Although weight loss can significantly improve physical healthy, its effects on mental health are less straightforward, according to a new study.

Researchers from University College London found that people who lost 5 percent or more of their initial body weight over four years showed significant changes in markers of physical health, but were more likely to report depressed mood than those who stayed within 5 percent of their original weight.

"We do not want to discourage anyone from trying to lose weight, which has tremendous physical benefits, but people should not expect weight loss to instantly improve all aspects of life," Sarah Jackson, lead author of the study, said in a statement.

For the study, researchers collected data from nearly 2,000 overweight and obese adults in the United Kingdom. Before adjusting for serious health issues and major life events such as bereavement, which can cause both weight loss and depressed mood, the people who lost weight were 78 percent more likely to report depressed mood. After controlling for these, the increased odds of depressed mood remained significant at 52 percent.

"Resisting the ever-present temptations of unhealthy food in modern society takes a mental toll, as it requires considerable willpower and may involve missing out on some enjoyable activities. Anyone who has ever been on a diet would understand how this could affect wellbeing," Jackson said. "However, mood may improve once target weight is reached and the focus is on weight maintenance."

Researchers suggest that healthcare professionals monitor patients' mental as well as physical health when recommending or responding to weight loss, and offer support where necessary.

"People who are trying to lose weight should be aware of the challenges and not be afraid to seek support, whether from friends, family or healthcare professionals," Jackson added.