The glycemic index (GI) is measure of how carbohydrates affect blood sugar. More specifically the GI measures how quickly 50 g of a carbohydrate affects serum glucose levels in the 2 – 3 hour period after eating. The GI was originally developed to help persons with diabetes manage their blood sugar but is currently being researched for its effect on weight loss, cardiovascular disease, and cancer.
The 3 categories of carbohydrates are sugars, starches, and dietary fiber. To determine the GI of a carbohydrate, its effect on blood sugar is compared to the effect of a reference food on blood sugar. Glucose and white bread are often used as the references because they cause the fastest and most dramatic rise in blood glucose.
Another related term is the glycemic load (GL) which accounts not only for how rapidly a food’s carbohydrates are converted to glucose but also how much carbohydrate is in a serving size. Many things affect glycemic load including food processing, how ripe a fruit is, how a food is prepared, and how long it’s been stored. It is important to note that foods with a high GI may not necessarily have a high GL. An example is watermelon, which has a high GI of 72; however, the average serving of watermelon only has a GL of 4.
Researchers consider glycemic load to be a more realistic and accurate indication of how a carbohydrate affects blood sugar levels than glycemic index.
Table 1. Values for Glycemic Index and Glycemic Load
When we eat foods with a high GI and a high GL, it causes a large spike in blood sugar levels, which over time can lead to insulin resistance and associated health problems including diabetes, obesity, and cardiovascular disease.
There are several online glycemic load calculators, e.g.
Or you can calculate it manually:
Glycemic Load = Glycemic Index X Carbohydrate* (grams) / 100
*Note: When inputting the grams of carbohydrate, do not include the dietary fiber
Example:
Apple Cinnamon Cheerios (Serving size ¾ cup or 30 g)
GI = 74
Total Carbohydrate = 24 g
· Dietary Fiber = 2 g (do not include this when calculating GI or GL)
· Sugars = 10 g
· Other Carbohydrates = 12 g
Calculation:
Glycemic Load = Glycemic Index X Carbohydrate (grams) / 100
GL = 74 x (10 + 12) / 100 = 16 –> medium GL