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Today in Your Health, an American epidemic of kidney stones.

INSKEEP: Doctors see a link between that problem and obesity. And we begin with ways to maintain a healthy weight. Some people count calories, carbs and fats. And some researchers think you should look at the glycemic food index. If you're wondering what's that, listen to NPR's Patti Neighmond.

PATTI NEIGHMOND, BYLINE: Here's the premise. Not all calories are alike. Obesity specialist Dr. David Ludwig with Boston Children's Hospital.

DR. DAVID LUDWIG: Simply focusing on the calorie content of food ignores a key notion, which is that foods affect blood sugar and hormones in dramatically different ways based on their composition.

NEIGHMOND: For example, some foods are slow to digest, like most fruits, nuts and vegetables. They take their time wending their way through the digestive system. And that means they use up more energy and calories. Other foods zip through our system rapidly.

LUDWIG: White bread, white rice, potato products, prepared breakfast cereals are all made up of sugar in a long chain. And the body can digest these foods into sugar in literally moments after eating.

NEIGHMOND: That quick digestion causes a surge of blood sugar and then a crash. That makes us feel hungry pretty quickly after eating. But with slow foods we feel satisfied longer. Here's Mike Rogers, who took part in a study headed by Dr. Ludwig. Rogers was all excited when part of his diet plan one night included a big pile of mashed potatoes.

MIKE ROGERS: Then I went and looked at the clock, because I was hungry again, figuring that it must be time for me to eat my snack. And I said, that can't be right. When did I - and I realized that it had been almost no time at all since I had finished this enormous plate of mashed potatoes.

NEIGHMOND: The reason Rogers was hungry so quickly is because potatoes, like corn and a few other vegetables, have a high glycemic index. So do most refined and processed foods. They're digested quickly, causing that surge, then crash, of blood sugar. Not only that, Ludwig says, high glycemic foods slow down metabolism.

LUDWIG: One of the most unfortunate aspects of weight loss maintenance is that it takes fewer and fewer calories to just stay the same, let alone keep losing weight. That as the body loses weight, it becomes more efficient and requires fewer calories.

NEIGHMOND: Ludwig recently completed his diet study, which compared low-carb, low-fat and low-glycemic diets. He found the low-carb diet burned the most calories, but it was hard to stick to. The most promising diet turned out to be the low-glycemic one. People like Mike Rogers were able to lose weight and keep it off. Rogers maintained a 40 pound weight loss by shifting to low-glycemic foods that are mostly natural and unprocessed.

ROGERS: I eat a lot less white rice and a lot less potatoes than I used to eat. And the other thing that I learned that I liked quite a bit was steel cut oats. I've never been a big fan of standard oatmeal, but steel cut oats has enough tooth in it that I really enjoy that.

LUDWIG: Dr. Ludwig is quick to caution his study was short and not conclusive. He's working now to design a long term study that looks at diet and weight loss maintenance over a number of years.

NEIGHMOND: Joy Dubost is a registered dietitian with the Academy of Nutrition and Dietetics. She says the problem with the low-glycemic diet is that it can be confusing because lots of factors affect how the body digests food.

JOY DUBOST: Factors like ripeness, the variety of the products, the way it was prepared, the types of sugar that are present or carbohydrates, the amount of fiber and fat present and just the way your body digests it.

NEIGHMOND: And eating too many low-glycemic foods that are also high in calories, sugar or saturated fats can also be a problem. Dubost urges moderation of carbs and fats. And equally important, the other end of the equation - exercise. She points to a study which found people who successfully maintained their weight loss added 60 to 90 minutes of moderate exercise every single day.

Patti Neighmond, NPR News.

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