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What’s the Deal With  the Glycemic Index?


When it comes to your diet, chances are you’re already paying attention to your intake of vitamins and nutrients, fats, and added sugars. Now you’re hearing buzz about how eating foods low on the glycemic index (GI) may boost your energy and your efforts to maintain a healthy weight. But what, exactly, does this scale mean—and how can you figure out how certain foods rank?

The GI measures how foods affect your blood sugar levels. It’s tested by feeding people a portion of food that has 50 grams of carbohydrates and then seeing how much that raises their blood sugar levels, say Willow Jarosh, RD, and Stephanie Clarke, RD, co-owners of C&J Nutrition. This is given a value on a scale of 0 to 100. A greater GI score means that food causes a higher spike in your blood sugar.

For people with diabetes, the GI may be helpful in keeping blood sugar levels steady. But people without diabetes might find it a helpful tool in their own diets, too.

How Going Low May Boost Your Health 

With low-glycemic foods, sugars enter the body more slowly. This keeps your blood sugar levels steady, which can keep you feeling full. Although research is mixed, some studies have found that eating a diet rich in low-GI foods may encourage weight loss. It may also help reduce the risk of heart disease—one study of more than 75,000 women showed that those who had high glycemic loads were nearly twice as likely to develop heart disease than those who had the lowest. But before you go all-in on a low-GI diet, a review of 15 studies found just a slight association between high GI diets and heart disease.

All the same, high-glycemic foods are often more processed, i.e., cookies and bread made with white flour, and processing removes many nutrients. “Lower-GI foods tend to be higher in fiber, protein, and/or fat,” Jarosh and Clarke say. That means eating a low-glycemic diet can mean fewer processed foods and more produce, fiber-rich whole grains, and protein. This may help boost your energy, weight loss efforts, and health.

How to Use the GI Scale 

While the GI scale can be useful, it’s not a perfect science. It doesn’t factor in portion size, because it looks at the response to specifically 50 grams of carbohydrates. So some healthy foods, such as unbuttered popcorn or watermelon, rank higher on the GI than others, like a chocolate-and-peanut candy bar—but that’s not taking into account how much you normally eat (and how many carbs are in that portion). “That’s where glycemic load comes in,” Jarosh and Clarke say. “It measures the change in blood sugar levels in response to a typical portion of food.”

What’s more, the GI value reflects the effect of each kind of food on its own. But when’s the last time you ate only one type of food at a meal? Think about that sandwich you had for lunch. The white bread on its own is high on the GI, but when you eat it with all your sandwich fixings, your blood sugar won’t spike as high as you might think it would from the bread’s GI value alone, Jarosh and Clarke say. 

How Do Your Foods Rank?   

As a rule of thumb, foods that are processed and cooked longer tend to fall higher on the GI; for example, instant oatmeal has a higher GI value than rolled-oat oatmeal. Foods that contain more fiber and fat usually rank lower. Here are some examples of where foods fall in the scale.

High-GI Foods (70 or more)
Mashed potato (87)
Cornflakes (81)
Instant oatmeal (79)
Boiled potato (78)
Watermelon (76)
White bread (75)
White rice (73)

Medium-GI Foods (56 to 69)
Brown rice (68)
Couscous (65)
Popcorn (65)
Boiled sweet potato (63)
Pineapple (59)

Low-GI Foods (55 or less)
Rolled-oat oatmeal (55)
Orange (43)
Chocolate (40)
Milk (39)
Boiled carrots (39)
Apple (36)
Lentils (32)

If you don’t want to constantly check the GI scale, here’s an easy way to meal-plan: Jarosh and Clarke suggest filling your half your plate with vegetables, one-quarter with less-processed carbs (think: whole grains, beans, fruit, and starchy vegetables), and one-quarter with protein. “This helps keep blood sugar levels and energy stable without having to do any mealtime math.”

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