Sports Science Update: Revisiting The Glycemic Index

New evidence suggests the glycemic index of foods doesn’t matter as much as you’ve been told.

Written by: Matt Fitzgerald

Twenty years ago, most Americans had never heard of the glycemic index. Today, it’s a familiar concept. We all know that the glycemic index is a measure of how quickly the blood glucose level rises after carbohydrate-containing foods are consumed. We know that most vegetables and whole-grain foods are considered low to moderate glycemic, while most sugary and starchy foods are considered high glycemic. And we know that the carbs in high-glycemic foods are more likely to be converted to body fat and that, over time, eating too many high-glycemic foods increases the risk of becoming overweight and insulin resistant.

It turns out, however, that we might not know as much about the glycemic index as we think we do. Nutrition scientists are now finding that the effect of foods on blood glucose levels may have more to do with individual biochemistry than with the foods themselves. For example, the glycemic index of white bread is 70. But in a recent study involving 14 subjects, the individual glycemic index scores of white bread ranged from 44 to 132. Sure, the average score was 70, but that score was irrelevant to most of the study participants’ bodies!

What’s more, the Tufts University Researchers who conducted this study also found a high degree of variation in the blood glucose response to specific foods within individuals depending on when they ate them—as much as 42 percent variation. That means a low-fat muffin could be a low GI food for you in the morning and a high GI food in the evening!

What does this mean for you? It means that it’s rather pointless to base your food choices based on foods’ glycemic index, which represents an average value that might not apply to you.

Is GI A Stand-In For Something Else?

The health benefits of a low glycemic index diet are also being questioned. Several studies have shown that switching to a low GI diet improves insulin sensitivity in individuals with type 2 diabetes, but the effect is small compared to that of exercise. And a recent study found that 18 months on a low GI diet had no effect on weight loss in Brazilian women compared to a high GI diet. What’s more, when confounding variables such as fiber intake are removed, the glycemic index of one’s habitual diet is a poor predictor of overweight, diabetes and cardiovascular disease. In other words, eating a high GI diet is no more likely than eating a low GI diet to make you fat or diabetic or to cause a heart attack.

Increasingly, experts believe that it’s not the glycemic effect of certain foods that makes them healthy but their fiber and perhaps also their antioxidant content. For example, a large, recent Dutch study found no association between glycemic load (which factors in both the glycemic index and the total amount of carbohydrate) in the individual diet and various cardiovascular disease risk factors. There was a link between the glycemic index of the individual diet and these risk factors, but the study also found that a lower GI diet was typically achieved through all-around healthier food choices (such as more fruit and fewer sweets).

Conventional wisdom holds that a high GI diet increases metabolic disease risk by causing repeated glucose and insulin spikes. But this study suggests that the truth is less complicated. High GI diets simply tend to contain lots of high-calorie foods that make people fat. Thus, the glycemic index is really just a stand-in for other food qualities that affect metabolic disease risk: specifically, calorie density and satiety. Foods that contribute to lowering the GI of one’s diet, such as fruit and vegetables, provide more satiety per calorie, whereas foods that contribute to increasing the GI of the diet, such as sweets, are more calorie-dense and less filling.

The Glycemic Index And Training

High GI carbohydrates are actually preferable for athletes before, during, and immediately after exercise. During exercise, the muscles burn carbohydrate faster than the body can possibly absorb carbohydrates consumed in food. Consuming carbs immediately before and during prolonged exercise has been shown to enhance performance by providing an extra fuel source to the muscles. But this benefit can only be realized if those carbs are absorbed quickly. They don’t do the muscles any good if they’re just sitting around in the stomach being processed. This is why sports drinks and energy gels contain sugars such as dextrose that are rapidly absorbed.

High GI carbs are also beneficial in the first hour after exercise, because they result in faster replenishment of the muscles’ depleted carbohydrate fuel stores. Also, when high GI carbs are consumed along with protein after exercise, the muscles are able to repair and rebuild themselves faster.

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About The Author:

Matt Fitzgerald is the author of Iron War: Dave Scott, Mark Allen & The Greatest Race Ever Run (VeloPress 2011) and a Coach and Training Intelligence Specialist for PEAR Sports. Find out more at mattfizgerald.org.