For more than two decades a debate has raged over carbs and the glycemic index. Diet experts and nutritionists just can't seem to agree. Some argue that the glycemic index is a valid tool for judging the effect of foods on health and body composition. Others disagree, insisting other factors must be considered.
As a result of the commotion, as well as the popularity of certain diets, foods with a high glycemic index are often vilified, while those with a low glycemic index are endorsed. The truth is, however, much of the controversy is the result of a misapplication of the research, which, in fact, has limited relevance in the real world.
Foods with a high glycemic index are often vilified, while those with a low glycemic index are endorsed. The truth is, however, much of the controversy is the result of a misapplication of the research.
What Is the Glycemic Index?
To begin, it's important to understand what the glycemic index actually is. A food's glycemic index, or GI, is a measure of its ability to raise blood sugar. This is determined by eating a standard amount of carbohydrate, typically 50 g, in a test food. The test food is consumed after an overnight fast, and blood glucose levels are measured for two hours afterward. The glucose response to the test food is then compared to that of dietary glucose, which has a glycemic index of 100. Foods with a GI of 55 or less are considered low-GI foods. Foods that range from 56 to 69 are moderate, and those with a GI of 70 or above are considered high.
Why Is the Glycemic Index Controversial?
The GI is controversial mainly because the test results many people base their opinions on are problematic.
In the tests, a food's GI is determined by consuming it in a fasted state. This automatically calls the relevance of the results into question because the situation is outside of a realistic context. In actuality the majority of a person's day is spent in a postprandial state -- that is, a fed state -- not in a fasted state. The full digestion and absorption of a meal can take four to eight hours or more, depending on the size of the meal. The overlap of absorption from meal to meal can heavily influence the GI of individual foods.
Another issue with the studies is that the carbohydrate source consumed during the GI test is given in isolation. In everyday life, however, meals are typically a combination of macronutrients, not a single, isolated macronutrient. And factors such as the fiber, protein and fat content of one food can affect the GI of any other food it is consumed with.
To top it all off, the GI is an average response to a given food. This varies widely with food subtypes and is even affected by the way a food is cooked.
What Does the Research Say About the Glycemic Index and Health?
As far as blood markers of health are concerned, low GI diets are generally superior to high GI diets.
But once more the limitations of the research must be considered. In many cases, the total amount of carbohydrate, protein, fat and fiber are not matched, so it's impossible to determine whether the positive health effects were solely because of the low GI value of the food or to another factor, such as levels of protein, fiber or fat of the food.
When studies attempt to match macronutrient content between comparison diets, the high GI groups are usually fed an artificially high proportion of candy and processed dessert foods to fulfill the condition. In contrast, the low GI diets typically contain more whole and minimally processed foods.
So again, it's not necessarily a food's high GI that makes it a bad choice. There are healthy whole foods with a high GI, and conversely, there are junk foods with a low GI, disproving the familiar lower-is-better rule.
James Krieger, a licensed nutritionist, researcher and author, and founder of Weightology, a weight management information website, agrees that it isn't the GI alone that determines the merit of a food.
"Some energy dense, high calorie, less healthy foods are actually low GI. Like a Snicker's bar," he said. "Likewise, some low calorie, more healthy choices are high GI, like baked potatoes."
Glycemic Index and Weight Control
A common belief in some circles is that high GI foods cause weight gain. This is simply not true. A chronic excess of total calories beyond what the body can use is what causes weight gain.
The usual suspect implicated in the weight-gain effect of high GI foods is a rise in insulin levels from a spike in glucose. But multiple well-controlled studies have failed to consistently find high GI diets more fattening than low-GI diets.
In fact, the article "Glycaemic Index Effects on Fuel Partitioning in Humans," published in the May 2006 issue of "Obesity Reviews," looked at a number of studies that compared the effects of foods with contrasting GI levels. The article's authors concluded that the studies they examined did not show a difference in impact on the body's fuel partitioning and that the insulin shifts the high GI foods caused were not enough to affect fuel oxidation.
Furthermore, most of the diets used to compared GI effects have been low protein, high carbohydrate, moderate fat. This type of setup is irrelevant to the dieting population, who tends to increase protein and restrict carbohydrates.
Emma-Leigh Synnott, a doctor, researcher, and food and nutrition consultant, emphasizes some of the problems with judging a food by its GI alone, saying the food's other components, such as fiber content and vitamin, mineral, and antioxidant levels, are often disregarded.
"If you base a food solely on the GI," she said, "one food that may be completely void of any micronutrients could be deemed a better choice than a more nutrient-dense, but higher GI, food," Synnott said.
Consider, for example, a serving of full-fat ice cream and a serving of broad beans. Synnott points out that the full-fat ice cream is a low GI food, while the beans are a high GI food. And yet it's clear which food is the more beneficial choice for your general health.
Synnott goes on to warn about falling victim to the belief that eating low GI foods will keep you satisfied longer or do much in terms of weight loss. She reiterates that weight loss and the maintenance of body composition are the result of several factors, including "calorie intake, adequate protein intake, and a few specifics like the adequate intake of essential fatty acids."
"So if you keep in mind that the GI doesn't take into consideration important factors such as serving size, calorie content, protein content, fatty acid composition, et cetera," Synnott said, "then you can see some obvious pitfalls in instituting the [low-GI] diet as a weight-loss tool. Essentially, using the GI of a food to help with your diet and body composition goals can be a futile -- and pointless -- exercise."
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