Bariatric surgery or weight loss surgery does not necessarily alleviate depression in obese or overweight people, according to a Yale University study.

The researchers urge medical personnel to measure depression levels six to 12 months after surgery to provide necessary help.

In the study, the researchers wanted to determine whether bariatric patients are still experiencing depressive symptoms and if such symptoms are increasing after post-surgery. This is the first study to examine patients with discernible worsening depressive symptoms six and 12 months following gastric bypass surgery.

For the study, 107 patients obese people completed questionnaires before they underwent gastric bypass surgery and then again six and 12 months after the procedure. The participants were asked to keep a track on their levels of depression, eating disorders, self-esteem and social behaviour. Among 107 participants, 94 were women and 13 were men, 73 were white and 24 had completed college.

The researchers found that most bariatric patients were in much better spirits and experienced a normal and improved mood at six and 12 months after surgery.

However, in some cases, negative mood changes started to leak in between six and 12 months after the operation. About 3.7 percent of patients felt discernibly more depressed 12 months post-surgery. Plus, 13.1 percent of patients suffered increasing depressive symptoms between six and 12 months after the operation; however, even more patients (13.1 percent) reported increases in depressive symptoms.

Besides depression, their levels self-esteem and social functioning were also significantly affected.

"The majority of patients whose mood had worsened discernibly experienced these mood changes between six and 12 months post-surgery, suggesting this may be a critical period for early detection and intervention, as needed," Valentina Ivezaj from Yale School of Medicine said in a press release.

"The increases in symptoms of depression are also notable given that they were associated with other difficulties including lower self-esteem and social functioning," Carlos Grilo from Yale School of Medicine said.

The researchers said that the increasing depressive symptoms reveal only subthreshold or mild mood disturbances.

The finding is published in the journal Obesity Surgery.