It's no mystery that eating fruits and vegetables does a body good, but does eating certain foods burn more calories than they contain?

Dr. Timothy Harlan, also known as Dr. Gourmet, said the notion has long been a theory, but it has not been proven.

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"There is not substantive research to support the concept of a negative-calorie food," he said.

He said the basis for the concept comes from the thermal effect of feeding, which describes the calories used in digesting foods. Ten percent of calories consumed in a meal are used for digestion.

"For instance, in a 400-calorie meal, the body will use about 40 calories for digestion and absorption of the nutrients," he said.

Harlan said there is clear and well-established research that shows diets high in fruits in vegetables result in weight loss. But, he said, it is not because they burn more calories than other foods, but primarily because of calorie density.

"The best part of the foods that are on many 'negative calorie' lists is that they are -- for the most part -- fresh foods that are great tasting and great for you. It's easy to lose weight by simply substituting these for what most folks eat today in processed, pre-packaged foods," he said.

That means eating fruits such as grapefruit, apples, oranges and strawberries; and vegetables such as lettuces, tomatoes and cucumbers. These foods aren't packed with fat. Harlan offered a list of foods that he says fits the bill.

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"What I like to stress is there (are) definitely low-calorie foods," said Keri Gans, spokeswoman for the American Dietetic Association. "They can benefit your diet whether you lose weight or not."

She said the great thing about fruits and vegetables is peoples' ability to eat them in large amounts.

But eating these foods might leave people hungry unless they add protein and fat. Gans said all fruits and vegetables are carbohydrates, and therefore the body burns them quickly.

"If you want to incorporate negative calories into a healthy weight-loss program, they need to be incorporated into a well-balanced diet," she said.

Gans added that the balance helps people feel full. For example, eating berries with low-fat yogurt or a salad with chicken keeps people satiated longer so they won't feel the need to overeat at the next meal.

She also stressed picking the right choices of food: low-fat dairy, whole grains and healthy fats.

Eating these foods that may not be as tempting or glamorous as chocolate, cake or ice cream, but they can help get stave off temptation.

Deborah Enos, the One Minute Wellness Coach and author of Weight a Minute, said the good news is if people increase the amount of fruits and vegetables they consume, they are probably not going want as much junk food.

She recommended staying away from foods that have refined ingredients such as white flour or sugar listed in the first five ingredients of the product and processed foods.

"It's not about just adding good stuff, it's about deleting bad stuff," she said. Other Links:

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