Gauging the Glycemic Index and Glycemic Load
The glycemic index (GI) is a tool to measure how individual foods are expected to impact blood-glucose levels. A food is scored on a scale of 0 to 100 according to how much it raises blood-glucose levels as compared with blood-glucose levels after the consumption of 100 grams of glucose.
GI tables separate foods into three categories: low (0–55), medium (56–69), and high (70–100). High GI foods tend to be digested and absorbed quickly and cause a steeper rise in blood glucose. Low GI foods are expected to digest slowly and produce a more gradual rise in glucose levels.
Foods are evaluated one at a time. To determine the GI of a food, 50 grams of digestible carbohydrate (total carbohydrate minus grams of fiber) are consumed on an empty stomach after an overnight fast. Blood glucose is checked every 15 to 30 minutes for the next two hours. Finger-stick blood samples are monitored. The blood-glucose responses of ten people are averaged to determine the GI of each food.
According to GI tables, apple juice has a low GI, yet it is commonly used to treat hypoglycemia. Carrots have a high GI, but most people can munch them without a significant rise in blood glucose. According to GI scoring, white rice can fall in the low, medium, or high GI range. It depends on the type of rice, how it’s cooked, and even whether it’s reheated after being previously cooked. Numerous GI tables and calculators can be found in print and online, yet there are many inconsistencies in the GI scores when you compare sources. It can get pretty confusing.
Here are a few more things to consider:
- Most people do not eat one single food at a time on an empty stomach. When foods are eaten in a mixed meal that contains carbohydrate, protein, and fat, the glycemic effect differs. Meals are blended and churned in the stomach, and foods are digested together, not as separate components. The glycemic index score doesn’t predict how blood glucose will respond when foods are consumed in a mixed meal.
- The 50 grams of carbohydrate used to derive the GI score don’t necessarily coincide with normal portion sizes. A slightly rounded cup of cooked rice provides 50 grams of carb, which is a fairly common portion size. However, it may take close to 6 or 7 cups of cooked carrots to reach 50 grams of digestible carb. Most people simply don’t eat that many carrots at one time. When a typical portion of carrots is consumed, such as a half cup, blood-glucose response is quite flat.
- GI scores are based on a relatively small sample size of people.
Because the portion sizes studied were not always consistent with usual intakes, an alternate system was developed. The glycemic load (GL) is based on the typical portion sizes consumed. Glycemic load ranks the blood-glucose effect of foods when eaten in normal portion sizes, not the arbitrary 50 grams of carbohydrate used in constructing the glycemic index tables. Glycemic load has more utility simply because it reflects usual portions. For example, watermelon has a high glycemic index but a low glycemic load as long as portion size is controlled.
The question remains: Should consumers be looking at GI or GL tables before writing out their shopping lists? There’s no solid consensus among healthcare professionals. If glycemic tables help you reach your blood-glucose targets, it can be argued that they work for you. Evidence is mixed when you review study results. The ADA’s 2016 Standards of Care note the complexity of the glycemic index concept. The ADA cites several studies related to GI, but the results are mixed and inconclusive. Some studies showed that use of lower glycemic load carbs improved A1C values by 0.2–0.5 percentage points. Other studies showed no appreciable effect. It may be that portion control and exercise make a more significant impact on diabetes control than using glycemic tables.