W. Thomas Johnson
A few days ago as I was taking a break and eating a banana at my desk, a coworker came in to my office to ask a question. As she was leaving my office she pointed at the banana and said, "You know that banana has 25 grams of carbohydrate." Until then I had never given much thought to how much carbohydrate was in a banana, but her statement made think twice. Eating a banana did not seem particularly harmful, because bananas are a low fat, low sodium food that supplies potassium fiber, vitamin A and folate. In view of the recent publicity regarding the health benefits of low carbohydrate diets, I think my coworker's comment for bananas was based on the general concept that carbohydrates are bad as presented in magazines, newspapers, some popular diet books and television.
Although there are justifiable concerns regarding carbohydrate consumption, it is important to realize that all carbohydrates are not created equal. When a food is eaten, its digestible carbohydrates are converted to glucose by the body to provide energy for cellular function. Glucose released from the digested carbohydrates also causes the pancreas to secrete insulin, which promotes absorption of the glucose by muscle, fat and other cells. But some carbohydrates in food are digested and converted to glucose faster than others. To account for differences in how rapidly carbohydrates in different foods are digested, and subsequently release glucose into the blood stream, scientists have ranked foods according to their "glycemic index."
The glycemic index of a food is measured by comparing the change in blood glucose after a person eats a portion of food containing 50 grams of available carbohydrate, to the change in blood glucose after a person eats a reference food - either glucose or white bread - also containing 50 grams of available carbohydrate. The reference foods are assigned a glycemic index of 100. This means that a food having a glycemic index of 50 causes about half the change in blood glucose over a time period (usually 2 or 3 hours) when compared to the reference food.
While the glycemic index gives an indication of how a food will affect blood glucose, it is based on the amount of the food that provides 50 grams of carbohydrate, and not on the amount of available carbohydrate present in a normal serving of that food. The concept of glycemic load was introduced by scientists at Harvard University to correct for the effect of serving size on the glycemic index, because not all serving sizes of a food contain 50 grams of available carbohydrate.
Glycemic load is defined as the glycemic index of a food, times the amount of available carbohydrate in the food, divided by 100. Foods having a high glycemic load cause blood glucose levels and insulin to rise faster and higher than foods having a low glycemic load. Furthermore, the spike in insulin release caused by high glycemic load leads to a rapid decline in blood glucose, which in turn causes a feeling of hunger and the need to eat. In addition to the undesirable effect that high glycemic load has on blood glucose and insulin release, the long-term consumption of foods having high glycemic loads is also a predictor of risk for developing type 2 diabetes and heart disease.
All of this means that one should eat foods having low glycemic load, which brings me back to the banana. A banana has a glycemic index of 52 and 24 grams of available carbohydrate. This gives a glycemic load of 12. In comparison, an apple having a glycemic index of 38 and 15 grams of available carbohydrate has a glycemic load of 6. Does this mean that I was properly chastised by my coworker for eating a banana? Although an apple may be a little better choice for a snack, eating a banana isn't all that bad either because foods with glycemic loads in the low teens and below are the ones that should be selected as part of a balanced diet.
Generally, fruits, non-starchy vegetables, nuts and legumes have desirable glycemic loads. Their sugars enter the blood stream gradually and trigger only a modest release of insulin. In considering low carbohydrate diets, it is important to remember that even though fruits, vegetables, beans and nuts contain carbohydrates, they also contain important vitamins, minerals and fiber. Removing these foods from your diet to achieve low carbohydrate intake also means that you are reducing your intake of important, essential nutrients. Although foods having high glycemic loads should be eaten infrequently, foods with glycemic loads in the low teens and below contain "good carbohydrates" along with other important nutrients and should be part of a healthy diet. The glycemic loads for many foods are available on the internet and finding a "good carbohydrate" is as easy as typing either glycemic load or glycemic index into your search engine.