If you have diabetes or are trying to manage your blood sugar, you may have heard that low-glycemic foods are better than high-glycemic foods. But what does the glycemic index actually tell you, and how much attention should you pay to it?
The glycemic index, or GI, is a system for ranking carbohydrate-containing foods according to how quickly they raise blood sugar. It can be a useful tool for comparing foods, but it has important limitations.
A food’s GI does not tell you how much carbohydrate you are eating, how nutritious the food is, or exactly how your own blood sugar will respond.
That is why the glycemic index is most useful when you treat it as one piece of the blood sugar puzzle rather than a set of rules about which foods you can and cannot eat.
Quick Answer: What Is the Glycemic Index?
The glycemic index (GI) ranks carbohydrate-containing foods on a scale from 0 to 100 according to how they affect blood glucose after eating.
Foods are generally classified as:
- Low GI: 55 or less
- Medium GI: 56 to 69
- High GI: 70 or higher
Lower-GI foods generally cause a slower rise in blood glucose, while higher-GI foods tend to cause a faster rise.
However, GI does not account for how much carbohydrate you actually eat. Portion size, total carbohydrate, food preparation, what you eat with the food, medications, activity and your individual response can all influence what happens to your blood sugar.
Key Takeaways
- The glycemic index measures how quickly carbohydrate-containing foods raise blood sugar.
- Low-GI foods have a GI of 55 or less, while high-GI foods have a GI of 70 or higher.
- A lower GI does not automatically mean that a food is healthier.
- Portion size and total carbohydrate can matter more than GI alone.
- Glycemic load combines GI with the amount of carbohydrate in a serving and can provide additional context.
- Processing, cooking, ripeness and meal composition can change the glycemic effect of a food.
- People can have different glucose responses to the same food.
- GI is best used alongside overall nutrition, portion awareness and your individual diabetes treatment plan.
How Does the Glycemic Index Work?
When you eat foods containing digestible carbohydrates, those carbohydrates are broken down during digestion and glucose enters your bloodstream.
But different carbohydrate foods are digested and absorbed at different rates.
Some produce a relatively rapid rise in blood glucose. Others are digested more slowly and tend to produce a more gradual response.
The glycemic index was developed as a way of comparing these differences.
GI values are determined under standardized testing conditions. Participants eat a portion of a food containing a set amount of available carbohydrate, and their blood glucose response is measured over a period of time. That response is then compared with a reference food, usually glucose.
The resulting number becomes the food’s GI value.
What Do Low, Medium and High GI Mean?
The standard GI categories are straightforward:
| GI Category | GI Value | General Effect |
|---|---|---|
| Low | 55 or less | Slower rise in blood glucose |
| Medium | 56 to 69 | Moderate rise |
| High | 70 or higher | Faster rise |
Foods such as beans, lentils and some minimally processed grains tend to have lower GI values.
Many refined grain products and highly processed starches tend to have higher values.
But there are plenty of exceptions, which is one reason you should not judge the healthfulness of a food by its GI number alone.
Why Does Glycemic Index Matter for Diabetes?
The glycemic index can help you understand why two foods containing carbohydrates may affect your blood sugar differently.
For example, a minimally processed grain may be digested more slowly than a highly refined grain product. Choosing the slower-digesting option may help reduce the size or speed of the rise in blood glucose after a meal.
For people with type 2 diabetes, research suggests that lower-GI eating patterns may provide modest improvements in blood glucose management for some people. However, study results vary, and GI should not be viewed as a stand-alone treatment for diabetes.
For people with type 1 diabetes, GI may provide additional information about how quickly a meal is likely to affect glucose, but carbohydrate counting remains particularly important when matching food with mealtime insulin.
If you take insulin, do not change the amount or timing of your insulin simply because a food has a particular GI value. Insulin needs depend on many factors, and changes should be based on the plan you have developed with your diabetes care team.
Does Low GI Mean a Food Is Healthy?
No. This is one of the most important things to understand about the glycemic index.
GI measures blood glucose response to carbohydrate. It does not measure overall nutritional quality.
A food can have a relatively low GI because it contains enough fat or other components to slow digestion, yet still be high in saturated fat, calories or added sugar.
Conversely, a nutritious food can have a moderate or relatively high GI and still fit into a healthy diabetes eating pattern.
Instead of asking only:
“Is this food low GI?”
also consider:
- How much carbohydrate is in my serving?
- How much fiber does it contain?
- Is it minimally processed?
- Does it provide useful nutrients?
- How large is my portion?
- What am I eating with it?
- How does my blood sugar actually respond?
This broader approach is much more useful than choosing foods according to GI numbers alone.
For a more complete look at building an overall eating pattern, see our Diabetes Food Guide.
What Foods Tend to Have a Lower Glycemic Index?
Many foods commonly recommended as part of a balanced diabetes eating pattern also happen to have a relatively low GI.
Examples include:
- Lentils and beans
- Chickpeas
- Barley
- Steel-cut or traditional oats
- Some whole or minimally processed grains
- Many whole fruits
- Non-starchy vegetables
Fiber and intact food structure can slow digestion, which is one reason beans, legumes and less-processed grains often have lower glycemic effects than highly refined carbohydrate foods.
However, you do not need to build your diet around memorizing GI numbers.
A simpler strategy is to emphasize minimally processed, fiber-rich carbohydrate foods while paying attention to portions and your own glucose response.
If you want to go beyond individual foods and learn how to build an eating pattern around slower-digesting carbohydrates, our guide to the Low Glycemic Diet covers that topic in greater detail.
What Foods Tend to Have a Higher Glycemic Index?
Foods that are heavily processed or contain rapidly digested starches often have higher GI values.
Examples can include:
- White bread
- Some refined breakfast cereals
- Instant potatoes
- Some varieties of white rice
- Highly processed grain products
But even here, GI requires context.
A food’s effect on your blood glucose depends not only on how quickly its carbohydrate is digested but also on how much carbohydrate you eat.
That distinction becomes clearer when you understand glycemic load.
Glycemic Index vs. Glycemic Load: What’s the Difference?
Glycemic index tells you how quickly a carbohydrate food tends to raise blood glucose under standardized conditions.
Glycemic load (GL) goes a step further by considering both the food’s GI and the amount of carbohydrate in the portion you actually eat.
The formula is:
Glycemic Load = (GI × grams of available carbohydrate in the serving) ÷ 100
This helps explain an apparent weakness in GI.
A food can have a relatively high glycemic index but contain relatively little carbohydrate in a normal serving. Its actual effect on blood glucose may therefore be smaller than its GI number makes it appear.
The reverse can also happen. Eating a large amount of a lower-GI carbohydrate food can still deliver a substantial amount of carbohydrate and significantly raise your blood sugar.
Glycemic Load Categories
Glycemic load is commonly classified as:
| Glycemic Load | Classification |
|---|---|
| 10 or less | Low |
| 11 to 19 | Medium |
| 20 or higher | High |
You do not need to calculate glycemic load every time you eat.
The more important lesson is simple:
How much carbohydrate you eat matters, not just how quickly that carbohydrate is digested.
That is particularly important for people with diabetes.
What Can Change a Food’s Glycemic Index?
GI values are useful for comparison, but they are not fixed predictions of what will happen every time you eat a food.
Several factors can change how quickly carbohydrate is digested and absorbed.
Processing
Processing can break down the physical structure of food, making starch easier to digest.
For example, less-processed oats generally produce a different glucose response than highly processed instant oats.
The same principle can apply to grains and other starchy foods. The more the original structure has been broken down, the more quickly the carbohydrate may be available for digestion.
Cooking
Cooking method and cooking time can affect starch structure.
Pasta cooked until firm, for example, generally has a lower glycemic effect than pasta cooked until very soft.
This does not mean you need to cook foods according to precise GI rules. It simply demonstrates that the same basic food can affect glucose differently depending on how it is prepared.
Ripeness
As some fruits ripen, their starches are converted into sugars.
A less-ripe banana, for example, contains more resistant starch than a very ripe banana and may produce a different blood glucose response.
That does not make ripe fruit “bad.” It simply illustrates why a GI value should be considered an estimate rather than a guarantee.
What You Eat With the Food
GI values are generally determined by testing foods individually. Most people, however, eat foods as part of meals.
Protein, fat and fiber can slow digestion and influence the overall glucose response to a meal.
For example, your glucose response to a serving of rice eaten alone may be different from your response to the same amount of rice eaten with vegetables and protein.
This is another reason that looking up the GI of every ingredient in your meal usually isn’t necessary.
Why Portion Size Can Matter More Than GI
One of the biggest limitations of the glycemic index is that it does not tell you how much carbohydrate you are consuming.
Imagine two carbohydrate foods.
Food A has a lower GI, but you eat a very large serving.
Food B has a higher GI, but you eat only a small amount.
Food A could potentially have the greater overall effect on your blood glucose because you consumed substantially more carbohydrate.
For people with diabetes, this is why total carbohydrate and portion size should not be ignored in favor of GI.
Carbohydrate counting and the diabetes plate method are two common approaches to meal planning. Which approach works best can depend on your medications, insulin use, glucose targets and personal preferences.
Can Two People Have Different Blood Sugar Responses to the Same Food?
Yes.
GI values describe an average response measured under standardized conditions. They cannot predict exactly what will happen to your blood glucose.
Your response may be influenced by factors including:
- Portion size
- Meal composition
- Physical activity
- Time of day
- Medications or insulin
- Stress
- Sleep
- Your individual metabolism
Even your own response to the same meal may not be identical every time.
This is why your glucose readings can provide information that a GI chart cannot.
If you monitor your blood glucose, comparing readings before and after meals can help you and your healthcare team identify patterns. A continuous glucose monitor (CGM), when appropriate, can provide even more information about glucose trends throughout the day.
The goal is not to test every food indefinitely. It is to learn which meals and portions tend to work well for you.
How to Use the Glycemic Index in Real Life
You do not need a GI chart beside you at every meal.
Instead, use GI as a tool for making comparisons when you have a choice between carbohydrate foods.
1. Start With Overall Food Quality
Choose mostly nutrient-dense foods such as vegetables, whole fruits, beans, legumes, whole grains, lean protein foods and healthy sources of fat.
Do not choose a nutritionally poor food simply because it happens to have a low GI.
2. Pay Attention to the Amount of Carbohydrate
GI does not replace carbohydrate awareness.
If you eat a large portion of a carbohydrate-rich food, its lower GI does not prevent the carbohydrates from affecting your blood glucose.
3. Choose Less-Processed Carbohydrates More Often
When practical, choose intact or minimally processed carbohydrate foods over highly refined versions.
For example, you might choose beans, lentils, minimally processed whole grains or traditional oats more often than refined breads and highly processed cereals.
4. Build Balanced Meals
Instead of eating carbohydrate foods by themselves, build meals that also contain vegetables, protein and healthy fats.
A simple approach is the diabetes plate method:
- Fill half the plate with non-starchy vegetables.
- Fill one-quarter with a protein food.
- Fill one-quarter with a carbohydrate food, preferably one that is higher in fiber.
This approach naturally incorporates several principles that can help with blood glucose management without requiring you to calculate the GI of an entire meal.
5. Compare Your Glucose Response
If you check your blood sugar, use your readings to learn how different meals affect you.
You may discover that a food considered low GI still raises your glucose more than expected, or that you tolerate a moderate-GI food well when you eat an appropriate portion as part of a balanced meal.
Your own results provide information that a GI table cannot.
Common Mistakes When Using the Glycemic Index
The glycemic index becomes much more useful once you understand what it cannot tell you.
Assuming Low GI Means “Eat as Much as You Want”
It doesn’t.
A large serving of a low-GI carbohydrate food can still contain a large amount of carbohydrate and significantly affect your blood glucose.
Assuming High GI Means “Never Eat This”
GI is not a good-food/bad-food rating system.
Some nutritious foods have moderate or higher GI values. Portion size and the rest of the meal matter.
Ignoring Overall Nutrition
GI tells you nothing directly about calories, saturated fat, sodium, vitamins, minerals or the overall nutritional quality of a food.
A healthy diabetes eating pattern requires looking beyond one number.
Using GI Instead of Carbohydrate Counting
This can be particularly important if you use mealtime insulin.
GI may provide additional information about how rapidly a carbohydrate food is likely to affect glucose, but it does not tell you the total amount of carbohydrate in your meal.
Follow the carbohydrate-counting and insulin instructions provided by your diabetes care team.
Expecting Your Blood Sugar to Match a GI Chart
GI values are averages, not personalized predictions.
Use them as a guide, then pay attention to how foods actually affect you.
Is a Low-Glycemic Diet Good for Diabetes?
A lower-GI eating pattern can be a reasonable option for some people with diabetes, particularly when it encourages higher-fiber, less-processed carbohydrate choices.
Research suggests that lower-GI or lower-glycemic-load eating patterns may modestly improve some measures of glucose control in some people with diabetes. However, studies have not produced identical results, and GI is only one characteristic of a healthy eating pattern.
There is no single diet that is right for everyone with diabetes.
Your overall eating pattern, the amount and type of carbohydrate you consume, nutritional quality, portion sizes, medications, physical activity and other health needs all matter.
If you have insulin resistance or prediabetes, you may also find our guide to an Insulin Resistance Diet helpful for understanding the broader role of food choices in insulin sensitivity.
Frequently Asked Questions
What is a good glycemic index for someone with diabetes?
There is no single GI number that every food must meet. Foods with a GI of 55 or less are classified as low GI, but people with diabetes do not need to eat only low-GI foods. Portion size, total carbohydrate, nutritional quality and your individual glucose response are also important.
Are low-GI foods always better for blood sugar?
Not necessarily. Low-GI foods generally raise glucose more slowly, but the amount you eat still matters. A large serving of a carbohydrate-rich low-GI food may have a substantial effect on blood sugar.
Is glycemic index more important than carbohydrates?
Usually, no. The amount of carbohydrate you eat is a major factor in how a food or meal affects blood glucose. GI can provide additional information about how quickly that carbohydrate may affect your glucose.
Is glycemic load better than glycemic index?
Glycemic load adds information that GI leaves out because it considers the amount of carbohydrate in a serving. However, you do not need to calculate GL for everything you eat. Portion awareness, overall food quality and your personal glucose response are often more practical.
Can I eat high-GI foods if I have diabetes?
Yes. Having diabetes does not mean every higher-GI food must be eliminated. Consider the portion, total carbohydrate, nutritional value and what else you are eating. Your healthcare team can help you determine how particular foods fit into your individual meal plan.
Do foods without carbohydrates have a glycemic index?
Foods containing little or no carbohydrate generally are not assigned meaningful GI values because GI measures the blood glucose response to carbohydrate-containing foods.
Can the glycemic index help with prediabetes?
GI can be one useful consideration when choosing carbohydrate foods, but preventing or managing prediabetes involves much more than choosing low-GI foods. Overall diet quality, physical activity, weight management when appropriate, sleep and other lifestyle factors can all play a role.
Final Thoughts
The glycemic index can help you understand an important fact about carbohydrates: they do not all affect blood sugar at the same speed.
But GI is not a scorecard that divides foods into “good” and “bad.”
A better way to use it is as one piece of information alongside portion size, total carbohydrate, fiber, nutritional quality and your own glucose response.
Choose nutrient-dense, minimally processed foods more often. Pay attention to portions. Build balanced meals. And if you monitor your blood glucose, use those readings to learn how different foods and meals affect you personally.
Most importantly, do not change your medication or insulin based on GI values alone. If you need help incorporating glycemic index principles into your diabetes management plan, talk with your doctor, registered dietitian or diabetes care and education specialist.