Answering Top Questions on Glycemic Index

Wondering what the glycemic index (GI) is all about? I’m answering your top questions on the GI of foods, as well as the science behind this concept.

You’ve probably heard the term glycemic index (GI) used in a variety of venues, from your neighborhood gym to the latest weight loss book. Eating a low-glycemic index diet is certainly a growing trend, but is there any proof that it can help you achieve better health?

What is the Glycemic Index?

It all starts with carbohydrates, such as breads, cereals, fruits and sugars, that provide energy for your body. The simplest form of carbohydrates is glucose, which fuels your body’s organs and tissues. When you eat a carbohydrate food, it raises your blood glucose (or blood sugar) levels. The glycemic index is a tool that helps rank different carbohydrate foods and their effect on blood glucose levels on a scale from 0 to 100. Low-GI foods contain slowly digested carbohydrates and produce gradual, low rises in blood glucose levels, while high-GI foods contain rapidly digested carbohydrates and produce a large, rapid rise and fall in blood glucose levels.

What is the GL?

The GI only tells you how rapidly a type of carbohydrate turns into sugar in the bloodstream; it doesn’t take into account the amount of carbohydrate found in a typical serving of that food. For example, some foods, like watermelon, may be high-GI, but contain lower amounts of carbs, and thus have a smaller overall effect on blood glucose. The glycemic load (GL), a calculation of the GI multiplied by the amount of carbohydrate in a serving of food and divided by 100, takes into account both the GI and the amount of carbohydrate in a particular food and how both impact blood glucose. For example, an 80-gram serving of raw carrots contains 8 grams of carbs, has a GI of 16 and a GL of 1. The equation would be 16 × 8 ÷ 100 = 1. But you don’t have to carry a calculator around with you to determine the GL of foods. The international GI database (www.glycemicindex.com) created by the Human Nutrition Unit of the University of Sydney contains both the GI and GL values for almost 2,500 foods.

What Kinds of Foods are High or Low in GI?

As a rule of thumb, high-GI carbohydrate foods, such as white bagels and corn flakes, are more processed and refined, while low-GI foods, like barley and beans, are less refined and higher in fiber. If you’d like to see how your favorite foods measure up, check out the international GI database.

What Happens When You Eat a High-GI or Low-GI Food?

When you eat a high-GI food, it’s a little like surging to the top of a roller coaster ride: your blood glucose rapidly rises, your insulin output spikes, and glucose uptake occurs in your muscle tissue and fat storage. Then, as if the roller coaster is plummeting down, your blood glucose drops and counter-regulatory hormones are released, putting you into reactive hyperglycemia (below normal glucose level.) It’s at this time that your appetite is stimulated and you may crave high-carb foods to help get your blood glucose levels back up. There’s no such roller coaster ride when you eat a low-GI food. Instead, your blood glucose rises slowly, insulin is released moderately, you don’t experience reactive hyperglycemia, and you feel satisfied longer.

Can the GI Provide any Health Benefits? 

One obvious benefit is better glucose control for people with diabetes. A review of randomized, controlled trials comparing low-GI diets with higher-GI diets in people with type 2 diabetes was published in the journal Nutrients in March 2018. The review found that low-GI diets were linked with significant improvements in the glucose biomarker HbA1c, as well as other biomarkers that indicate improved glucose control. In fact, the American Diabetes Association recommends that the GI can provide glucose control benefits in addition to watching the amount of carbs you eat.

Low-GI dietary patterns have even been linked with reduced risk of developing type 2 diabetes. But the health benefits don’t stop there; current research is exploring many other advantages, such as heart health and weight control. Jenny Brand-Miller, PhD, Professor of Human Nutrition at the University of Sydney and one of the world’s leading GI experts, says “There is very convincing evidence that low-GI diets improve cholesterol levels, which means that they help reduce the risk of cardiovascular disease.” She explains that low-GI diets reduce oxidative stress (damage caused by free radicals), and that the viscous fiber, such as those found in oat products and legumes, are commonly found in low-GI foods and may be behind their cholesterol-lowering benefits.

A meta-analysis of studies published in the February 2019 journal Obesity Reviews suggests that a low-GI diet may effectively result in weight loss, improve Body Mass Index (BMI), and help lower total and LDL cholesterol. “There is increasing evidence that low GI diets aid weight control,” adds Brand-Miller. This hypothesis has been supported by several studies, including a study published in the May 2019 Journal of Medicinal Food, which found among overweight participants, who lowered body weight by 12 percent, 18 percent body fat, nearly 9 percent waist circumference, experienced significant reductions in total and LDL cholesterol and triglycerides, and reduced blood pressure. Brand-Miller notes that low-GI carbs that are slowly digested reach lower parts of the small intestine and stimulate L cells, which produce a hormone called GLP-1 that is known to enhance satiety. Low-GI carbs also reduce insulin levels after eating and facilitate the body’s use of fat as a source of fuel during mild to moderate exercise.

Are There any Drawbacks to the Low-GI diet? 

The GI may not be an entirely perfect dietary tool; some concerns include:

• Charts are required in order to look up food values.
• Only carbohydrate-containing foods are considered, not fat or protein.
• Some nutrient-rich foods are high-GI, while some nutrient-poor foods are low-GI, which encourages elimination of some healthy foods from the diet. For example, chocolate has a lower GI than oatmeal.
• The GI of a food is different when it is eaten alone, compared to when it is combined with other foods, such as in a sandwich or meal.
• The effect of a food’s GI varies from person to person as a result of individual glucose responses.  

Tips for Eating a Low-GI Diet

Overall, there are still plenty of reasons to strive for eating more low-GI foods. Here are a few tips to get you started.

1. Use the GI to help you choose the right starchy foods; don’t bother worrying about the GI of proteins, fats, nuts, and most fruits and vegetables.
2. Fill half your dinner plate with non-starchy vegetables or salad.
3. Don’t completely eliminate an entire plant food like potatoes from your diet. If you’re going to have a high-GI food, eat a smaller portion and combine it with low-GI foods.
4. Include healthy fats and lean proteins at each meal to lower the GI of the meal.
5. Remember to make nutrient-rich food choices; it’s not only about how low you can go with the GI.
6. Switch to a low-GI bread full of roughly ground grains.
7. Swap refined breakfast cereals, such as flakes, for a lower-GI choice like oats.

Written by Sharon Palmer, MSFS, January 2011; Updated on February 3, 2020.

For low-GI recipes, check out some of my favorites:

Heirloom Bean Cassoulet with Root Vegetables
Greek Butter Bean Salad
Moroccan Chickpea Sorghum Bowl