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How to Use The Glycemic Index to Improve Endurance Performance

 

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I recently got an email question asking how to use the glycemic index to improve endurance performance. This is a great question. So, in this blog post I want to describe the glycemic index (GI) and how you can use this concept to improve your daily nutrition plan.

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What is the GI?

GI is a numerical ranking used to measure the rate of digestion and absorption of carbohydrate foods and their effect on blood glucose (sugar).  A food ranking high on the GI produces a large, momentary spike in glucose after it is has been consumed. By contrast, a food with a low GI causes a slower, sustained rise in blood glucose.

The concept of GI was first established in 1981 by Jenkins and colleagues as a way to classify carbohydrate containing foods for improvement of glucose control in diabetics. The researchers had subjects consume 50 grams of food and monitored the blood glucose response for 2 hours. This blood glucose response was then compared to 50 grams of a reference food, either glucose or white bread. Findings were used to establish a table ranking 62 common foods based on glycemic response (the GI was born). GI scores were classified as either low (below 55), medium (56-69), or high (greater than 70). See examples below.

Low GI Moderate GI High GI
skim milk banana watermelon
lentils raisins instant mashed potatoes
kidney beans pineapple french fries
plain yogurt popcorn jelly beans
firm pasta whole wheat bread white bread
apple brown rice table sugar

 

Since their initial work, extensive GI tables have been established and can be easily accessed.

 

Several factors affect a food’s GI such as physical form (liquid or solid), the amount of fiber, and preparation method (raw or cooked).  In general, highly processed foods containing refined sugars (such as crackers and corn syrup) will have a higher GI. It should also be noted that the GI for any food may vary significantly between individuals, so it’s important to test foods for yourself to determine their effects.

 

How Do I Use the Glycemic Index For Endurance Performance?

The GI can be a useful tool to help athletes select the right type of carbohydrates to eat before, during, and after exercise. Selecting foods that are high or low GI can speed up or slow down the availability of carbohydrate.

Daily nutrition for endurance athletes should consist of moderate to low GI foods. Remember that the GI of a meal is the combinations of foods that are consumed so look at the overall meal not just one food item. By adding fat and in particular a protein source to every meal and snack will lower its GI.

 

Moderate and high GI foods are recommended prior to, during and after exercise. Higher GI foods are easily consumed, digested, and absorbed by the body allowing rapid availability of energy. Examples of high GI foods commonly used during exercise include sports drinks and energy gels/bars. Eating a high GI snack immediately (with 45 minutes) after exercise elevates blood glucose concentrations. Post-exercise muscle glycogen resynthesis is of high metabolic priority for the exercised muscles (to replenish depleted glycogen stores). The elevated glucose levels from the consumed from high GI food also stimulates insulin secretion. Post-exercise, insulin helps to promote glycogen storage. Insulin also increases protein synthesis by increasing amino acid uptake by the muscle. Lastly, insulin also enhances blood flow into muscle, thus facilitating the removal of metabolic byproducts from exercise (lactate and carbon dioxide).

 

The general guidelines above work well for most but not for everyone that exercises. There is a great deal of individual differences in how well one digests and processes foods. Experiment with meal timing and food choice to find out what works best for you. Never try anything new prior to an important competition or workout.

 

Is Eating A Low Carbohydrate Diet and a Low GI Diet the same?

The supposition of low carbohydrate diets is that throughout the day, insulin levels are also lower, allowing (or promoting) greater use of fat as the source of fuel. This is a similar hypothesis of what is occurring with low GI diet plans. However, low GI diets essentially do not restrict carbohydrates, they are just very selective of the carbohydrates chosen for consumption.

 

Summary

Take a look at just one of your meals and determine its GI. How did you do? Would you rank it a low, moderate or high GI meal? Let me know in the comments section or on the facebook page. I’d love to answer your questions so please continue to send them my way!

 

Bottom line for the GI: Keep meals low to moderate but increase the GI of foods in and around your exercise.

 

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