Ask the Expert


Question:
I am curious about the glycemic index (GI) compared with the glycemic load of a particular food and the impact on blood sugar. My understanding is that the GI can be high, but the food may have less of an effect on blood sugar given that its total amount of carbohydrate is low. For example, carrots have a high GI but only 5 to 6 grams carbohydrate per 100 gram weight. Carbohydrate management would look different if this is the case and would follow the general guidelines of distribution of carbohydrates, which doesn’t take into account the GI of the food but the total amount of digestible carbs at a meal or snack.

Marybeth Judy, MS, RD, LD
Bernard, Me.

Response:
This is a great question, and you are correct. We know that when it comes to glucose control, it’s about both the quality and quantity of carbs. The glycemic index compares the potential of foods containing the same amount of carbohydrate to raise blood glucose. But we know that it’s not just about the quality of carbs; it’s also how much carbohydrate consumed that affects blood glucose levels and insulin responses.

The glycemic load (GL) of a food is calculated by multiplying the glycemic index by the amount of carbohydrate in grams provided by a food and dividing the total by 100. GL = (GI x the amount of carbohydrate)/100. Most experts agree that it’s the best way to predict blood glucose values because it combines both the quality and the quantity of carbohydrates into one number. As a rule of thumb, glycemic loads below 10 are considered low and glycemic loads above 20 are considered high. Dietary glycemic load is the sum of the glycemic loads for all foods consumed in the diet.

Here’s an example using an apple: It has a GI of 40 and contains 15 grams of carbohydrate. GL = 40 x 15/100 = 6 g

Here are some examples of other foods comparing GI to GL.

  • Shredded Wheat Cereal — GI: 69; GL: 57
  • Bagel — GI: 72; GL: 38.4
  • Cornflakes — GI: 84; GL: 72.7

Compare those with the following relatively high GI vegetables and fruits:

  • Beets, boiled — GI: 64; GL: 6.3
  • Bananas — GI: 53; GL: 12.1
  • Sweet Potatoes — GI: 54; GL: 13.1
  • Carrots— GI: 71; GL: 7.2

Although the GL is more accurate, many people find the GI to be an easy tool to help them look for better quality carbs. I always urge people to use the GI as a guideline.

— Sharon Palmer, RD, is a contributing editor at Today’s Dietitian and freelance food and nutrition writer in southern California.