June 11, 2002 – NY TImes
Fear Not That Carrot, Potato or Ear of Corn
By JANE E. BRODY
When someone at a party said, “I don’t eat carrots — they have too much sugar” and his companion replied, “I don’t eat corn or potatoes for the same reason,” I realized there was an urgent need to put the so-called glycemic index into proper perspective before anyone else decided to avoid delicious, nutritious foods packed with health-protective substances.
The glycemic index is a measure of how much carbohydrate-containing foods raise a person’s fasting level of blood glucose and consequent need for insulin in two hours.
According to this index, carrots have a value above that of table sugar. Instant rice, baked potatoes, corn flakes, watermelon and white bread are even worse. A baked potato is on a par with a Mars bar. But does this mean these foods are to be avoided at all costs, and especially by people with diabetes?
The answer, according to those well versed in the field, is “not at all.”
Dr. Daniel Nadeau, an endocrinologist and medical director of the Health-Reach Diabetes Center in Hampton, N.H., said in an interview: “The glycemic index is best used as a research tool. It has tragically given foods like carrots, brown rice and corn a bad name. People should eat the whole range of colorful low-calorie foods and leave the glycemic index in the laboratory where it belongs.”
Others, like Dr. David S. Ludwig, director of the obesity program at Children’s Hospital in Boston, maintain that the glycemic index has value in helping people choose healthful diets, but that in doing so it is important to appreciate its limits, prime among them that the index does not take into account the caloric density of a food.
A glycemic index ranking is defined as the blood glucose response after a person consumes 50 grams of available carbohydrates in a food. Depending on what else is in the food, including water, it can have more or less carbohydrate per serving than a standard reference food like white bread or sugar. Thus, even though carrots have an index number of 71 and sugar has 65, it would take six or seven servings of carrots to match the blood glucose effect of one-fourth cup of sugar. That is because measure for measure, sugar has far more carbohydrate.
Looked at another way, reaching a caloric intake of 400 calories requires eating 12 servings of carrots or four five-ounce potatoes or three cups of corn — but only one Mr. Goodbar. You don’t have to know much about nutrition to realize which of these foods more quickly satisfies an appetite.
Furthermore, even if the raw ingredients are the same, highly processed foods typically have much higher glycemic index numbers than foods that are minimally or not at all processed. Thus, coarse stone-ground flour has far less effect on blood glucose than refined white or even finely ground whole wheat flour, Dr. Ludwig said. And steel-cut oats have a much lower index number than the more processed quick-cooking or five-minute oats.
In addition, factors other than the nature and amount of carbohydrate in a food can influence its glycemic effect. Since the blood glucose response is determined by how rapidly a food is digested and absorbed, a densely formed food like pasta has a much lower glycemic index number than fluffy white bread, and foods like ice cream that contain fat, which slows absorption, have lower numbers than some fruits, which contain little or no fat.
A more helpful gauge is a value like the glycemic load, which refers to the blood glucose effect of a standard serving of a food rather than a fixed amount of carbohydrate. The glycemic load value for carrots puts them in a far more favorable light, on a par with milk and well below spaghetti, apples and even lentils.
But for now, tables of glycemic load are not generally available to the public or to health experts who dispense dietary advice.
Value of the Glycemic Index
Still, useful lessons can be learned from the existing glycemic index, which Dr. Ludwig described recently in The Journal of the American Medical Association and in an interview. The glycemic index and glycemic load of the average American diet, he said, “appear to have risen in recent years because of increases in carbohydrate consumption and changes in food-processing technology.” These increases, in turn, could account in part for rising rates of obesity and diabetes among Americans of all ages.
Since foods are more often consumed as complete meals rather than eaten in isolation, it is necessary to consider all the kinds and amounts of carbohydrate-containing foods a person consumes at one time.
The blood glucose response, Dr. Ludwig pointed out, can be twice as high after a meal loaded with high-glycemic-index foods than a meal with low ones. Thus, a large mixed salad with a slice or two of turkey on stone-ground whole-grain bread would have minimal effect on blood sugar and insulin demand, whereas a snack of a cookie and soft drink would have a major effect.
Why is this important? Dr. Ludwig explained that a sharp rise in blood glucose results in a surge of insulin to restore a normal blood glucose level. Insulin is a hormone with many roles. In addition to promoting entry of glucose in body cells, it fosters the deposition of dietary fat into fat cells, a process that can lead to bulges in the wrong places.
As if that were not bad enough, the common result of an insulin surge is a drop in blood glucose to below-normal levels, and those low levels set off hunger signals. But even after enough carbohydrate is consumed to restore the levels, Dr. Ludwig said, hunger commonly persists, and people overeat.
A Bad Cycle
That overeating, in turn, stimulates the release of more insulin and sets off a cycle that can end in obesity. In addition, excess demands for insulin resulting from too many high-glycemic-index meals may eventually exhaust the pancreatic cells that produce this hormone, resulting in diabetes.
Two of three observational studies of tens of thousands of people found that the risk of developing diabetes was greatest among those who consumed the highest glycemic load.
High glycemic foods may also increase cardiac risk, Dr. Ludwig said, because high blood glucose levels after meals can cause oxidative damage and inflammation that result in increased blood pressure and clot formation and reduced blood flow.
Further, people who follow diets with low glycemic indexes have lower blood levels of triglycerides and harmful low density lipoprotein (L.D.L.) cholesterol and a healthier ratio of total cholesterol to protective high density lipoprotein (H.D.L.) cholesterol, Dr. Ludwig noted.
Although diets high in protein and fat have minimal effects on blood glucose and insulin demand, Dr. Nadeau, Dr. Ludwig and responsible nutritionists do not endorse such plans because they are extremely limiting and deprive people of foods rich in micronutrients, phytochemicals and dietary fiber, all of which protect health.
Dr. Ludwig says he eats potatoes once or twice a week. As a side dish of an otherwise low-glycemic-index meal, foods like potatoes and rice are fine, he said. And no one should be deprived of a wide range of colorful fruits and vegetables, regardless of their glycemic index value. Nearly all have a low glycemic load, so when eaten as part of meals, they may have an effect on blood sugar comparable to that of foods with low glycemic index values.
Copyright 2002 The New York Times Company