What You Should Know About The Glycemic Index

It’s widely known that eating a diet abundant in whole foods such as fruit and vegetables is good for our health. However, I sometimes hear people express concern about the glycemic index (GI) of some fruits and vegetables. With only 9% of U.S. adults meeting the recommended daily vegetable intake (2-3 cups) and just 12% meeting the recommended fruit intake (1.5-2 cups),(1) the GI may be yet another barrier to promoting adequate consumption. In this article, I’ll be covering what exactly the GI is and why you shouldn’t worry too much about it when building your meals and snacks.

What is the GI?

The GI ranks carbohydrate-containing foods based on how they affect blood glucose. These complex mathematical measurements are then used to calculate a value and classify foods as being low-, medium- or high-GI(2) based on the anticipated effect it may on blood sugar levels when consumed on its own. A low-GI food falls in between 1 to 55; medium-GI is 56 to 69 and high-GI is 70 and higher. Since originating in the 1980s, the GI has been used primarily as a tool for individuals with diabetes. However, there are several methodological weaknesses and substantial variability when determining a foods GI value according to a recent study of 63 otherwise healthy U.S.-based volunteers - which cannot be applied to individuals with chronic disease - and the way it ranks foods has been criticized.(3) How a specific food affects your blood sugar will depend on the individual, what’s all consumed in combination with the food (i.e. protein, fat), and can even vary depending on cooking time, preparation, and ripeness of the food.

Where GI really falls short is that it measures the effect of a single food on blood sugar levels when majority of the time, we are consuming meals that contain multiple foods all at once. Consuming carbohydrates in combination with protein and fat makes GI difficult to predict and may reduce the impact that some medium or high-GI foods are anticipated to have on blood sugar.(4)

Should you use the GI to guide your meal or snack selection, or follow a low-GI diet?

According to a survey(5) fielded by Today’s Dietitian and sponsored by Potatoes USA, nearly three-quarters (73%) of nutrition professionals report they do not use the glycemic index when counseling their patients and clients. What’s more, nutrition professionals surveyed believe the GI leads to misinformation about the healthfulness of fruits and vegetables (69% and 37%, respectively) and 60% strongly agree that eliminating high-GI foods from the diet and allowing only low-GI foods can cause people to exclude perfectly healthy foods. The overwhelming majority (83%) surveyed report that the GI is not part of the nutritional guidelines or standards to guide the food and/or meal selection of patients in their professional place of practice. When it comes to weight loss or weight maintenance, the Academy of Nutrition and Dietetics does not recommend a low-GI diet as part of a comprehensive weight management program.(6)

Don’t let the GI cause carbohydrate confusion or confusion around the healthfulness of foods. There are many issues that come from just basing a food off it’s GI. Whether a food has a high- GI or low-GI, it doesn’t say anything about its nutritional composition. For example, while potatoes are high-GI, they are a nutrient-dense vegetable and high-quality complex carbohydrate that provide 26 grams of carbs, 3 grams of plant-based protein, 27 mg vitamin C and under-consumed nutrients like potassium (620 mg) and fiber (2 g) per 5.3 oz. skin-on serving.

A serving of soda or ice cream both have a lower GI values(4) than a serving of potatoes! Several other fruits and vegetables, such as winter squash, pineapple, and parsnips, are considered to have a medium to high-GI.(4)

So, what should you do instead? It’s important to focus on consuming a healthy, well-balanced diet that works for you and meets your individual needs while containing a wide variety of whole food sources such as complex carbohydrates, lean protein, and unsaturated fats. All foods – especially all fruits and vegetables – can fit.

What about the GI and athletic performance?

As for athletic performance, the GI was never intended to be used by athletes and is not a guideline I advise on when educating my athletes about what to eat before, during, or after exercise. In fact, a randomized controlled trial of 9 male endurance runners found that there was no difference on running performance or carbohydrate and fat oxidation after having a low-GI or high-GI pre-exercise meal when carbohydrates are consumed during physical activity.(7) It’s important to note that these results cannot be applied to other sports or training levels and more research is needed. 

Rather than focus solely on the GI of a food around the times you’re training, it’s more important to focus on the type, timing, and amount of carbohydrate,(8) which is very individualized. These recommendations will vary depending on your activity, personal food preferences, and what foods are tolerated best based on your own experiences.

This blog post was sponsored by Potatoes USA. Find helpful information and  performance recipes on PotatoesFuelPerformance.com for fueling inspiration to power your athletic goals.  

References:

1.     Only 1 in 10 Adults Get Enough Fruits or Vegetables. CDC website. https://www.cdc.gov/media/releases/2017/p1116-fruit-vegetable-consumption.html. Updated November 16, 2017. Accessed May 19, 2020.

2.     Do potatoes have a high glycemic index (GI)? Potatoes USA website. https://www.potatogoodness.com/glycemic-indexfact-check/. Accessed May 19, 2020.

3.     Matthan NR, et al. Estimating the reliability of glycemic index values and potential sources of methodological and biological variability. The American Journal of Clinical Nutrition. 2016;104(4):1004-1013.

4.     Atkinson FS, Foster-Powell K, Brand-Miller JC. International tables of glycemic index and glycemic load values: 2008. Diabetes Care. 2008;31(12):2281-2283.

5.     Glycemic Index Survey Key Findings. Potatoes USA website. https://www.potatogoodness.com/wp-content/uploads/2020/05/Glycemic-Index-Survey-Key-Findings-FINAL.pdf. Accessed May 19, 2020.

6.     Adult Weight Management (AWM) Low Glycemic Index Diets. Academy of Nutrition and Dietetics website. https://www.andeal.org/template.cfm?template=guide_summary&key=626&highlight=glycemic%20index&home=1. Accessed May 19, 2020. 

7.     Wong SH, et al. Effect of preexercise glycemic-index meal on running when CHO-electrolyte solution is consumed during exercise. International Journal of Sport Nutrition and Exercise Metabolism. 2009;19(3):222-242.

8.     Kerksick CM, Wilborn CD, Roberts MD, et al. ISSN exercise & sports nutrition review update: Research & recommendations. J Int Soc Sports Nutr. 2018;15(1):38.