Glycemic Index: What It Means, GI Food List and How to Use It

01Definition

What is the glycemic index?

MedlinePlus defines the glycemic index as a measure of how quickly a food can make your blood sugar (glucose) rise. Not all carbohydrates work the same in the body: some trigger a quick spike, while others work more slowly. The GI gives each food a number that reflects how quickly it raises blood glucose compared with pure glucose.

Mayo Clinic describes it as a classification of carbohydrate-containing foods by their potential to raise blood sugar. Foods with high values tend to raise blood sugar higher, faster and for a longer time than foods with lower values.

02The scale

How the GI scale works

The scale goes from 0 to 100. Pure glucose has the highest value and is given 100. Only foods that contain carbohydrates have a GI. Oils, fats and meats do not have a GI, although, as MedlinePlus notes, in people with diabetes they can still affect blood sugar.

This is an important point: a food without a GI value is not the same as a food with a GI of zero. It simply was not measured, because the GI only applies to foods with carbohydrates. Mayo Clinic adds that the index only measures foods with a high amount of carbohydrates and does not include low- or no-carbohydrate foods.

03Categories

Low, medium and high GI foods

MedlinePlus groups foods into three bands and gives these examples:

• Low GI (0 to 55): bulgur and barley; pasta and parboiled (converted) rice; quinoa; high-fiber bran cereal; steel-cut or rolled oatmeal; carrots, non-starchy vegetables and greens; apples, oranges, grapefruit and many other fruits; most nuts, legumes and beans; milk and yogurt.
• Moderate GI (56 to 69): pita bread and rye bread; couscous; brown rice; raisins; honey; pineapple.
• High GI (70 and higher): white bread and bagels; most processed cereals and instant oatmeal, including bran flakes; most snack foods; potatoes; white rice; sugar; watermelon.

These lists are a general guide, not fixed rankings. The same food can move up or down depending on ripeness, processing and cooking, as explained below.

04What changes GI

Why the GI of a food can change

MedlinePlus lists several factors that change the GI of a food:

• Ripeness: the GI of a piece of fruit depends on how ripe it is.
• Processing: processed foods generally have a higher GI. Fruit juice and instant potatoes have a higher GI than whole fruit and whole baked potatoes.
• Cooking: pasta cooked al dente has a lower GI than soft-cooked pasta.
• Fat and fiber: foods higher in fat or fiber tend to have a lower GI.
• Variety: foods of the same type can differ. Converted long-grain white rice has a lower GI than brown rice, while short-grain white rice has a higher GI than brown rice. Quick oats or grits have a high GI, but whole oats and whole-grain breakfast cereals have a lower one.

Mayo Clinic adds that the GI does not consider how food is prepared, and that packaged foods generally do not list a GI value on the label, which makes it hard to estimate.

05Portions

GI, portion size and the amount of carbohydrate

The GI tells you how fast a carbohydrate food tends to raise blood sugar, not how much it will raise it in a real portion. MedlinePlus is clear that portion size still matters, because calories still matter, and so does the amount of carbohydrate in the meal, even when it is made of low GI foods. Mayo Clinic also points out that the index does not take into account how much a person eats.

You may also see the term glycemic load, which combines the GI with the amount of carbohydrate in a portion. The two sources used for this guide do not define or use it; their practical message is the same, though: look at the amount of carbohydrate and the portion, not only at the GI number.

06Meals

Thinking in whole meals

People rarely eat a single food on its own. Mayo Clinic notes that the GI does not show how a combination of foods affects blood sugar differently from single items. MedlinePlus gives two meal-planning tips:

• choose foods with a low to medium GI
• when you eat a high GI food, combine it with low GI foods to balance the effect on your glucose, because the GI and its impact can change when foods are combined

Look at the nutritional value of the whole meal as well as the GI. Some high GI foods are rich in nutrients, so MedlinePlus suggests balancing them with lower GI foods rather than cutting them out. And a low GI does not make a food healthy: Mayo Clinic warns that the GI does not rank foods by nutrients, and some low GI foods are high in calories, sugar or saturated fat.

07Diabetes

Using the glycemic index with diabetes

If you have diabetes, high GI foods can make blood sugar harder to control. MedlinePlus presents the GI as another tool alongside carbohydrate counting: for many people, carb counting together with healthy food choices and a healthy weight may be enough to regulate diabetes. If you have trouble regulating your blood sugar or want tighter control, talk with your provider about adding the GI to your plan.

Mayo Clinic considers healthy eating principles, portion control and counting carbohydrates to be reliable ways to manage blood sugar, and recommends talking to a registered dietitian. A simple food sheet can make that conversation more useful:

• the food and how it was prepared (raw, cooked al dente, mashed, juiced)
• the amount you ate
• what else was on the plate
• your blood sugar reading, if you check it as advised by your care team

In our nutrition practice in Bilbao we help adults plan meals that take carbohydrates, portions and GI into account (consultations are in Spanish). We do not adjust diabetes medicines or insulin; that stays with your doctor. For more ideas, see our diet for type 2 diabetes and diet for prediabetes guides.

08Weight and health

Does a low GI diet help with weight or heart health?

MedlinePlus says that eating low GI foods can help you gain tighter control over your blood sugar and that following a low GI diet also may help with weight loss. Mayo Clinic reports that eating a lot of high GI foods is linked to a higher risk of diabetes, high blood pressure and heart disease, but that studies do not consistently show that eating lower GI foods reduces these risks.

In practice, the sources support using the GI as a guide within a balanced diet, not as a promise of weight loss or disease prevention.

09Sources

Where the sources differ

How useful the GI is. MedlinePlus is more positive: low GI foods can help tighten blood sugar control and may help with weight loss. Mayo Clinic is more cautious: lower GI foods have not consistently been shown to reduce the risks linked to high GI diets, and the index is hard to follow in daily life.

Main tool. MedlinePlus treats carbohydrate counting as the main tool for many people with diabetes and the GI as an addition for those who need tighter control. Mayo Clinic names healthy eating, portion control and carb counting as the reliable methods and does not single out the GI.

Both agree that portion size, the amount of carbohydrate, preparation and the whole meal matter as much as the GI number.

10Summary

In summary

The glycemic index ranks carbohydrate foods from 0 to 100 by how quickly they raise blood sugar compared with glucose. Low GI is 55 or less, medium 56 to 69 and high 70 or more. Foods without carbohydrates have no GI, which is not the same as a GI of zero.

A food’s GI changes with ripeness, processing and cooking, and with what it is eaten with. The GI does not measure portion size or nutrients, so a low GI food is not automatically healthy. Used together with carb counting and portion control, it can help some people with diabetes; any change to medicines stays with the care team.