Carbohydrate foods have been categorized in two ways to help people concerned about their blood sugar make informed decisions about what, or how much, to eat.
Both the glycemic index (GI) and glycemic load (GL) have been used for diabetic patients as well as people who are following a very low carb diet, like keto, to decide whether or not to include certain foods in their diets.
What is the Glycemic Index?
The glycemic index is a rating system that goes from 1 to 100, with 50 grams of pure glucose ranking as the control at 100. Foods are categorized as low (55 or under), medium (under 70), or high (70 or over) based on how they are expected to make someone’s blood sugar levels respond after consumption.
Foods that are lower in fiber and higher in starch or refined carbohydrates will rank higher on the GI scale, while whole grains or high-fiber foods will rank lower.
The problem with the glycemic index scale is that it doesn’t consider the portion that you eat. An extra-large portion of a low-GI food would still produce a glucose spike, while one bite of a high-GI food probably wouldn’t do much in the big picture. The glycemic index alone is not enough to determine whether or not it will be problematic.  To solve this problem, and to provide a more standardized system for understanding how foods impact glucose levels, the glycemic load was developed.
What is the Glycemic Load and How Is It Different?
The glycemic load is a system that takes into account the type of carbohydrate that a food contains, how much fiber, protein, or fat that it contains (the macronutrient profile), and the serving size. These factors together help it give a more accurate picture of how it will impact a person’s glucose levels after eating it. Working with a glycemic load calculation produces a more consistent clinical outcome when tracking how carbs affect a person’s glucose and insulin levels. 
The glycemic load has a rating scale, too. Foods under 10 are low, under 20 are medium, and 20 or greater are high. For diabetes and general purposes, the aim is to keep your glycemic load under 100 each day.
How Do You Calculate Glycemic Load?
The glycemic load is calculated using the glycemic index number and multiplying it by the number of grams of carbohydrates in the portion that you’re eating, and then dividing by 100.
A raw apple has a GI of 40 and is 16 grams of carbohydrates per serving. You, however, are planning to only eat half of the apple. The glycemic load, then, for one-half of a medium-sized apple is 40 x 8 = 320. Now, this number is divided by 100 to calculate the glycemic load of this food and serving size: 3.2.
How Do You Use Glycemic Load with a Keto Diet?
Most ketogenic diets are going to contain 20 grams of carbohydrates or less, although some can eat more depending on other factors like weight, muscle, intake of protein, and more.
Ultimately there is not a one-size-fits-all carb count for people on a ketogenic diet, but you can use the glycemic load to determine whether or not certain carbohydrate-rich foods are going to sabotage your efforts to get into, or stay in, ketosis.
Do you follow a low G.I. diet?
Let us know your experience with low G.I. foods and how they affect your blood glucose.
Aimee McNew, MNT is a nutritionist and researcher who focuses on women’s health, thyroid, prenatal, and postpartum wellness. She has worked in private practice and written on nutrition-related topics for a decade and is the author of The Everything Guide to Hashimoto’s Thyroiditis (Simon & Schuster, 2016). She is currently working on her next book.
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