What is the Glycemic Index?

Although it may seem like diet trends are constantly changing, the reality is that most popular diets are simply iterations of the low-carb, low-fat, or low-calorie diets that came before them. And while all these diets can work (at least in the short-term), the restriction and deprivation typically required often makes them unsustainable in the long-term.

I want to make it clear that I’m not here to tell you to stop following your low-carb, low-fat, or low-calorie diet if you’ve experienced long-term success. I’m speaking to individuals interested in the science and art of ‘macronutrient moderation’ who no longer wish to demonize fat or carbs and instead wish to learn how to adopt a flexible pattern of balanced eating. And speaking of balanced eating, I’d like to introduce you to the glycemic index.

What is the Glycemic Index?

The glycemic index (G1), which was first described in 1981 by Dr. David A. Jenkins and colleagues (1), is a scale that ranks carbohydrate-containing foods and beverages by how much they raise blood glucose (sugar) after being consumed. Foods with a high GI are digested quickly and result in a in a large spike in both blood sugars and insulin (2). In contrast, low GI foods take longer to digest and result in smaller fluctuations in blood glucose and insulin levels (2).

To determine a food’s glycemic index, the following test is performed (2):

Glucose vs Time

 

  1. Ten or more healthy people are fed an amount of the food containing 50 gra
  2. ms of available carbohydrate (i.e. total carbohydrate minus fibre).
  3. For the next 2 hours, the impact of the food on the individual’s blood sugar is measured.
  4. This process is then repeated on a separate day with the same 10 people consuming an equal amount of carbohydrate in the form of pure glucose.
  5. A GI value is calculated for each person by dividing their blood sugar from the test food by their blood sugar for the pure glucose. The final GI number is the average of all these individual values.

How are foods categorized using the GI?

The GI ranks food on a scale of 0-100 as either “low”, “medium”, or “high” based on their glycemic impact. The following table demonstrates how foods within different categories are classified using this system. Note that most non-starchy vegetables (such as lettuce and tomatoes) are not assigned a GI value because they contain so few available carbohydrates. Similarly, meat, fish, poultry, and fats are not assigned a GI value since they do not contain carbohydrates. To find out the specific GI of a food, check out the University of Sydney’s comprehensive database.

Grains and Starches

Low GI (55 or less)
Medium GI (56-69)
High GI (70 or more)
Heavy mixed grain bread

Sourdough bread

Whole grain tortilla

All Bran Buds

Oat bran

Steel cut oats

Barley

Pasta (cooked al dente)

Quinoa

Rice (converted or parboiled)

Sweet potato

Whole grain wheat bread

Pumpernickel bread

Oats (quick, large flake)

Rice (basmati, wild, or brown)

Couscous

Rice noodles

White/whole wheat bread

Naan bread

Special K

Corn Flakes

Potatoes (instant mashed or white)

Rice (sticky or instant white)

Millet

Pretzels

Soda crackers

 

Fruits and Vegetables

Low GI (55 or less)
Medium GI (56-69)
High GI (70 or more)
Apple

Banana (green, unripe)

Berries

Orange

Peach

Pear

Peas

Banana (ripe, yellow)

Beets

Cherries

Grapes

Pineapple

Parsnips

Banana (overripe, brown)

Carrots

Watermelon

Calcium-rich Foods

Low GI (55 or less)
Medium GI (56-69)
High GI (70 or more)
Cow’s milk

Plant-milks (other than rice)

Yogurt

  Rice milk

Protein-rich Foods

Low GI (55 or less)
Medium GI (56-69)
High GI (70 or more)
Beans

Lentils

Lentil soup (pre-made)  

Table adapted from: Diabetes Canada (2018). Glycemic Index Food Guide. Available from: http://guidelines.diabetes.ca/docs/patient-resources/glycemic-index-food-guide.pdf

Glycemic Index vs. Glycemic Load

A term that is commonly referenced when discussing a food’s GI is the glycemic load or GL. While the GI only measures quality of carbohydrate in food, the GL is a measure of both quality and quantity and is determined by the following formula (2):

GL = (GI of a food x amount of carbohydrate in that food)/100

In other words, the GL is the amount of carbohydrate in a food adjusted for its potential to raise blood sugar levels; it allows us to compare the glycemic effect of different foods.

For example, the GI of an apple is 40 and it contains 15 grams of carbohydrate. Thus, it’s GL would be: (40 x 15)/100 = 6 g.

In contrast, a small baked potato has a GI of 80 but also contains 15 grams of carbohydrates. Its GL is: (80 x 15)/100 = 12 g.

Thus, we can reasonably determine from the above example that eating a small baked potato will have a greater impact on our blood sugar compared to eating an apple, even though they contain the same amount of carbohydrates.

What are the health benefits of following a low GI diet?

Since it’s development, there has been a great amount of research published on the health effects of the GI with findings suggesting that a low GI dietary pattern may have a positive influence on outcomes of diabetes, cardiovascular disease, and weight. The results of large, population-based studies suggest that a combination of a low GL and high fibre diet results in a two-fold reduction in risk of type 2 diabetes in both men and women (3, 4). Low GI diets have also been associated with better blood sugar control, improved cholesterol, and weight loss among those living with diabetes (5). In addition, eating a low GI/GL diet has been shown to increase between-meal fullness in both adults and children (6-8) and may promote a greater loss of weight and body fat (5).

Practical Steps for Implementing a Low GI/GL Diet

The GI is but one of many tools that can assist you in making more nutritious choices on the road to better health. Keep in mind that it is your overall dietary pattern and lifestyle (including sleep, stress, and activity levels) that are the strongest predictors of wellness. Consensus guidelines advocate for a dietary pattern that consists of unprocessed, whole-ingredient foods including fruits, vegetables, whole grains, legumes, nuts, and seeds (5). If you rarely to never include such foods, this would be a logical and reasonable starting point.

At the end of the day, there is not enough convincing, high-quality data to suggest that the complete avoidance of carbohydrates is necessary for the achievement of long-term health and wellness. In fact, a long-term study that looked at mortality risk among 15,428 adults found that a moderate carbohydrate intake of 50-55% total energy was associated with the lowest mortality risk, and low carbohydrate diets that favouried animal-derived protein and fats resulted in a greater risk of mortality compared to those that favoured plant-based sources of fat and protein (9). Findings such as these speak to the importance of balance and moderation and suggest that the best health outcomes are achieved when we focus on the quality of our overall diet.

Susan Macfarlane is a Registered Dietitian Nutritionist utilizing cronometer.com to help run a busy private practice in Ontario, Canada and blogs regularly for cronometer.com

References

  1. Jenkins DJ, Wolever TM, Taylor RH, Barker H, Fielden H, Baldwin JM, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr Mar.1981;34:362e6. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/6259925
  2. The University of Sydney. About Glycemic Index. May 2017. Article available from: http://www.glycemicindex.com/about.php
  3. Salmeron J, Ascherio A, Rimm EB, Colditz GA, Spiegelman D, Jenkins DJ, et al. Dietary fiber, glycemic load, and risk of NIDDM in men. Diabetes Care Apr. 1997;20:545e50. Abstract available from: http://care.diabetesjournals.org/content/20/4/545
  4. Salmeron J, Manson JE, Stampfer MJ, Colditz GA, Wing AL, Willett WC. Dietary fiber, glycemic load, and risk of noninsulin-dependent diabetes mellitus in women. JAMA. Feb 12 1997;277:472e7. Abstract available from: https://jamanetwork.com/journals/jama/article-abstract/414104
  5. Augustin LS, Kendall CW, Jenkins DJ, Willett WC, Astrup A, Barclay AW, et al. Glycemic index, glycemic load and glycemic response: An International Scientific Consensus Summit from the International Carbohydrate Quality Consortium (ICQC). Nutr Metab Cardiovasc Dis. 2015 Sep;25(9):795-815. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/26160327
  6. Ludwig D. Dietary glycemic index and obesity. J Nutr. 2000; 130(Suppl.):280Se3S. Abstract available from: https://academic.oup.com/jn/article/130/2/280S/4686354
  7. Ludwig D, Majzoub J, AL-Zahrani A, Dallal G, Blanco I, Roberts S. High glycemic index foods, overeating and obesity. Pediatrics.1999;103:E261e6. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/10049982
  8. Ball SD, Keller KR, Moyer-Mileur LJ, Ding YW, Donaldson D, Jackson WD. Prolongation of satiety after low versus moderately high glycemic index meals in obese adolescents. Pediatrics. Mar 2003;111:488e94. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/12612226
  9. Seidelmann SB, Claggett B, Cheng S, Henglin M, Shah A, Steffen LM, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. Lancet Public Health. 2018 Aug 16. pii: S2468-2667(18)30135-X. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/30122560
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