For overweight and obese people, losing weight delivers clear health benefits. Blood pressure goes down, and so do tryglecerides, the fatty cholesterols that can clog up blood streams. The risk of heart disease is lower.

But while weight loss is a clear-cut plus for physical health, mental health is a murkier subject. According the the results of a recent study published in PLoS One, in some cases, weight loss even seems linked to depression.

In the past, clinical trials aimed at weight loss have found that people become happier when they drop pounds. As the University College London authors of this new study pointed out, however, those are controlled, supportive environments. It could be that the moral support, not the weight loss itself, is improving people's mood.

In this new longitudinal study, the researchers analyzed data collected over four years from nearly 2,000 overweight and obese adults in the U.K. who were over 50 years old. Those who lost more than five percent of their original body weight, the team found, were more likely to report feeling depressed.

Depression itself can cause weight loss, however, so the researchers controlled for individuals who suffered from clinical depression or other serious illnesses. They also controlled for individuals who had undergone an extreme loss in the past four years, such as the death of a spouse. After applying all of those controls, they found that the people who lost weight were just over 50 percent more likely to be depressed than those who had not.

This doesn't mean that losing weight causes depression, the authors point out. It could be that people felt depressed because of the constant strain of having to resist their favorite foods or that their mood would even out after they reached their weight loss goal. Even if the cause of the depression is unclear, though, the team writes that the results send a message that "people should not expect weight loss to instantly improve all aspects of life."