What is the Glycemic Index?

This is an interesting question and one that has relevance to most individuals. It’s especially interesting considering the overload of questions and discussion about carbohydrates relative to sports nutrition or supplementation.

When considering the role of carbohydrates as a fuel source throughout any form of intense exercise, both endurance and resistance athletes should care about glycemic index and glycemic load. Even if you don’t consider yourself an athlete anymore, a good bit of data exists to suggest that you should know what the glycemic index is because it can have a large impact on your results as well as overall health.

How Is The Glycemic Index Measured?

The GLYCEMIC INDEX of a food relates to an assigned number that represents the extent to which ingestion of the food changes the amount of glucose in your blood [1]. The glycemic index scale, in theory, ranges from 0 to 100, with 100 being the score assigned to glucose. Glucose is the fuel system of your body and what all carbohydrates break down to, to be used as energy for your muscles, brain, and daily functions.

What Do The Glycemic Index Numbers Mean?

Common food sources of carbohydrates (e.g., bread, bagels, pasta, grains, etc.) are ingested and the changes in blood glucose are recorded and compared to glucose and a number is assigned. The higher the number, the more glucose levels in the blood increase and the lower the number, the less glucose levels change in the blood. In an effort to simplify things, foods with a glycemic index higher than 90 are considered ‘High’, 70 – 90 is considered ‘Moderate’ and <70 is considered ‘Low’ [2].

What’s The Purpose Of The Glycemic Index?

Initially, glycemic index was developed as a means to categorize carbohydrates, but its utility has been criticized as many factors can impact the glycemic index of a food. For example, how fast a food is ingested, its form (cooked vs. raw), the amount of other nutrients (fat, protein, fiber, etc.) and any other alterations made to the food before you stick it in your mouth can all impact its glycemic index [2].

What Is Glycemic Load?

A major concept associated with glycemic index was its ability to predict and/or reflect the impact of consuming various carbohydrate sources. As it was investigated more and more, the term glycemic load was developed and continues to be popular today. Glycemic load takes into account how much carbohydrate is ingested in addition to the glycemic index of the food being consumed. Thus, glycemic load = (Glycemic index of a food) x (grams of carbohydrate found in the food consumed).

As you can see, a food that has a glycemic index of 99 (that’s really high) may be perceived as having a tremendous impact on blood glucose levels, but if you only ingest one gram, its overall impact will be much lower than if you ingest 50 grams of it. Along the same lines, ingesting 50 grams of what is considered to be a low glycemic index food (a GI of 50) has a much higher glycemic load (50 x 50 = 2,500) than ingesting ten grams of a food with a high glycemic index rating of 90 (90 x 10 = 900).

How Does The Glycemic Index Impact My Results?

I’ll admit glycemic index and glycemic load are terms often reserved for nutritionists, dietitians, and clients who may struggle with their weight or have shown evidence of developing problems managing their blood glucose levels (i.e., diabetes). But they will impact your results. If you’re an endurance athlete, studies have shown that post-exercise ingestion of high-glycemic carbohydrates can speed up recovery and replace stored carbohydrate (glycogen) in your muscles.

For example, in an excellent review by Jentjens and Jeukendrup [3]they discussed a study which (Keins et al.) compared the impact of ingesting high-glycemic index foods to low-glycemic index foods for their ability to facilitate recovery of muscle glycogen. After completing an exercise bout that depleted all of the glycogen stored in their muscles, participants ingested 70% of their daily calories in the form of either high glycemic or low glycemic carbohydrates. As expected, plasma insulin levels were almost 100% higher in the first six hours after consuming the high glycemic index foods. Which means that their blood glucose levels were higher and thus able to replenish glycogen stores more efficiently and effectively to ensure proper recovery of the muscle. 

What Is The Role Of Insulin?

Insulin is secreted by the pancreas and transports glucose and amino acids (from protein) from our blood to inside tissues that need it (your exercising muscles). Increases in insulin are closely associated with increases in muscle glycogen, so ingesting a diet which increases insulin should increase the ability of your muscles to re-build the glycogen that was burned up during exercise. This is exactly what the researchers found as muscle glycogen synthesis rates were 61% higher after consuming a high-glycemic index carbohydrate when compared to a low-glycemic index carbohydrate [3].

The Glycemic Index And Resistance Training

So who cares about carbs if your idea of a workout is slinging steel instead of pounding the pavement? Protein is your friend, right? Protein is important, but you can’t overlook the importance of carbohydrates. To start, studies have shown that just six sets of leg extensions to muscular failure using 70% of a person’s maximum effort (your leg workouts are more than one exercise, right?) can decrease muscle glycogen in the exercising leg by 30% [4] and when higher intensities (70% vs. 35%) are used the rate at which glycogen is broken down for fuel is almost two times greater than at lower intensities [5].

The need to replace lost glycogen is important and ingesting a high-glycemic carbohydrate source is critical to your results.

Our body needs glucose and insulin levels to be increased to start replacing glycogen and the greater both of these are increased, the faster glycogen is replaced [3]. While high glycemic index foods stimulate massive increases in glucose and insulin (a good thing for recovery of glycogen), small amounts of them minimize their effect.

Use The Glycemic Index To Your Advantage

In conclusion, your body has a limited supply of carbohydrates stored inside it. During exercise, the glycogen found in your liver and muscle is used preferentially as a fuel source. For most individuals, large amounts of glycogen can be lost after an hour of INTENSE exercise and most studies agree that at moderate levels of exercise, your body will run out of glycogen after three hours of exercise [9].

As exercise progresses, the level of cortisol increases in the blood, which has a powerful ability to chemically rip apart muscle so it can be used as a fuel source or for repair (not exactly the best scenario). When exercise is halted, ingesting a healthy dose (45–75 grams for a man and 25-60 grams for a female) of high-glycemic carbohydrates which results in an overall high glycemic load is a critically important consideration for all types of athletes.

Carbohydrate ingestion sharply increases glucose, which just as sharply increases insulin. An increase in insulin works doubly by rapidly decreasing cortisol inside the blood while the high amounts of glucose and insulin will rapidly stimulate the recovery of muscle and liver glycogen. This insulin spike will also help shuttle amino acids from protein out to your muscles to aid in the repair process. Ingesting a high-glycemic carbohydrate and rapid assimilating protein post training will deliver highly favorable results.

The detrimental effects of exercise are a necessary evil, for sure, but using the glycemic index and glycemic load of foods to your advantage can help to facilitate your recovery and optimize how well your body can respond to training.

-Dr. Chad Kerksick is an Associate Professor of Exercise Science at Lindenwood University with a PhD in Exercise, Nutrition and Preventive Health. His research and expertise center upon study the impact of exercise and nutrition interventions on health and performance. 

References

1. Foster-Powell, K., S.H. Holt, and J.C. Brand-Miller, International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr, 2002. 76(1): p. 5-56.
2. Shils, M.E., et al., eds. Modern Nutrition in Health and Disease. 9th ed. 1999, Lippincott Williams & Wilkins: Baltimore, MD.
3. Jentjens, R. and A. Jeukendrup, Determinants of post-exercise glycogen synthesis during short-term recovery. Sports Med, 2003. 33(2): p. 117-44.
4. Pascoe, D.D., et al., Glycogen resynthesis in skeletal muscle following resistive exercise. Med Sci Sports Exerc, 1993. 25(3): p. 349-54.
5. Robergs, R.A., et al., Muscle glycogenolysis during differing intensities of weight-resistance exercise. J Appl Physiol, 1991. 70(4): p. 1700-6.
6. Galgani, J., C. Aguirre, and E. Diaz, Acute effect of meal glycemic index and glycemic load on blood glucose and insulin responses in humans. Nutr J, 2006. 5: p. 22.
7. Wolever, T.M. and C. Bolognesi, Prediction of glucose and insulin responses of normal subjects after consuming mixed meals varying in energy, protein, fat, carbohydrate and glycemic index. J Nutr, 1996. 126(11): p. 2807-12.
8. Wolever, T.M. and C. Bolognesi, Source and amount of carbohydrate affect postprandial glucose and insulin in normal subjects. J Nutr, 1996. 126(11): p. 2798-806.
9. Romijn, J.A., et al., Substrate metabolism during different exercise intensities in endurance-trained women. J Appl Physiol, 2000. 88(5): p. 1707-14.