Just when you think you’ve figured out all the buzzwords for diets (low-carb, high carb, low-fat, high-protein), a new one gets thrown into the soup.
The latest term that’s making people reevaluate the content of their kitchen pantries (and their soups!) is glycemic index. It’s a way of ranking foods—primarily carbohydrates—by how quickly they raise levels of blood sugar and insulin (the hormone that helps convert blood sugar to energy) in the body.
Proponents of the glycemic index claim that foods that cause blood sugar and insulin to spike (those with a “high-GI” ranking) stimulate hunger and inhibit the body’s ability to burn fat. They also argue that a low-GI diet not only helps people maintain a healthy weight but also prevents heart disease, diabetes, and certain cancers.
Critics, however, say making dietary changes based on the glycemic index is an interesting, but unproven, idea. “It’s certainly true that certain food types will influence malfunction and absorption of nutrients,” says Steven Smith, MD, an endocrinologist at the Mayo Clinic in Rochester, Minnesota. “But glycemic diets are no more beneficial than any other diet if you don’t contend with your behavior.”
Sorting fiction from fact about glycemic index diets is a bit challenging. Here’s what the glycemic index is—and what’s currently known about its effects on health.
How it works
The glycemic index essentially divides carbohydrate foods into two categories: those, like white bread, that dramatically affect blood sugar and those, like whole oats, that don’t. Glycemic index (GI) values are relative: Each food is compared to a “reference” food (usually white bread or glucose) that’s been given an arbitrary value of 100. An apple, for example, has a GI score of 40, which means its carbohydrates are absorbed into the body at a pace 40 percent lower than the carbohydrates in white bread or glucose.
Low-GI foods are defined as having a score of 55 or below. High-GI foods—70 or higher.
Low-glycemic foods (1-55)
sweet potatoes, beans (except for broad beans), lentils, peanuts, pumpernickel bread, oatmeal, tomato soup, peas, cherries, apples, oranges, sweet corn, brown rice
Moderate-glycemic foods (56-69)
whole wheat bread, banana, table sugar, honey, popcorn, mango, cornmeal, couscous, raisins, beets
High-glycemic foods (70-100)
mashed potatoes, white rice, white bread, watermelon, broad beans, millet, pretzels, jelly beans, parsnips, rutabagas, french fries, pumpkin, french baguette
Factors that affect a food’s glycemic index
- Fiber content. The more fiber in a food, the lower its GI value.
- Grain coarseness. Finely ground grain is more quickly digested—and thus has a higher GI value—than coarsely ground grain.
- Ripeness. The riper a fruit or vegetable, the more sugar it contains—and the higher its GI value.
- Processing. Refined grains—ones that have had their bran and germ removed—have a higher GI value than whole grains.
- Time of day. The speed at which carbohydrates impact blood sugar varies during the day. A muffin devoured in the afternoon will likely produce a higher GI score than one eaten in the morning.
This scoring system presents several problems, however. Many factors affect how the body breaks down carbohydrates. The riper a banana, the higher its GI. The longer pasta is cooked, the higher its GI. Furthermore, we tend to eat meals rather than individual foods. Combine a high-GI food (French baguette) with a low-GI one (tomato soup) and the meal’s overall GI effect will be mixed.
In addition, a low GI score doesn’t make a food healthy. Some candy bars have GI rankings in the 40s or below.
Your best bet, then, is to use the glycemic index only as a general guide. Focus less on the actual GI numbers than on replacing highly processed carbs (white bread, white rice, cookies and cakes) with whole grains, beans, and most fruits and veggies. Doing so will automatically lower your diet’s overall GI score.
A low-GI diet may help you shed unwanted pounds—and keep them off. After you eat these foods, you’re more likely to feel full and satisfied. You’re also less likely to get a return of the munchies—the downfall of many dieters.
In one Australian study, young overweight adults doubled the amount of fat they lost during a 12-week period by swapping low-GI carbs for highGI ones. The effect was even more striking in women: their fat loss rate increased by 80 percent.
Some evidence suggests that a low-GI diet keeps the body’s resting metabolic rate (the number of calories you burn when you’re sitting still or otherwise not active) revving at a higher speed—a definite aid to holding fat at bay.
Low-GI meals also appear to accelerate the breakdown of fat during exercise. And, if eaten an hour or so before a workout, they may boost your physical endurance helping you reach and maintain your weight goal. When you’re not fatigued, you may trod on the treadmill or elliptical trainer longer, burning off even more calories.
Type 2 diabetes
A low-GI diet may be helpful in preventing type 2 diabetes, a chronic disease that develops when the body fails to effectively process or control the level of insulin in the blood. High-GI foods increase the body’s demand for insulin, which puts stress on the pancreas (where insulin is produced), eventually hobbling it from doing its job. The result: insufficient amounts of insulin—and diabetes.
The findings from several studies involving different groups of women, including African American women and women living in China, give credence to this theory. They found that women whose diets contained the greatest amount of high GI-foods were much more likely to develop type 2 diabetes than those whose diets contained the least amount of these foods. In fact, in the Chinese study, women who consumed 300 grams or more of rice daily were 78 percent more likely to be diagnosed with diabetes than those who ate less than 200 grams per day.
Other evidence suggests that replacing high-GI foods with low-GI ones may help people who have diabetes better manage their blood sugar levels—and disease itself. But not all experts agree. If you have diabetes, you should talk with your doctor before making any dietary changes.
A healthy heart needs a good cholesterol profile, and when it comes to cholesterol, a low-GI diet may be just the thing. People who switch to these kinds of carbs tend to see a drop in their total cholesterol and LDL (bad) cholesterol, as well as a rise in their HDL (good) cholesterol—all positive signs for a healthier heart.
When scientists looked at data from the large, long-running Nurses’ Health Study, they found that women who ate the greatest amount of high-GI foods were twice as likely to develop heart disease as those who ate the least amount. Other research has linked a high-GI diet to an increased risk of stroke, particularly among overweight women.
Not all studies have found an association between a low-GI diet and heart disease, however. It may be that such a diet helps only certain groups of people—those with diabetes, for example. More research is needed.
Some studies have linked a low-GI diet to a reduced risk of developing common cancers, including endometrial, breast, colon, ovarian and prostate. These findings suggested to scientists that chronically elevated insulin levels—the kind caused by eating lots of high-GI foods—might create a biochemical environment within the body that encourages tumors to take hold and flourish.
But more recent research has failed to confirm that hypothesis. Indeed, a large study published earlier this year in the American Journal of Epidemiology found that the amount (high or low) of glycemic index foods in people’s diets is not a good predictor of who will or will not develop cancer.