Obese people who participate in a weight loss program based on exercise and lifestyle changes end up less depressed, according to a new review.
But how many pounds they actually shed didn't seem to matter, and it's not clear that weight loss itself played a role, one expert said.
Obesity is a well-known risk factor for many medical conditions, including depression. Previous research has suggested that by losing even a small percentage of their body weight, heavy people can improve their physical and mental health, even if they remain obese.
However, some weight loss medications have been linked to higher rates of depression and suicidal behavior. To complicate things even further, people on antidepressants often gain weight.
The authors of the new report "don't want to imply that weight loss is a substitute for treatment of clinically significant depression," said Dr. Anthony Fabricatore, whose findings appear in the International Journal of Obesity.
"But for people who have some symptoms of depression and are overweight (or) obese, there's some relief that comes with weight loss," he added. Fabricatore, then at the University of Pennsylvania School of Medicine, now works for the diet and weight loss company Nutrisystem.
The researchers, all of whom have ties to drugmakers, reviewed a collection of 31 previous studies that had explored the relationship between weight loss in a structured program and changes in symptoms of depression.
In each of the studies, obese patients were randomly assigned to different kinds of weight loss programs, including diet-only or exercise-only programs or programs based around counseling and behavioral change.
Some patients took medications to boost weight loss, and others, used for comparison, got no treatment whatsoever. A total of almost 8,000 people were part of the studies.
The researchers on those original studies scored depressive symptoms before and after the weight-loss program, and noted how much weight participants lost during the program.
Because most weight loss studies do not include people suffering from clinical depression, Fabricatore and his colleagues focused on symptoms related to depression and mood, rather than tracking who had been diagnosed with clinical depression.
As a whole, people in almost every kind of weight loss program not based on medication saw improvements in their mood.
"Lifestyle modification" programs, such as those based on counseling on diet and exercise, had the most positive effect, and programs including workouts also relieved symptoms of depression. Treatment with weight loss medication, however, had no impact on participants' mood.
How much weight each person lost -- or didn't lose -- was not linked with their changes in mood during the weight loss program.
Fabricatore said that because people in weight loss programs such as these generally lose about 5 to 10 percent of their initial body weight, it could be that after a certain point, more weight loss doesn't improve mental health. "There may be a threshold value that once you lose a certain amount of weight, your mood's going to improve and losing more weight isn't necessarily going to make it better," he said.
Shedding pounds might also not be the only explanation for improvements in mood, said Dr. Patrick Smith, researcher at Duke University Medical Center in Durham, North Carolina, whose study was included in the review.
"There are some studies that have shown that improvements in fitness and weight loss and increased time exercising will improve your mood," he told Reuters Health. And then there are the possible positive psychological changes from these programs, such as improved body image, regardless of weight loss, he said.
Finally, "the other possible explanation is that many of these weight loss programs are conducted in a group manner, which provides some social support and some validation from other people," which could help improve symptoms of depression, Smith explained.
The authors stressed that their findings don't necessarily apply to people who've been diagnosed with clinical depression. Whether those patients can also improve their mood through participation in weight loss programs is still unclear.