The term Glycemic Index is one that’s heard more and more these days. So much so that it suggests that most people actually know what it means.
In reality, the Glycemic Index concept is not all that easy to grasp but it is one worth understanding, since it relates to the overall quality of the diet and also has implications for weight management.
The Glycemic Index looks at the effects of carbohydrate-containing foods on the sugar levels in your bloodstream. Whenever you eat and digest carbohydrate-rich foods—like fruits, vegetables, grains and sweets—the end result is a rise in your blood sugar (blood glucose). This sugar in your blood is important. It’s the primary fuel for your brain and muscles, and in large part keeps you going mentally and physically throughout your day.
Not all carbohydrate-containing foods cause your blood sugar to rise to the same degree, and this is where the Glycemic Index (or GI) comes in. The GI ranks foods according to how much and how rapidly they cause the blood sugar to rise after they’re eaten.
The first paper on Glycemic Index was published over 30 years ago,1 in which a small group of healthy people were used to establish the index. The volunteers were fed each of the 62 foods in whatever amount was necessary to supply 50 grams of carbohydrate. This varies a lot from food to food—it takes about 60 baby carrots, but a mere handful of cooked white rice. Their blood sugar measurements were then taken several times over a 2-hour period. The effect of each food on blood sugar was compared to the effect of 50 grams of pure glucose (the form of sugar in your bloodstream), which was given a value of 100. So, foods that caused the blood sugar to rise quickly and steeply had a number closer to 100, while foods that caused a less dramatic rise in sugar had a lower GI.
The highest GI foods are those that are low in fiber but starchy or sugary—such as like white bread, sweet breakfast cereals, noodles, fruit juices and white rice. Since they are digested and absorbed relatively quickly, these high Glycemic Index foods tend to cause fairly large and rapid rises in blood sugar.
Now, this burst of sugary energy might sound like a good thing. After all, we need sugar in the blood to fuel our activities but not in such large surges. That’s because a quick spike in your blood sugar is often followed by a steep drop. And suddenly you’re craving something sugary to boost your blood sugar levels back up. Then the cycle starts all over again. If you wind up snacking on sugary foods all day long, there’s a good chance you’ll take in more calories than you need, which will be put into storage on your belly and thighs.
On the other hand, the lowest GI foods are those carbohydrate-rich foods that are whole and unprocessed. So, vegetables, whole fruits, beans, and most 100% whole grain foods—like brown rice, rolled oats, barley, quinoa and 100% whole grain bread—have relatively low Glycemic Index rankings. That’s because they’re high in fiber, which means they take longer to digest so your blood sugar rises more gently after you eat them.
Rather than a big spike in blood sugar, these wholesome foods lead to a slower release into your bloodstream, which provides you with more sustained energy. Thanks to their high-fiber content, they’re also more filling. So, a diet that emphasizes low GI foods can be a good strategy for weight control.
If you use the GI as a guide to choosing what to eat, it can steer you towards foods that are less ‘carb heavy’ (like whole grains and veggies), with fewer calories per bite. But you should know that this isn’t always the case. Some foods (like ice cream) have a low Glycemic Index because their high fat content slows digestion, which means they don’t cause a big spike in blood sugar after they’re eaten. On the basis of GI alone, you might conclude that ice cream was a good thing to include in your low GI diet.
On the other hand, some healthy foods have a high Glycemic Index value, which can be a bit misleading if you don’t consider portion size. Take watermelon, for example: you’d need to eat 5 servings of watermelon to get the 50 grams of carbohydrate needed to determine the GI. But a typical serving doesn’t contain nearly that much, and it doesn’t contribute much to the overall carbohydrate load of your diet. If you were to focus on GI values alone, you might end up unnecessarily omitting some healthy fruits.
That’s why it’s better to look at the Glycemic Index of your diet as a whole, rather than getting hung up on individual foods.
To cut back on your high GI foods and reduce the carbohydrate load of your diet overall, here are some switches you can easily make.
Instead of white rice and potatoes, switch to brown rice or other whole grains like cracked wheat, barley, millet or quinoa. Or substitute beans, lentils or sweet potatoes. Rather than drinking a lot of calories from high Glycemic Index fruit juices, eat whole fresh fruits instead. Have berries on cereal or a whole piece of fruit for a snack or dessert. Switch from refined white breads, crackers and snack foods to products that are made with 100% whole grain—or try nuts instead of chips for snacks.
Whole and lightly processed low GI foods are more bulky and filling than their refined cousins, which means they retain their natural vitamins, minerals and healthy antioxidant phytonutrients, too. That means that you get more nutrition for your calories. By swapping out the high GI foods and replacing with more low GI items, you can greatly reduce the overall carbohydrate load of your diet—which can help you with calorie control while providing a healthy nutrient boost.
1Jenkins D et al. Am J Clin Nutr 34:362; 1981