Glycemic Index – a reliable indicator?
Dear Coach,
I’m re-thinking my mashed potatoes for Thanksgiving. I’d like to make them, but I’ve read that they are bad for us because they’re high on the glycemic index. Doesn’t that mean they’re high in sugar? Vicky
Dear Vicky,
I have good news for you. Make those mashed potatoes….just be careful how you top them! The Glycemic Index (GI), introduced by David Jenkins of the University of Toronto in 1981, measures the rise in blood sugar over the two to three hours following the consumption of an amount of food that contains 50 grams of carbohydrate. The rise is then compared to the rise in blood sugar caused by eating white sugar or white bread. The final result, the GI, is expressed as a percentage. Carbohydrate foods that break down quickly during digestion have the highest GI. Meat, poultry, fish, cheese and eggs do not have GI values because they contain little or no carbohydrate.
Sugars actually have a very low GI; if you chose foods based strictly on their GI, you would be choosing M&Ms (33) or Nestlé’s Quick (35) over parsnips (97); chocolate cake (38) instead of carrots (92)—or cheese-topped pizza (30) rather than potatoes (85). Corn chips would score better than watermelon, and ice cream would score better than wheat bread. Candies and ice cream are high in simple sugar and fat content, so they’re absorbed from the gut into the body slowly; therefore these widely acknowledged “junk foods” have lower GI values than so-believed “health-foods,” like whole grain breads and brown rice.
The use of GI as the only factor in determining the value of a food is impractical, since low GI does not necessarily equate to healthy food. There are many other qualities of a food that are important, such as the starch structure, fiber content, food processing and other macronutrients in the meal. Also, only one food is tested at a time, and that’s not how people eat. For example, a baked potato topped with chili or salsa would have a totally different GI than if the potato were consumed alone. No one has tested the infinite combinations of foods that comprise our daily meals. Blending or grinding a food disrupts the fiber, making carbohydrates more easily absorbed, thereby increasing the GI. Food ripening affects the GI. Cooking and storage change the GI of foods. And, it is almost impossible to determine the GI values for foods prepared outside the home. Therefore, even with great effort you will likely be way off your target GI.
Theoretically, low GI foods can lower glucose and insulin responses, improve lipid profiles and increase insulin sensitivity. Thus, some researchers believe that the glycemic index has a significant influence on the risk of heart disease, diabetes and obesity. However, much research shows otherwise. A recent study shows the futility of using glycemic index to reduce the risk of cardiovascular disease or insulin resistance. Researchers enrolled 163 overweight patients in a 5-week controlled feeding study. They were assigned to one of four diets which covered meals, snacks, and calorie-containing beverages. Once the participants completed one 5-week diet they were switched to another. All participants ate at least two of the four diets.
The researchers reported that diets including low GI carbohydrate foods did not improve insulin sensitivity, lipid levels, or blood pressure as compared to high-GI carbohydrate diets. They concluded, "In the context of an overall DASH-type diet, using glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance."
Researcher Lawrence Appel stated in an interview that studies that looked at the relationship between low GI foods and weight loss reported inconsistent results and that using GI as an anti-obesity strategy was "unwarranted." He suggested that people should "Get back to the basics," which include eating whole plant foods, and avoiding sweets and foods high in saturated and Trans fat.
We also know from a mountain of research that FAT, not carbs, causes diabetes. That’s why avoiding healthy high GI foods in favor of a low carb, high protein, high fat diet would be unwise. The high GI carbohydrates found in potatoes and carrots don’t make them any more unhealthful than do the simple sugars found in fruit. But the topic of diabetes will be for another column. Until then, enjoy your starches.
(Sources: Sacks F, Carey V, Anderson A et al. "Effects of High vs Low Glycemic Index of Dietary Carbohydrate on Cardiovascular Disease Risk Factors and Insulin Sensitivity." JAMA 2014;312(23):2531-2541; https://www.drmcdougall.com/misc/2006nl/july/glycemic.htm; wellness forum health briefs, glycemic index; Annie-Rose Harrison-Dunn "No clear benefit of low GI diet for heart health, say 'surprised' researcher." Food Navigator December 17 2014)