Diet and exercise have always been key factors when striving for a fit, disease-free lifestyle. In recent years, carbohydrates have gotten a lot of attention too. Eating plans like the South Beach Diet or Atkins are designed to help people lose weight by limiting carbohydrate intake. These popular diets are based on what is known as the “glycemic index,” a tool that was developed by researchers about 20 years ago to measure how much a carbohydrate-containing food is likely to raise blood sugar. But is the glycemic index (GI) a scientific instrument for designing a healthy meal plan, or simply a nutritional catch-phrase?

“The GI gets a lot of attention, but it should probably not be used to design a meal plan because there are too many confounding factors that affect the GI of a food,” says Keith-Thomas Ayoob, Associate Professor in the Department of Pediatrics at Albert Einstein College of Medicine. “You may know the GI of a food, but that tells you nothing about the GI of a meal.”

All foods that contain carbohydrates, from fruit to pasta, can be tested for how they affect blood sugar levels after they’ve been eaten. The glycemic index is determined by having people eat a specific amount of a single food, and then measuring the change in blood sugar levels compared to levels achieved when those same people eat a control food, like white bread or pure glucose. Pure glucose has a glycemic index of 100, while an apple measures 40. A carbohydrate has a low GI if it falls at 55 and under, medium at 56 to 69, and high at 70 and above.

There is also the related concept of glycemic load, or GL, which considers both the glycemic index and the amount of carbohydrate in a serving of food. Glycemic load is calculated by multiplying the grams of carbohydrate in a serving of food by that food’s glycemic index – an approach that is somewhat more accurate for determining a food’s effect on blood sugar. While boiled carrots have a high glycemic index (92), they contain far fewer carbs (8 grams) than many other items in a single serving. The glycemic load of carrots (8 x .71) is 6. By comparison, white bread, with a GI of 71 (and 13.5 grams of carbs), has a glycemic load of 9.5.

Although these calculations seem straightforward, they are anything but. Many variables can affect GI results, and therefore GL results as well. The ripeness of a food, its variety and how it is cooked are all factors that influence a food’s glycemic index. So, it’s actually possible that a banana could have one GI in the morning, and a very different GI in the afternoon. In addition, GI changes based on how a particular food is consumed. A high GI food like jam, combined with a low GI food like high fiber wheat bread, will produce a more moderate response in blood sugar levels than consuming a spoonful of jam on its own. The GI of pasta varies according to the shape of the pasta, how long it is cooked, and what it is served with. In other words, foods consumed as a single snack – versus the same foods consumed as part of a meal – will score very differently.

“Sound complicated?” Ayoob says. “It is. Which is why it is very difficult to teach to the general public.”

And what about the relationship between GI and diabetes? Some theories relate the reduction of GI and blood sugar levels to the reduction of insulin levels and subsequent contribution to weight loss. The American Diabetes Association, however, does not support this claim. Instead, they recommend sticking to current total carbohydrate monitoring, and focusing on foods with good nutrient density – or foods with a favorable amount of nutrients in proportion to their caloric content. Fruits, vegetables, whole grains and legumes all fall into this category, and interestingly, all have a naturally low glycemic index.

"The GI may help with fine tuning, and may help someone eat more whole grains and whole foods in general, however, someone using the GI exclusively may omit foods unnecessarily that they could consume in moderation,” Ayoob says. “The GI can give some general guidelines, but it’s still going to come down to this: eating the right foods, and eating the foods right.”

Ayoob says that for now, there is little evidence that the GI provides an advantage for most Americans over the general recommendation to consume whole grains, fruits, and vegetables, along with lean meat and moderate, healthy fats.

He adds, “Nutrition is a dynamic field, not a static one. We need to be open to the results of well-done research as it becomes available.”