by Cassandra Johnson, MS, RDN, Registered Dietitian, Boston, MA
The glycemic index (GI) and glycemic load (GL) are tools originally developed to help people with diabetes better manage their blood sugar levels. The GI and GL are related but different measurements, and both numbers rank foods according to how glycemic they are – how quickly they raise blood sugar.
You may already know that eating foods that contain carbohydrates – such as grains, fruits, vegetables, beans and dairy products – will naturally raise blood sugar. But did you know that not all carbohydrate foods have the same impact? For example, eating a serving of white rice will cause a faster rise in blood sugar than eating brown rice, even though both foods contain the same amount of carbohydrate.
The Glycemic Index (GI) for a food is measured by how quickly blood sugar rises after a person eats a serving that contains 50 grams of carbohydrate, such as a cup of rice, or one medium bagel. (Pure glucose has a GI of 100 and is the reference point for other foods.)
High glycemic foods (GI >70) will raise blood sugar higher and more quickly than low glycemic foods (GI <55). A food’s GI depends on many factors, including how much protein, fat, or fiber the food contains, how ripe it is or how long it has been stored, and how the food has been cooked or processed. (See the table below for some examples.)
For example, a baked potato has a much lower GI when eaten with its skin, because the fiber in the potato skin slows down digestion. Similarly, carrots that have been cooked or juiced are more quickly absorbed and therefore have a higher GI than raw carrots do.
It’s important to remember that the GI should not be used to judge the overall nutritional value of a food. For example, chocolate covered peanuts have a low GI of 32, but would not be considered healthy additions to your daily diet. Similarly, there are many nutritious foods that have high GIs, such as potatoes and sweet potatoes.
How much you eat of a given food also affects your blood sugar response. You might easily eat a cup of rice or a bagel, but how likely are you to eat the 4½ cups of diced watermelon it would take to equal 50 grams of carbohydrate?
The Glycemic Load (GL) was designed to account for portion size. It includes the GI and the overall carbohydrate content of a serving of a given food:
Glycemic Load = (Glycemic Index * net carbohydrate content of the serving)/100
For example, a typical serving of watermelon might be ¾ of a cup, which has about 6 grams of carbohydrate. The GL for that serving of watermelon is 4:
Glycemic Load = (72 * 6)/100 = 4
Eating a serving of a low GL food (GL <10) will have less impact on blood sugar than a serving of a high GL food (GL >20). Watermelon may be a high GI food, but when eaten in a reasonable serving size, it is low glycemic load.
You can look up glycemic index and glycemic load for different foods here: http://www.glycemicindex.com/foodSearch.php
Using the Glycemic Index and Glycemic Load
Though not all nutrition experts agree, there is general consensus that research suggests that eating a low GI/GL diet may help with weight management and may prevent diabetes, cardiovascular diseases, and certain cancers.
If blood sugar levels are a concern for you, choosing low GI/GL foods can help. Other ways to support a healthy blood sugar include exercising, eating a balance of protein, fiber, and fat along with carbohydrate at each meal, and being mindful of portion sizes, particularly with high GI/GL foods.
Thirsty for “Low GI Juices”?
Fruits and higher GI vegetables such as beets, carrots and sweet potato add flavor and nutrients to your juice, but their glycemic impact can add up when juiced in large amounts. If you’re looking for a low GI juice, try balancing them with lower GI ingredients. Look for fresh juices that emphasize vegetables with low (or no) glycemic impact: cucumbers, peppers, celery, and leafy greens such as spinach, kale, chard, escarole or romaine.