The idea that all carbohydrates are not created equal has become the foundation of many popular diets. Some argue that foods like white bread and potatoes, which have a high so-called glycemic index because they spike blood sugar and insulin, should be avoided in favor of more healthful carbs like whole grains and non-starchy vegetables.
But rigorous new research from the National Institutes of Health suggests that for people who already follow a healthful diet, the glycemic index may not be very important.
The study, published in JAMA, found that diets containing low glycemic foods did not lower cholesterol and other heart disease risk factors compared to diets containing mostly high glycemic foods. Nutrition experts argue that low glycemic diets improve blood sugar levels and insulin sensitivity. But the new study found that low glycemic diets actually made insulin sensitivity worse.
“The dogma out there is that a high glycemic index is bad,” said Dr. Robert Eckel, a past president of the American Heart Association and a professor at the University of Colorado, who was not involved in the research. “I hope that ultimately the glycemic index will be left on the shelf.”
Developed in 1981, the glycemic index reflects the extent to which carbohydrate-containing foods raise a person’s blood sugar and subsequent need for insulin. The idea is that low glycemic foods are better for health, warding off diabetes and weight gain, because the carbs they contain are digested at a slower rate.
According to the index, a baked russet potato has a greater impact on blood sugar, and thus a higher glycemic index, than a baked sweet potato. A plain baguette has double the glycemic index of whole grain bread, and grapes and bananas have twice the glycemic number of grapefruit.
The glycemic index indicates how quickly a particular carbohydrate raises blood sugar, but not how much of it is in a typical serving of a given food. So proponents of the index have also come up with another system, known as glycemic load, which takes portion sizes into account.
Last year, a committee of scientists led by the chair of the Harvard School of Public Health called for glycemic values to be included on food labels and emphasized in dietary guidelines. The committee said it was crucial that the public understand the glycemic response to foods “given the rapid rise in diabetes and obesity.”
But the evidence from studies has been mixed, and many question its usefulness. The 2010 Dietary Guidelines for Americans encourage people to eat whole grains and “naturally occurring carbohydrates.” The guidelines caution that “strong evidence” shows no relationship between the glycemic index of foods and body weight, even though a large clinical trial in 2010 showed otherwise.
In the new study, researchers wanted to find out whether diets that were similar in calories and carbohydrates but composed of either high or low glycemic foods had different effects on cardiovascular health. So they recruited 163 people who were mostly overweight and had high blood pressure, putting them at greater risk of heart disease and Type 2 diabetes.
The subjects were rotated through four diets for five weeks at a time, with all of their food provided to them. The researchers said they devised each diet to be heart healthy, with plenty of fruits, vegetables, beans, fish, poultry, lean meat and grains. Two of the diets were slightly higher in carbs than what the average American eats, and two were slightly lower.
The researchers then altered the types of carbs they contained. The low glycemic diets included things like whole grain bread and cereal, apples, steel-cut oats, and non-starchy vegetables. The high glycemic diets allowed things like white bread, carrot and bran muffins, instant rice and instant oatmeal, and sweet snacks like honey, bananas, and apricots in heavy syrup.
When the overall amount of carb intake was lowered, cardiovascular risk factors like cholesterol, triglycerides and blood pressure moved in the right direction. But when two diets had similar amounts of carbs and calories, the low glycemic approach did not improve insulin sensitivity, cholesterol or blood pressure levels.
Dr. Frank M. Sacks, the lead author of the study and a professor at Harvard Medical School and Brigham and Women’s Hospital, said that other trials carried out even longer had reached similar results. He said that people should eat whole grains, fresh produce and high fiber foods because of the nutrients they contain. But unless someone has diabetes and must monitor their blood sugar levels, people who are already following a healthful diet do not need to worry about the blood-sugar impact of one type of fruit or grain versus another.
“The takeaway is a good message for people,” he said. “They can pick foods that are part of a healthy dietary pattern without wondering if they’re high or low glycemic. They don’t have to learn that system.”
Dr. David Ludwig, the director of the New Balance Foundation Obesity Prevention Center at Boston’s Children’s Hospital, said that short term clinical trials of increased fiber and whole grain intake often fail to show any benefit. But public health recommendations are based on long-term trials and high quality observational studies. And unlike very low carbohydrate diets, which show quick results, low glycemic diets may require more time, he said.
“A low G.I. diet may be more like the tortoise than the hare,” he added. “It takes longer, but gets you there in the end.”
The developer of the glycemic index, Dr. David Jenkins, a professor of nutrition at the University of Toronto, added that the study was limited because it did not include people with Type 2 diabetes, a rapidly growing population that has the most problems with blood sugar control.
“If they had done that,” he said, “they would have made it easier to see some of the cardiovascular benefits.”