Edited by Kate Findley, The Great Courses Daily
Not all carbs are created equal, and whole grains are almost always better than refined grains and sugars. The glycemic index, though, offers a precise tool for evaluating carbohydrates. Professor Anding explains.
The Glycemic Index
A new carbohydrate evaluation tool called the glycemic index has emerged over the past few years. It provides insight into the body’s use of carbohydrates; but more specifically, it indicates how quickly your blood sugar rises after the ingestion of a particular food.
Most carbohydrates are absorbed in a mono- and disaccharide form (the simplest forms of sugar), so when it reaches the blood is what impacts blood sugar. As we digest these foods and break them down into their component parts, they reach the bloodstream at variable rates.
The glycemic index measures how quickly your blood sugar rises after the ingestion of a particular carbohydrate food. Each food is compared to its reference standard, which is either white bread or pure glucose.
When you look in your guidebook, you need to pay attention to the reference food to which the glycemic index is compared. One of the problems we have in implementing the index is that we’re not universally using the same reference food.
Although somewhat controversial and complex, major studies indicate that those who eat a diet of high glycemic index carbohydrates have the greatest risk of many chronic diseases. That means that the carbohydrate in that food was dumped into the blood supply at a rapid rate, and your body produces insulin to metabolize or to utilize that particular monocarbohydrate.
Insulin is an anabolic hormone. Anabolic is the phase of metabolism where molecules are built up and organized. The development of heart disease, high blood pressure, and cancer are all thought to have this anabolic or insulinogenic response.
However, scientists know that many factors influence the glycemic index. Some foods have a mixed content of carbohydrate, protein, and fat. For example, the glycemic index of white bread might be high, but when you mix it on a hamburger bun with protein or fat, it leaves the stomach slowly and therefore exhibit a lower glycemic response.
The organization of the carbohydrate also has an effect. If it’s in a phenyla, or long straight chain, it’s going to take longer to break down. If the carbohydrate is arranged in an amylopectin, or branched chain configuration, it can raise the blood sugar more quickly.
For example, many of the glycemic index tools online will list a different value for a white potato and a red potato. The red potato has more phenyla, and the white potato has more amylopectin, so it changes the glycemic index.
Classifying Glycemic Index
Food such as oatmeal, most whole grains, non-tropical fruits like apples and pears, legumes, dried beans and peas, and minimally processed foods, in general, have a lower glycemic index.
The International Table of Glycemic Index and Glycemic Load Values from the American Journal of Clinical Nutrition provides a glycemic index of some processed foods versus non-processed foods. For example, the glycemic index of oatmeal is 58.
The value for cornflakes is 81. Since oatmeal has a lower glycemic index, it is going to have possibly better health benefits than eating cornflakes, even if the carbohydrate levels are identical, in terms of how the body metabolizes or processes that carbohydrate.
Under fruits and vegetables, for example, apples are 38, and a baked potato is 85, which is a significant difference. Lentils have an index of 29, angel food cake is 67, and jelly beans are 78.
The dietary guidelines for Americans encourage consumers to eat minimally processed foods and limit sweeteners in their diet. However, most public health organizations within the United States have stopped short of endorsing the glycemic index as a way of evaluating food, partially because of the complexity of that guide.
Many studies have shown a relationship between high glycemic index foods and hypertension, cataracts, obesity, heart disease, and diabetes. The challenge is for those in public health to translate those guidelines so that consumers can use them. Simple suggestions are increasing fresh fruits and vegetables, whole grains, and complex carbohydrates.
There are times, though, when a high glycemic index food is desirable.
“For example, in the athletic world, where I want that carbohydrate to get back in that muscle as fast as I can get it there,” Professor Anding said. “Why? Because maybe I have to run another race in two hours and I need to make sure I’ve got adequate carbohydrate for exercising muscle.”
Fueling the body is a primary function of carbohydrates. When training Rice University athletes, Professor Anding said that she has even given them sports drinks and even jelly beans in between races.
However, major studies indicate that those who eat a diet with a high glycemic index as a chronic diet—meaning they’re not using it for athletic performance—experience more issues with chronic illnesses.
Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.