Dear Doctor:

I’ve been reading that choosing foods that are low on the glycemic index is better for your health. What is the glycemic index and why should I pay attention to it?

Dear Reader: The glycemic index, also referred to as the GI, is a system that rates foods based on how quickly the glucose contained within them is absorbed into the bloodstream. Developed to help people living with diabetes to maintain steady blood glucose levels, the GI has become a useful tool for anyone who wants to avoid blood sugar spikes.

The food we eat is broken up into three main categories — protein, fat and carbohydrates. Of the three, carbohydrates are our main source of energy. The body turns carbohydrates into glucose, a type of sugar, which powers bodily functions. It gives you energy for both mental (your brain is a heavy user of glucose) and physical activity.

However, not all carbohydrates behave the same way after they are ingested. Some, like sweets, baked goods and some fruits and cereals, will cause a sharp rise in blood glucose levels. More complex carbohydrates, like beans or legumes and most vegetables, are digested and absorbed slowly due to their fiber content. When you eat foods that are low on the glycemic index, it leads to a gradual and controlled rise in blood sugar. Foods high on the glycemic index cause blood sugar levels to rise and fall quickly.

The GI is based on a scale of 0 to 100, with pure glucose assigned a value of 100. The more rapidly that foods release their load of glucose into the bloodstream, the higher they rank on the scale. Candy and processed cereals have high glycemic index values. The lowest values are assigned to foods in which glucose is digested and absorbed slowly. For example, cauliflower, spinach, green beans and mushrooms all have a GI of 0.

Why does this all matter?

Chronically high blood sugar levels have been tied to a wide range of health problems, including the risk of developing diabetes, heart disease, nerve damage, cataracts and kidney disease. By learning the GI values of the foods that you eat, you can take control of your diet. You can add foods low on the GI scale that will steady your blood sugar, and eliminate — or at least limit — the foods that make your blood sugar jump.

It’s important to note that a balanced and healthful diet will be made up of foods from all parts of the GI spectrum. There are several nutritious foods with a high GI value, such as sweet potatoes, which have a GI of 70. Meanwhile, foods with little nutritional value, such as milk chocolate, will rank lower on the GI scale, at about 40.

Foods high in carbohydrates are essential to a healthy diet, but quality matters. Stick with fresh vegetables, fresh fruit, whole grains, legumes and beans. Steer clear of processed foods, which are usually high in refined carbohydrates like white sugar or white flour. And before you make a radical change to the way you eat, be sure to check with your physician.