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Dr K: Low glycemic diets good for your health

DEAR DOCTOR K: Diabetes runs in my family, and my mother says I should eat a “low glycemic index” diet. Can you explain what this is?

DEAR READER: Carbohydrates (“carbs,” for short) are one of the main types of nutrients in food. Common sources of carbs include bread, pasta, cereals, fruit, milk, vegetables and beans.

The carbs we eat are mostly too big for us to digest. Carbs are long strings of a certain type of molecule. Think of them as a string of pearls. When they hit the gut, digestive enzymes start to chop them up. It is the one-pearl and two-pearl strings that are the sugars that get digested and travel from the gut into the blood. As a result, the energy that is in those tiny pieces reaches every cell in our body.

Some of the carbs we eat are easy for our gut to reduce into tiny pieces. Other carbs take much longer to chop up. When carbs are easily converted into sugars, blood sugar levels rapidly rise to higher levels. When carbs are converted more slowly, blood sugar levels rise slowly to relatively lower levels.

Carbs that are chopped up slowly are said to have a low glycemic index. Carbs that are easily digested and absorbed, and raise blood sugar levels rapidly, have a high glycemic index.

Of course, we don’t eat pure carbs (except for table sugar). We eat foods that contain carbs, and some foods contain more carbs than others. So the amount of sugar that enters our bloodstream after a meal depends both on how much carbohydrate they contain and how easily those carbs are absorbed (the glycemic index). Multiplying the amount of carbs in a food by the glycemic index of those carbs determines the “glycemic load.”

For example, the glycemic index of a carrot is 131. In comparison, a serving of mashed potatoes has a glycemic index of only 104. However, a half-cup serving of carrots has only about 4 grams of carbohydrate. The same quantity of mashed potatoes has more than 18 grams of carbohydrate. That’s why the glycemic load for a serving of carrots is 11, while that of a serving of potatoes is 20.

In a study of tens of thousands of women over several decades, a research team at Harvard Medical School found that those who had low glycemic loads (mainly from eating whole grains) cut their risk of weight gain in half.

On my website (AskDoctorK.com) there is a table that gives the glycemic index and glycemic load of common foods. You may well find that some of the foods you love most have healthy glycemic loads.

(This column is an update of one that originally ran in June 2012.)

Correction: Recently I wrote that the Mantoux skin test is the “only” way to diagnose an inactive TB infection. I should have said the “most common” way. There also is a blood test, which has some advantages over the skin test. I regret the omission.

(Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.)

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