Glycemic Index: Use with caution

When the GI first gained national attention, carrots were kicked off some menus, along with watermelon and bananas. Consumers were misled into believing these low-calorie, highly nourishing foods were fattening. For such high-fiber foods, it makes no sense to look only at GI when considering their impact on blood sugar.

One food, however, received more negative attention than any other. In fact, it presents an anomaly that strikes at the very heart of GI as a means of evaluating carbohydrates. The potato, a naturally nutrient-dense food with a long history of sustaining populations, was lumped in with highly processed foods because of its high GI. Ironically, potato chips have a low GI.

Science vs. philosophy

High-GI foods spike insulin levels and leave us hungry soon after we eat. Or so it is said. But a 1995 study tested 38 foods for their ability to satisfy hunger. Data were calculated as with glycemic index and compared to a standard — white bread — which was assigned a value of 100. A score of 150 means the food is 1.5 times as satisfying as white bread. By these standards the most satisfying food is boiled potatoes at more than 300, more than three times that of white bread, although with a similarly high GI ranking.

The study found no significant relationships between blood glucose, insulin response and satiety among the 38 foods tested. Moreover, there was a tendency for subjects to eat less food two hours after eating the high-GI foods, calling into question the very premise upon which the link between GI and obesity is founded, that low-GI foods are more sustaining. A 2003 study found that a low GI value was not associated with a slow digestion of carbohydrates.

In yet another study, reported last December, results from nearly 1,000 adult volunteers responding to a questionnaire showed no association of glycemic index, glycemic load or carbohydrate intake with insulin sensitivity and insulin secretion. However, higher fiber intake, not GI, was positively associated with insulin sensitivity, reduced body fat and a healthy insulin response.

GI came on the heels of two diet fads: the low-fat fad and the low-carb craze. Supporters of GI view it as a more balanced and reasonable approach to health. But others point out the incongruity created by forcing GI values to be predictive of weight control and health.

"GI has enjoyed increasing popularity despite the lack of research to support its efficacy as a dietary tool for weight loss, disease prevention or health promotion,” says Beals. “Until large-scale studies are done using GI in a variety of circumstances and disease conditions, Americans should strive to follow the recommendations set forth in the U.S. Dietary Guidelines."

Is the appeal of GI in the science or the philosophy? Some healthy carb sources — fruits, vegetables, whole grains and beans — happen to have a low GI, and many nutrient-dense carb sources — potatoes, carrots, dried fruits and watermelon — have a high GI. Are there lessons locked in the often contradictory GI numbers? Or has the attention GI has gathered, pro and con, unveiled a serious need in the modern diet for more focus on an overall healthy eating philosophy and less attention to numbers?