Many people on the weight-loss drug orlistat may not keep up the diet changes considered necessary for the treatment’s success, a small study suggests.

Orlistat, sold under the prescription name Xenical, aids weight loss by blocking the absorption of dietary fat in the intestines. People on the drug are advised to limit their fat intake to no more than 30 percent of their daily calories. Higher fat intake increases the risk of gastrointestinal side effects like loose stools and incontinence.

It’s been argued that orlistat may aid weight loss, in part, because those side effects encourage people to stick with a low-fat diet for the long haul.

But the new study, of 44 obese adults, found that orlistat users had a slightly higher average fat intake than non-users over two months. And that average exceeded the recommended 30-percent limit.

Dr. Mette Svendsen and colleagues at Ulleval University Hospital in Oslo, Norway, report the findings in the Journal of Human Nutrition and Dietetics.

The study included 44 men and women who’d taken part in a three-year clinical trial comparing orlistat with a placebo, in addition to lifestyle changes. After the trial ended, participants could opt to continue on the drug or not; two months later, Svendsen’s team surveyed users and non-users about their diet habits.

On average, they found, orlistat users were getting 33 percent of their daily calories from fat, versus 28 percent among non-users.

The findings, according to Svendsen’s team, suggest that orlistat did not have a “policing effect” on patients’ diet choices. And this, they write, “may hamper the effect of the drug.”

Larger studies are needed to look at the diet habits of people who choose to take the anti-obesity drug for long-term weight maintenance, the researchers conclude.

SOURCE: Journal of Human Nutrition and Dietetics, February 2009.