Is the glycemic index reliable and of any use?

The glycemic index has crept upon our food experience. Once a term used by people in white lab coats, reserved to diabetes clinics, it’s now part of popular weight-loss programs (South Beach, Zone) and disease prevention plans; the low glycemic index claim is now a selling point plastered on food labels.

Haven’t immersed yourself in the glycemic index yet? It’s never too late. Or maybe it’s still too early. We’ll get to that in just a bit, but first, a little introduction.

Glycemic index, explained

The glycemic index, developed initially for the treatment of diabetes, ranks carbohydrate-containing foods based on how they affect blood glucose levels when eaten in isolation. The glycemic index charts were developed by testing people’s blood glucose after ingestion of 50 grams (almost 2 ounces) of carbs from different foods.

Carbs that break down quickly, like white bread and potatoes, have a high glycemic index, and those that release sugar into the blood stream more gradually, like fruit, have a low glycemic index. Foods are scored on a scale of 0 to 100.

Since blood glucose fluctuations elicit a cascade of hormonal reactions, which can potentially affect satiety, hunger, blood lipids and inflammation, the glycemic index of a meal sounds like useful thing to control.

We’re all different

A new study, led by Nirupa Matthan from Tufts Medical Center, and published in the American Journal of Clinical Nutrition, looked at the blood glucose reaction of 63 healthy people after they were given 50 grams of glucose, either in white bread or in a glucose drink – each experiment was repeated in each volunteer twice.

This is basically a testing of the glycemic index of the bread, and if the glycemic index is a characteristic of the food itself, the results should be similar between the volunteers and between tests.

But that’s not what the researchers found. There was a 20 percent change in measured glycemic index of the exact same food between tests in the same individual, and there was 25 percent difference between people.

The glycemic index of the same exact bread ranged from 35 to 103, making it a both low and high on the glycemic index. 

In other words glycemic index wasn’t at all consistent, even when the food was eaten in isolation and in a controlled environment.

Is the glycemic index useful?

According to this new study, the glycemic index is not a dependable measurement.

It is also just partial data.

Eating 50 grams of carbs from white bagels is doable. But when it comes to other high glycemic foods, watermelon, for instance (glycemic index - 80) a normal serving will have just a few grams of sugar; you’d have to eat almost 2 pounds of watermelon to load yourself up with 2 ounces of carbs, and therefore, in reality, watermelon’s high glycemic index is quite irrelevant. To anticipate glycemic responses you’d also have to familiarize yourself with glycemic load – which also takes into account the amount of carbs consumed.

On top of that, although the type and proportion of sugar or starch in a food affects glycemic index, so do a lot of other factors in the meal—the way the food’s been cooked, the presence of fiber, protein, fat, alcohol or acid will all affect the glucose response. Exercising and activity around a meal will affect it, too.

So is it worth bothering with? At this point the evidence of glycemic index centered diets’ helpfulness for weight loss or disease prevention is inconsistent (it is helpful for the management of diabetes.)

If you don’t have diabetes, it’s much easier to follow the well-proven guidelines of limiting added sugar (sugary drinks are especially harmful), eating more plants and whole grains, and minimizing hyper-processed foods.

These simple principles happen to also yield a low glycemic diet – they are a practical shortcut to the same end.

Dr. Ayala