There’s been a buzz around the glycemic index (GI) and low GI diets in the past few years, particularly for people with diabetes or those simply interested in better managing their diet.
At its core, the glycemic index is a tool to help some people better understand and plan their food choices — but there can be a learning curve to understand what exactly GI means, its benefits and limits, and who the index could potentially benefit.
Below, Mayo Clinic expert and endocrinologist Pankaj Shah, M.D., explains everything you need to know about the glycemic index.
What is the glycemic index?
Every time you eat, you set off a cascade of digestive processes in your body. One effect: an increase in blood sugar, known as glucose.
“After we eat, the glucose concentrations in the blood go up and then they come down,” Dr. Shah says.
The glycemic index measures how a certain food impacts blood sugar over a certain time period. Foods on the lower end of the scale will raise blood sugar less over that period, while foods that rank higher raise blood sugar more.
Only foods that contain carbohydrates can rank on the scale. A low carb or carb-free food — like meat — is not measured on the glycemic index.
It’s important to note that the scale is comparative. Researchers compare a certain food against the effect of a “standardized” food like sugar or white bread, Dr. Shah explains.
For example, researchers measure changes in blood glucose over a two-hour period after participants eat 50 grams of sugar. They also measure the blood glucose over a two-hour period after participants eat 50 grams of banana.
The glycemic index is a ratio of these two measures. The standard food — in this case, sugar — is given a score of 100. Bananas have a GI score of about 50, meaning they raise blood sugar by about half as much as sugar does in a two-hour period.
On the glycemic index scale, foods are generally ranked as low GI (0 to 55), medium GI (56 to 69), or high GI (70+). Foods with a high GI are easy for the body to digest and absorb, causing a quick rise in blood sugar. Common high GI foods include:
- Certain grains like white rice.
- White bread.
- Potatoes.
- Processed foods like quick oats or pretzels.
Low GI foods, on the other hand, take longer for the body to digest and cause a slower, more gradual rise in blood sugar. These foods may be high in fat, protein or fiber. Low GI foods include:
- Green vegetables.
- Most fruits.
- Proteins like beans, chickpeas, peanuts and lentils.
- Some dairy like whole milk and Greek yogurt.
In short, glycemic index is a tool to understand how the foods you consume impact blood sugar levels.
Taking glycemic index into account is sometimes recommended for people with diabetes, who must monitor their blood sugar levels to manage their condition. However, the glycemic index may be a helpful guide for those without diabetes who want to lose or manage their weight, create healthier habits, or manage their risk for certain conditions, like diabetes or heart disease.
Understanding glycemic index and its limits
There are factors that impact the glycemic index, says Dr. Shah. Food preparation, processing and cooking methods (like freezing or blending) all impact how a food affects your blood sugar.
“If you take quick oats for example, they have a much higher glycemic index than traditional cooked steel cut oats,” says Dr. Shah. “This is due to preparation. In many cases, the grain is soaked and the outer part of the grain is removed. This makes it much easier to cook, but it has a much bigger impact on blood sugar levels.”
Critically, the glycemic index doesn’t account for how much of a food you eat or what you combine it with. As Dr. Shah points out, most of us don’t eat 50 grams of white bread for lunch — we want a sandwich with meat and cheese and lettuce and anything else we crave. Glycemic index can only offer the value of components, rather than the entire meal.
To help address portion size, it can be helpful to think about glycemic load (GL), which Dr. Shah says is the amount of glucose generated by the amount and GI of the food.
Finally, the glycemic index doesn’t rank foods on their holistic nutritional value, meaning a food may have a low GI, but be high in calories or saturated fats.
Using the GI in your daily life
Clearly, there’s nuance to using the glycemic index. But what’s important to understand is where foods generally fall on the scale, why and how GI may impact your overall well-being.
There is evidence that those who have diets rich in high GI foods may have an increased risk of diabetes, blood pressure and heart disease. It’s generally believed that consuming more food with a higher glycemic index increases the total glycemic load per day, and therefore also increases calorie intake and potentially, weight.
Research has not consistently shown, however, that lowering the GI index of your diet can reduce the risk of these conditions. Likewise, there is some mixed research on how exactly glycemic index and glycemic load impact A1C levels. However, some people do use a low-GI diet as part of their diabetes treatment plan or to lower their risk of diabetes complications.
In general, food packages do not include the glycemic index on the nutrient information so it can be challenging to shop or meal prep with GI in mind. If you are considering adopting a low-GI diet, consult with your healthcare team to create the best plan for your health and lifestyle.
How glycemic index might help you eat better
“Glycemic index is very good to analyze in terms of research,” says Dr. Shah.
Its role for consumers is more complex. For some people, keeping an eye on glycemic index is a helpful tool to manage their health.
But Dr. Shah says it’s important to approach food holistically, rather than focusing on a single component of a meal (whether that’s carbs, fat or protein content) or any single tool (be that the glycemic index, calorie counting or tracking macro nutrients).
“It’s also important to be mindful eaters and think about what we are eating, when we are eating, and our enjoyment of the food and taste. Intentional eating and mindfulness are really important here, too,” says Dr. Shah.
As always, consult with your healthcare team to determine what a healthy diet looks like for you.
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