Carbohydrates have long been studied as an energy source and the debate as to what quantities and types of carbohydrates are optimal for performance and health is ongoing. Some types of carbohydrates, most often termed 'simple' carbohydrates, are converted more easily into fat in the body, raise blood glucose levels rapidly, and are linked to certain health issues such as obesity, cancer, and diabetes, to name a few (1). Other carbohydrates, sometimes referred to as 'complex' carbohydrates, are not converted to fat easily and are also not linked with negative health problems (1). Due to the important effects that carbohydrates have on the body, glycemic index and glycemic load should be taken into consideration when planning for fitness, nutrition, sport performance, and overall health.
Developed about 30 years ago, the glycemic index ranks carbohydrate-containing foods based on the effect that they have on blood glucose (sugar) levels. The glycemic index uses a scale from 0 – 100, with 100 being equivalent to ingesting 50 grams of pure sugar. To find the glycemic index of a particular food, a portion of the food that contains 50 grams of available carbohydrate is fed to test subjects (1). The subjects' blood sugar response to that food is then measured and compared to the blood sugar response after ingesting 50 grams of glucose. For example, a baked potato is considered a high glycemic index food with a score of about 85, meaning that when a plain baked potato containing 50 grams of carbohydrate is eaten, it raises blood glucose 85% compared to when the same amount of pure sugar is consumed. This means that baked potatoes are digested and absorbed rapidly into the bloodstream. On the other hand, a carbohydrate-containing food that would be digested and absorbed much more slowly would be grapefruit, with a glycemic index score of 25.
The main limitation of the glycemic index is that it does not provide information based on realistic serving sizes and therefore does not account for the amount of carbohydrate that a normal meal or food item contains (2). A more recent approach to measuring the effect of carbohydrate on the body is to calculate the glycemic load. The glycemic load takes into account the actual carbohydrate content of a meal or food by multiplying the glycemic index by the amount of carbohydrate in the serving then dividing by 100 (3). The glycemic index reference point of pure sugar would have a glycemic load of 50, for a 50 gram serving.
If we look at the examples of foods listed above, we can now look at their glycemic load. The serving size for the grapefruit is ½ of a large grapefruit, which would be about 166 grams. The amount of carbohydrate in that serving is about 11 grams, putting its glycemic load score at 3, which is very low. Interestingly, the baked potato, which was considered to be a high glycemic index food, falls more into the moderate category in terms of glycemic load, if appropriate serving sizes are consumed. A medium-sized, approximately 173 gram baked potato has about 33 grams of available carbohydrate, putting its glycemic load score at 28.
Both the glycemic index and glycemic load provide us with information on how our bodies deal with carbohydrates. Because glycemic load takes into account the serving size of a food as well as the actual carbohydrate content of that serving, the glycemic load is possibly a more practical method to use. It is important to remember that there are other factors that determine the actual blood sugar response, such as simultaneous ingestion of fats and/or proteins which will lower the glycemic response of a food. Also, it is important to know that athletes, those who are trying to lose weight, and people with diabetes may all have different needs when it comes to the choices of carbohydrates to be eaten. Athletes may need high or low glycemic load foods depending on the timing with their workouts. On the other hand, people who have diabetes or are trying to lose weight are encouraged to consume mostly low glycemic load foods the majority of the time.
References:
1. Campbell, B., (n.d.). Glycemic load vs. glycemic index.
2. Powers, S., & Howley, E. (2007). Exercise physiology: Theory and application to fitness and performance (6th ed.). Toronto, ON: McGraw-Hill.
3. Wong, S., & O'Reilly, J., (2010). Glycemic index and glycemic load: Their application in health & fitness. ACSM's Health & Fitness Journal, 14(6), 18-23.
Not All Carbs Are Created Equally
