When we eat carbohydrates (including starch and sugars), they are digested and converted into glucose, a simple sugar, by our bodies. Glucose is then absorbed and therefore enters into the blood stream providing energy for our daily activities. Glycemic Index is a standardized system of ranking foods based on their effect on blood glucose levels over 2 – 3 hours compared to a reference food. Foods that are digested and absorbed faster will have a higher glycemic index. This GI food-ranking approach, which originated in Canada, is popular in Australia and gaining ground in Europe and the United States.
Glycemic Index Confusion
Most of the literature use Glucose as the reference food and therefore as the reference food glucose has a glycemic index of 100 and no other foods will have a glycemic index values more than 100. However, a few other literature use white bread as the reference food and hence some foods may have glycemic index values up to 140. Therefore be sure to check the reference food when looking at a glycemic index table.
Glycemic Index Range
- Low: 55 or less
- Intermediate: 55 – 70
- High: 70 or higher
To find out the GI value of a specific food, go to this Glycemic Index database by University of Sydney.
Factors Affecting Glycemic Index
A number of factors determine a food’s glycemic index. One of the most important is how highly processed the carbohydrates are. Whole-grain foods tend to have a lower glycemic index than their refined products. For example, white rice, which its bran and germ have been removed, has a higher glycemic index than brown rice, which is less processed.
Other Factors affecting Glycemic Index
Fiber Content: Fiber, especially soluble fiber, slows down the digestion of starch, therefore high fiber foods have lower glycemic index. For instance, apples with skin have a lower glycemic index than apples without skin.
Fat, Protein: Fat and protein slow down foods from leaving the stomach, therefore foods containing fat and protein such as beans and milk have lower glycemic index values.
Acid Content: Lemon juice or vinegar can also lower the glycemic index.
Maturation: Unripened fruits have lower glycemic index values than ripe fruits.
Glycemic Index and Weight Loss
The glycemic index was developed for helping people with diabetes choose foods that would not cause big swings in blood glucose levels. The glycemic index is very popular in books on weight loss even though there is little evidence that foods with a low glycemic index cause weight loss. Most of the low glycemic diets promoted by these books are indeed low in calories. Somehow, if one eats 500 less calories than s/he needs, 1 lb/week of weight loss can be achieved – regardless of a high carb or low carb diet. An easy way to lose 500 calories a day without restricting food is to engage in physical activities.
Glycemic Index Key Message: Short term drastic weight loss is often not lasting. The traditional weight loss method emphasizing a well-balanced diet with a variety of foods including whole grains, fruits & vegetables, dairy and meat/fish, low in total and saturated fats and trans fats in addition to being physically active can result in long lasting weight loss and improved health. This doesn’t sound too exciting but it works!
Scientific Study Update: a scientific article published in the American Journal of Clinical Nutrition Aug 2004 confirmed that despite the ineffectiveness of a low glycemic index diet on weight loss, it can reduce LDL cholesterol (the Bad cholesterol) by 10%.
A Cochrane review published in January 2009 studied 11 randomized controlled trials. Researchers found that following a low glycemic index diet helps people with type 1 and type 2 diabetes to improve their blood sugar control significantly.
- Top 10 Diet & Nutrition Myths – Debunked by Dietitians
- Low-Fat vs. Low-Carb: Diets Going Head to Head
- All About Pre-Diabetes Eating – Podcast
- Low Carb Diets – Do they work?
- Trans Fats 101 – What and Where?
Gloria Tsang is the author of 5 books and the founder of HealthCastle.com, the largest online nutrition network run by registered dietitians. Her work has appeared in major national publications, and she is a regularly featured nutrition expert for media outlets across the country. The Huffington Post named her one of its Top 20 Nutrition Experts on Twitter. Gloria’s articles have appeared on various media such as Reuters, NBC & ABC affiliates, The Chicago Sun-Times, Reader’s Digest Canada, iVillage and USA Today.