The glycemic index (GI) is a way to measure the impact that carbohydrate-containing foods have on blood sugar, or blood glucose.[1] People are interested in understanding the glycemic index (and the glycemic load) of food so they can construct a healthier diet. Blood sugar affects many aspects of health, including the risk of getting cancer, cardiovascular disease, and diabetes, so watching the GI of foods in your diet can improve your overall health prospects.

GI values range from zero to one hundred. The glycemic index was designed to measure how equal amounts of carbohydrates raise blood sugar compared to pure glucose – which is a 100 on the GI scale. Food with a high GI value will make blood sugar levels rise (and fall) quickly, while food with a low GI value has a slow and steady effect on blood sugar.

A GI value under 55 is low. Foods that have a low GI value include beans, cruciferous vegetables, grapefruit, and tomatoes. A GI value between 56-69 is moderate. Moderate GI foods include pasta, green peas, sweet potatoes, orange juice, and blueberries. A GI value over 70 is high.[2] Examples of high GI foods include refined sugar, potatoes, white bread, dried fruit, and watermelon. However, the glycemic index value of foods should be used along with its glycemic load value, explained below, which provides a more accurate understanding of how a single serving will affect your blood sugar.

Food with a high GI value will make blood sugar levels rise (and subsequently fall) quickly, while food with a low GI value has a slow and steady effect on blood sugar.

Factors That Affect Glycemic Index Values

A GI value can be affected by several factors. Processing and refining, for example, will result in a higher GI value for the same food. A whole baked potato has a lower GI value than instant mashed potatoes. Processed orange juice has a higher GI value than fresh squeezed.

Eating different foods together can also affect GI values. Research has shown that when you eat a high fiber diet – particularly soluble fiber – it can counter some negative effects of a high-carbohydrate diet.[3] (Just to ensure there’s no confusion – no, eating a pound of lettuce won’t cancel out eating a pound of sugar.) The riper a fruit or vegetable, the higher its GI value. Also, your physiology, age, metabolism, and health conditions all influence how foods affect blood sugar.

The Relationship Between Types of Carbs & Glycemic Index

Carbohydrates are a primary source of energy for the human body, and there are two basic types: simple carbohydrates and complex carbohydrates.

Simple carbohydrates, chemically known as monosaccharides or disaccharides, are digested quickly and have an immediate effect on blood sugar. Common examples include refined sugar (sucrose) or high fructose corn syrup, as found in soft drinks. In general, foods high in simple carbohydrates have high GI values. Simple carbs cause blood sugar to rise rapidly. When blood sugar rises, the pancreas secretes insulin, which makes cells absorb glucose. As cells absorb the circulating glucose, its level in the blood falls.

Most complex carbohydrates, also known as polysaccharides, are metabolized slowly and do not have such a dramatic effect on blood sugar. Foods high in complex carbohydrates include whole grain bread, vegetables, and legumes. Complex carbohydrates typically have a low GI value and usually provide other nutrients, such as fiber, vitamins, and minerals, that slow digestion time.

Generally, consuming complex carbs results in a more steady level of blood glucose, versus the spike and drop that comes from simple carbs. However, not all complex carbs have a low GI, and it depends on the type of fiber present. Foods high in soluble fiber tend to have lower GI values, but foods with insoluble fiber have higher GI values. Insoluble fiber does not slow down digestion in the way soluble fiber does. The starch in potatoes is also a complex carbohydrate with a high GI value. Starchy foods can act like simple carbs because they are broken down into glucose molecules during digestion.

Interestingly, if you eat a meal with low-GI foods, it lowers your body's blood sugar after eating (postprandial glycemic response) not only for that meal but also for the one after. This is called the second meal effect, and scientists have studied how this influences disease risk for health conditions related to blood sugar.[4]

What Is Glycemic Load?

You might be tempted to avoid all high-GI value foods since high blood sugar is linked with so many health concerns. However, the portion size for each food interacts with and affects blood sugar. You can eat high-GI foods like carrots because the serving portions are relatively small (only 6 grams of carbs per serving) and you would have to eat a lot of carrots to spike your blood sugar. Because of this, researchers developed the glycemic load (GL) index, which more thoroughly represents the impact food has on blood sugar. GL takes into account not only the GI value but also the grams of carbohydrates in a serving of food.[5]

Calculating Glycemic Load

To calculate glycemic load, you have to know the glycemic index of a food as well as the number of carbohydrates found in a serving, typically in grams. You calculate Gl as follows:

GL = (GI x grams of carbohydrate per serving) / 100

In other words, the glycemic load for any particular food is equal to the glycemic index for that food multiplied by the number of carbohydrates in a single serving, divided by 100.

FoodGlycemic Load

Low-GL Foods0-10

Medium-GL Foods11-19

High GL-Foods20

To give an example, watermelon has a glycemic index of 72 but since a typical serving only provides 6 grams of carbohydrates a quick calculation reveals a glycemic load of less than 5.

A can of soda, on the other hand, may have a GI value of 65 but if it provides 40 grams of carbohydrates, that means the glycemic load is 26, which is very high.