Twenty years ago, most Americans had never heard of the glycemic index, which is, of course, a measure of how quickly the blood glucose level rises after carbohydrate-containing foods are consumed. Researchers began to focus on the glycemic index in the early 1980s. They found that the body processes equal amounts of high GI and low GI carbs quite differently, and that these differences might have important implications for health. Their excitement slowly leaked out of the laboratory into society at large.
In 2002, with the publication of The New Glucose Revolution, the glycemic index burst into the collective consciousness as the low-carb diet craze sank toward its inevitable demise. Diet-crazy America latched onto the glycemic index is the new skeleton key of weight management. The New Glucose Revolution and the many similar books that followed it taught us that high-glycemic foods increase appetite, causes carbohydrate cravings and sugar addiction, promote fat storage and lead to the development of diabetes.
There was never much proof that any of this was true, but subsequent research has made it quite clear that the glycemic index is a borderline-useless tool for weight management. Here’s the truth about the glycemic index:
The Real-World Factor
One of the most obvious faults in the system that is used to generate GI values is that it poorly matches real-world eating patterns. First, GI scores are determined in a fastest state, whereas you and I seldom eat carbohydrate-containing foods in such a state. This is significant, because what you ate in your last meal can affect how your body processes the carbs you eat in the next. Second, carbohydrate-containing foods are consumed alone when GI scores are determined, but you and I seldom eat a single food in meals. Coca-cola has a very high glycemic index score, but if you consume a Coke with a hamburger and french fries, you’re having a low glycemic index meal. Third, glycemic index values are determined with equal amounts of carbohydrate in all foods. But since foods have wildly variant carbohydrate content, testers have to eat huge amounts of low-carb foods versus tiny amounts of high-carb foods in testing, which is not necessarily what we do at home.
No Hunger, No Cravings
Most dieters still believe that the drop in blood sugar that follows the spike in sugar that follows consumption of a high GI food (or “rebound hypoglycemia”) triggers hunger. It does not. Research has shown that the link between blood sugar levels and hunger is relatively weak compared to the volume of food in the stomach. A 2005 study by Brazilian and Indian researchers found no difference between the effects of high and low GI foods on hunger levels.
Nor does the phenomenon of rebound hypoglycemia trigger a specific craving for carbohydrates, as many dieters believe. In fact, a recent Tufts University study suggests that nobody ever craves carbohydrate specifically. Rather, the body craves calories, and it so happens that many high-calorie foods get most of their calories from carbohydrate.
Not a Better Diet
The promise of the various low-glycemic popular diets is that by ridding your diet of high GI foods you will crave sugar less and be less hungry and consequently lose weight. However, several studies have shown that there is no association between the glycemic load of one’s diet and body weight. If you simply lower the GI of your diet without consciously eating less or moving more, you will not lose weight.
No Diabetes Link
The popular mythology of the glycemic index also includes the notion that the repeated insulin spikes caused by eating high GI foods eventually causes the pancreas, which produces insulin, to “wear out,” leading to the development of type 2 diabetes. However, most studies have found no link or, at most, a weak link between consumption of high glycemic foods and diabetes.
In any case, any real correlation between consumption of high GI carbs and type 2 diabetes risk is almost certainly not indicative of a causal connection. Rather, the real causal connection appears to be between fiber intake and diabetes, and it so happens that high-fiber foods typically have low glycemic indices. This connection was clearly demonstrated in a study linked to the gigantic Nurses Health Study. In a population of 284,000 women it was found that the more whole grains there were in the diet, the lower the risk of type 2 diabetes was, regardless of what else was eaten.
Type 2 diabetes is not a disease of impaired insulin production caused by poor diet; it is a disease of impaired tissue insulin sensitivity caused by overweight and inactivity. This fact is clearly demonstrated by the fact that most cases of type 2 diabetes are fully reversible through exercise. That’s because exercise promotes weight loss and increases tissue insulin sensitivity. Indeed, a recent study found that physical fitness effectively lowers the glycemic index of any given food. Which brings me to my final point…
The Individuality Factor
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!
Athletes Need High GI Foods
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.