"Our goal was to understand what we see as an important, but under-researched aspect of obesity - sexual quality of life," said lead researcher Martin Binks, director of behaviour health at Duke University's Diet and Fitness Centre in Durham, North Carolina.

Previous research by Binks' team suggested that obese people experienced significantly high rates of difficulty in the quality of their sex lives. However, "little is known about the impact of weight loss on these factors," the researcher said.

In their study, Binks and his colleagues examined sexual quality of life data from a weight-loss trial. The trial included 161 obese women and 26 obese men. Data was collected every three months over two years. To measure sexual quality of life, the researchers used items from a standard questionnaire called the Impact of Weight on Quality of Life.

Among the factors covered by the survey: feeling sexually unattractive, lack of sexual desire, reluctance to be seen undressed, difficulty with sexual performance, avoidance of sexual encounters, and lack of enjoyment of sexual activity.

"At the beginning of the trial, both male and female participants indicated that they were experiencing significant difficulty in all areas of sexual quality of life," he noted.

Women reported more difficulty than men, Binks said. "For women, not wanting to be seen undressed and not enjoying sexual activity were particularly troublesome," he said.

"Improvements in the quality of sexual life were directly related to weight loss and seemed to reach their maximum at about 12 percent weight loss," Binks said.

Sixty-seven percent of the women said they felt sexually unattractive at the start of the study. "That prevalence dropped to 26.4 percent at one year and remained stable," Binks said. "Not wanting to be seen undressed went from 62.7 percent to 34.3 percent," he added. There were similar reductions in the other areas, Binks said.

While there were similar improvements among men, there were too few men in the study to be able to draw definitive conclusions, Binks said. For men, not wanting to be seen undressed and not enjoying sexual activity were the two most important issues, just as they were for women.

"A 10 percent reduction in weight significantly improves most health issues," Binks concluded. "It appears that sexual quality of life improves in a similar way to (other) weight-related issues."

"Body image is a big part of sexuality," said Lona Sandon, a spokeswoman for the American Dietetic Association and an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Centre, in Dallas.

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