The Lowdown on Glycemic Index
What is Glycemic Index?
Carbohydrates are a macronutrient found in many foods, including bread, pasta, rice, fruit, and vegetables. When we eat carbohydrates, they are broken down into glucose, which is used by the body as fuel. The speed at which carbohydrates are converted into glucose varies, depending on the type of carbohydrate, the processing of the food and our genetics (1).
Glycemic index is a ranking system that measures how quickly a carbohydrate-containing food raises blood sugar levels compared to pure glucose, which has a value of 100 (2).
High-GI foods, such as white bread, white rice, and sugary drinks are rapidly absorbed into the bloodstream and cause a rapid increase in blood sugar levels. Low-GI foods, such as whole grains, fruits, and vegetables, are more slowly absorbed into the bloodstream and cause a more gradual increase in blood sugar levels (3).
Why is Glycemic Index important?
The glycemic index is important for several reasons. First, high-GI foods can cause a rapid spike in blood sugar levels, which can lead to weight-loss resistance, insulin resistance, type 2 diabetes, and other metabolic disorders (4). By choosing low-GI foods, people can help regulate their blood sugar levels and reduce their risk of developing these issues.
Second, the glycemic index can be a useful tool for weight management. High-GI foods are often high in calories and can cause overeating, while low-GI foods are often high in fibre, which promotes satiety and can help with weight loss (5).
Third, the glycemic index can be an important consideration for athletes and other people who need to maintain stable blood sugar levels for optimal performance.
Low-GI foods provide sustained energy and can help maintain blood sugar levels during prolonged physical activity, while high-GI foods can cause a rapid drop in blood sugar levels and lead to fatigue and decreased performance (6).
How is Glycemic Index determined?
The glycemic index of a food is determined by feeding a group of people a standardised amount of the food and measuring their blood sugar levels over a two-hour period. The glycemic index value is then calculated by comparing the rise in blood sugar levels caused by the food to the rise in blood sugar levels caused by pure glucose, which has a value of 100 (7).
Factors that can affect glycemic index values include the type of carbohydrate, the processing of the food, the presence of fibre and other nutrients, and the individual's metabolic response to the food (8). As a result, glycemic index values may vary between individuals and may not always accurately reflect the impact of food on blood sugar levels.
Which foods have a high and low Glycemic Index?
Foods with a high glycemic index include:
- White bread and bagels
- Rice cakes
- Baked potatoes
- Corn flakes
Foods with a low glycemic index include:
- Oatmeal
- Whole grain bread
- Sweet potatoes
- Apples
- Berries
How is Glycemic Index measured?
The glycemic index is measured by feeding participants a portion of food containing 50 grams of carbohydrates, and then measuring blood sugar levels over the next two hours. The area under the curve (AUC) of blood sugar levels is then calculated and compared to the AUC of the reference food, usually glucose or white bread.
Limitations of the Glycemic Index
While the glycemic index can be a useful tool for managing blood sugar levels, there are some limitations to its use.
For example, the glycemic index does not take into account portion size, as the glycemic index is measured based on a standard portion size of 50 grams of carbohydrates. Additionally, the glycemic index may not accurately reflect the impact of mixed meals or meals that contain both carbohydrates and fat or protein - which is likely to reduce blood sugar response.
Conclusion
The glycemic index (GI) is a measure of how quickly carbohydrates in food are converted into glucose in the body and how much they raise blood sugar levels.
The GI scale ranges from 0 to 100, with high-GI foods having a value of 70 or above and low-GI foods having a value of 55 or below.
The glycemic index is a useful tool for understanding how different foods impact blood sugar levels, and can be a helpful guide for individuals with diabetes or those seeking to maintain healthy blood sugar levels. By choosing foods with a low glycemic index, we can promote stable blood sugar levels and support overall health.
References
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Jenkins DJ, Wolever TM, Taylor RH, Barker H, Fielden H, Baldwin JM, Bowling AC, Newman HC, Jenkins AL, Goff DV. Glycemic index of foods: a physiological basis for carbohydrate exchange. The American journal of clinical nutrition. 1981 Mar 1;34(3):362-6.
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Foster-Powell K, Holt SH, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. The American journal of clinical nutrition. 2002, Nov;76(1):5-56.
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Augustin LS, Kendall CW, Jenkins DJ, Willett WC, Astrup A, Barclay AW, Björck I, Brand-Miller JC, Brighenti F, Buyken AE, Ceriello A. Glycemic index, glycemic load and glycemic response: An International Scientific Consensus Summit from the International Carbohydrate Quality Consortium (ICQC). Nutrition, Metabolism and Cardiovascular Diseases. 2015 Nov 1;25(11):795-815.
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Livesey G, Taylor R, Livesey H, Liu S. Is there a dose-response relation of dietary glycemic load to risk of type 2 diabetes? Meta-analysis of prospective cohort studies. The American journal of clinical nutrition. 2013 Feb 1;97(2):584-96.
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Ludwig DS. The glycemic index: Physiological mechanisms relating to obesity, diabetes, and cardiovascular disease. Jama. 2002 May 8;287(18):2414-23.
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American Diabetes Association. Glycemic index and diabetes. Diabetes Care. 2004;27(7):1781-3.
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Brand-Miller JC, Holt SH, Pawlak DB, McMillan J. Glycemic index and obesity. The American journal of clinical nutrition. 2002 Dec 1;76(6):281S-5S.
