TUESDAY, Jan. 17 (HealthDay News) -- Feeling younger in middle age may be as easy as eating a low-calorie, nutritionally balanced diet, U.S. researchers report.

These types of diets help slow cardiac aging, according to researchers at the Washington University School of Medicine in St. Louis.

Their study included 25 people, aged 41 to 65, who'd consumed a low-calorie diet (about 1,400 to 2,000 calories a day) an average of six years.

Using ultrasound imaging, the team found the hearts of the people on the low-calorie diets appeared more elastic than those of other people the same age who ate a typical Western diet (about 2,000 to 3,000 calories per day).

The hearts of the people on the low-calorie diet were also able to relax between beats in a way similar to the hearts of people 15 years younger. The study appears in the Jan. 17 issue of the Journal of the American College of Cardiology.

"This is the first study to demonstrate that long-term calorie restriction with optimal nutrition has cardiac-specific effects that ameliorate age-associated declines in heart function," principal investigator Dr. Luigi Fontana, an assistant professor of medicine, said in a prepared statement.

Previous research in mice and rats found their lifespans increased by about 30 percent when they were put on calorie-restricted diets. This diet also protected the animals against cancer and atherosclerosis.

Fontana and his colleagues previously found that people on a very low-calorie diet had low blood levels of cholesterol and triglycerides, blood pressure scores similar to much younger people, less body fat, and reduced risk of diabetes.

This latest study also found that markers of inflammation indicative of primary aging were much lower in people who ate the calorie-restricted diets.

"It's very clear from these studies that caloric restriction has a powerful, protective effect against diseases associated with aging," Dr. John O. Holloszy, a professor of medicine and co-investigator on the latest study, said in a prepared statement.

Calorie counting wasn't the only important factor, he added. "Caloric restriction does not mean eating half a hamburger and half a pack of french fries and drinking half of a sugary beverage. These people have very good nutrition. They eliminate calories by eating nutrient-dense foods," Holloszy said.

The diets resemble a traditional Mediterranean diet, which includes a wide variety of fish, fruit, vegetables, whole grains, beans and olive oil. Calorie-restricted diets avoid refined and processed foods, soft drinks, desserts, white bread and other sources of "empty" calories.