Unless you have diabetes or peruse nutrition literature for fun, you probably wouldn’t be able to explain exactly what the sciencey-sounding term “glycemic index” means. But chances are, you’ve heard the phrase and maybe even seen it on food labels, and wondered if it was something worth paying attention to.
“Traditionally, the glycemic index has been something we primarily use for people with diabetes,” Colleen Tewksbury, Ph.D., M.P.H., R.D., a senior research investigator and bariatric program manager at Penn Medicine and president-elect of the Pennsylvania Academy of Nutrition and Dietetics, tells SELF. “But most individuals without diabetes find the same principles apply to them and understand the concept intuitively.”
Here’s what the glycemic index actually is, the science behind it, and how it may be useful for you.
What the glycemic index actually is
Whenever you eat carbohydrates, your body gets to work breaking them down into its favorite form of fuel, a simple sugar molecule called glucose, which enters the bloodstream and raises the concentration of glucose in your blood. Your body has an organ called the pancreas, and when you eat carbs, it secretes a hormone called insulin to help the glucose molecules exit the bloodstream and enter your body’s cells so they can use them as energy.
How quickly a carb gets broken down into these little glucose molecules varies widely depending on the kind of carb it is, and what other nutrients you’re consuming with it. Something that’s more or less all sugar, like soda or fruit juice, is already pretty close to pure glucose, so it enters your bloodstream almost instantly, spiking your blood sugar levels. More complex carbs like an apple or a slice of whole grain bread contain starch, a more complicated structure (also a carb) that takes more work to break down to glucose, along with fiber, which slows down the absorption of glucose into the bloodstream, as SELF previously reported. As a result, these foods end up causing a more gradual change in your blood sugar levels.
The glycemic index, or GI, is a simple tool to help people evaluate how quickly a carb will enter their bloodstream by assigning it a number between 1 and 100, according to the Boden Institute of Obesity, Nutrition, Exercise and Eating Disorders and Charles Perkins Centre at the University of Sydney, which maintains the largest official international GI database. This value is determined based on how quickly the carb raises people’s blood sugar on average, Whitney Linsenmeyer, Ph.D., R.D., nutrition and dietetics instructor in the Doisy College of Health Sciences at Saint Louis University and spokesperson for the Academy of Nutrition and Dietetics, tells SELF.
A food with no carbs and little-to-no blood sugar impact (like butter or meat) would score zero, while pure glucose ranks at 100, the Boden Institute says. High-GI carbs (over 55) are digested, absorbed, and metabolized the fastest, so they cause a rapid response in blood sugar and insulin secretion, while low-GI carbs (under 55) have the opposite effect, causing minimal impact on your blood sugar, Linsenmeyer explains. So if you were looking at a graph charting your blood sugar and insulin response after eating a carb, a low GI carb would cause a relatively long, gradual curve, while a high-GI carb would cause a more dramatic spike.
What determines a food’s glycemic index
The GI of a food is determined by a bunch of factors. Some of them are unsurprising, like fiber content and fat content, since these nutrients are known to slow down the rate at which carbs break down to glucose and enter the bloodstream. Others are more unexpected.
For instance, the more a food has been cooked or processed, the higher the GI generally is, the American Diabetes Association (ADA) explains. That’s because heat and physical refining have already begun to do some of the work of breaking down the carbohydrate structures, which shaves off the time your body has to spend doing that, Tewksbury says. So, for instance, mashed potatoes will have a higher GI than a whole baked potato, which will have a higher GI than a raw potato. Even al dente pasta will have a slightly lower GI than well-cooked pasta, according to the ADA.
When it comes to fruits, the length of time they’ve spent ripening, the higher their GI is, per the ADA. (Ripening is essentially a process of fruit starches breaking down into sugars, Tewksbury says, which explains why a green banana is hard and meh-tasting, and a ripe banana is soft and sweet.) Acid content is another one—the higher the acidity, the more slowly sugars hit the bloodstream (so the lower the GI), per Merck Manuals.
To calculate the specific GI value of any one food, researchers measure a group of 10+ people’s blood sugar response for two hours after eating 50 carbs of the test food, and compare it to the group’s blood sugar response to 50 carbs of the reference food, pure glucose, over two hours, the Boden Institute explains.
One issue that’s come up in comparing the GI of different foods is the fact that, in real life, many foods don’t typically get eaten in whatever quantity equates to 50 carbs. Take watermelon, for instance. The GI value is 80, but you’d have to eat a whole lot of watermelon to consume 50 carbs’ worth. So scientists came up with a related value called the glycemic load, which reflects how much the typical serving of a food impacts blood sugar (as opposed to 50 carbs’ worth), the Mayo Clinic explains. While glycemic load is useful in practice, GI is generally is more widespread, which is why we’re sticking to that here.
Why the glycemic index even matters
Because people with diabetes either don’t make their own insulin and have to inject it themselves (in the case of type 1) or don’t respond well to the insulin their body does make (in the case of type 2), they have to monitor and manage their blood glucose levels in order to avoid having high or low blood sugar. So eating low-GI foods can be helpful in maintaining steady blood sugar levels. Although people with diabetes tend to experience bigger spikes and drops in their blood sugar and feel the symptoms of them more acutely, people without diabetes can also feel the effects of rapid rises and falls in their blood sugar caused by high-GI foods, Tewksbury says, like tiredness and hunger.
“People can pick up on those feelings of their blood sugar rising and then crashing afterwards, so they can identify those high-GI foods and how they make them feel on their own without knowing the science behind it,” Tewksbury says. “They’ll notice that low-GI foods like oatmeal keep them fuller longer, and that high GI foods like cake keep them less full for not as long,” she explains. Or you might notice that adding peanut butter to a slice of toast keeps you fuller longer than the bread alone.
The evidence that a low-GI diet works (for some people)
As we mentioned, diets based on the glycemic index can help people with type 1 and type 2 diabetes maintain blood sugar control, according to the U.S. National Library of Medicine. This comes as no surprise given these diets encourage the foods that have a more gradual and predictable effect on blood sugar and discourage the foods that cause blood sugar to spike drastically.
Diabetes isn’t the only medical condition that’s been shown to benefit from a low-GI diet, though. There’s some pretty strong evidence that if you have high cholesterol or are trying to lose weight for health-related reasons, a low-GI diet may be more helpful than other types of diets.
A 2007 Cochrane review of six randomised controlled trials (RCTs) containing 202 overweight or obese participants found that people who followed a low-GI diet saw a greater decrease in their body weight, fat mass, total cholesterol, and LDL cholesterol than people who followed a higher-GI diet. (The study lengths ranged from five weeks to six months, with follow-ups as long as six months afterwards.) And the studies that compared people on low-GI diets without calorie restrictions to people on low-fat, calorie-restricted diets found that the low-GI dieters did as well or better on all these same measures of body weight and lipid profiles—even though they could technically eat as much as they wanted (as long as it was low-GI). Necessary disclaimer here, as with all reporting on weight loss: The research shows that the “best” diet for losing weight and maintaining that weight loss is the one that you’re likely to stick to for the long-haul, which is significantly easier said than done. And an increasing number of experts believe now that focusing on weight loss as your primary goal can be counterproductive and possibly even harmful, and that you might be better able to improve your health outcomes by focusing on other objectives—like managing your blood sugar or cholesterol, for instance.
The glycemic index’s limitations
It’s important to remember that it almost never makes sense to base your diet or approach to food off one single element. In the same way a low-fat, low-calorie, or low-carb diet is not automatically healthy, neither is a low-GI diet.
There are lots of high-GI foods that are very nutritious, and lots of low-GI foods that are not necessarily such great choices. For instance, “almost anything deep fried is going to be lower GI because of the large fat content,” Tewksbury explains. “That doesn’t mean it‘s going to be a healthier choice than a piece of fruit.”
Meanwhile, low-sugar versions of candy bars advertised to people with diabetes as “low-GI” will typically have the same amount or more calories and are often higher in fat, Tewksbury says. So if you have diabetes and avoiding a blood sugar spike is a priority for you, then it makes sense that you might want to reach for that product over a regular candy bar—but know that it’s not really offering any additional benefits.
But even for people with diabetes, a low-GI diet is not necessarily the best choice for everyone. As the American Diabetes Association (ADA) points out, carbohydrate-counting is still the most important blood-sugar management tool. (And some research indicates that the overall amount of carbs in a food is more important for predicting blood sugar response than the GI of the food.) So the ADA suggests thinking of the GI as a way to help fine-tune blood sugar management, and using whatever meal plan fits into your lifestyle and helps you meet your goals.
That applies to everyone. Whether you find the glycemic index helpful or not, it’s the overall quality and variety of foods you eat that’s most important.