer.

References

1. Gao R, Chilibeck police. Meal food glycemic index 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 in 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, the digestion rate of carbohydrate-rich foods and their potential association 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. Prince G. GLUCOSE | Glucose tolerance and the glycemic (glycemic) 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 crossover trial. Nutrients. 2023;15(5):1174. doi:10.3390/nu15051174
8. Mandal, A. Limitations of the glycemic index. News-Medical. 2023; Accessed 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 fiber on the glycemic response: a review and future perspectives. Food. 2022;11(23):3934.. doi:10.3390/foods11233934
10. Penaforte FR, Japur CC, Pigatto LP, Chiarello PG, Diez-Garcia RW. Short-term effects 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

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