Overweight can cause depression, researchers say, and the effects are largely psychological.
While previous studies have shown that obese people are more likely to be depressed, it was not clear whether this depends on a reduction in the burden of depression or retreat.
Currently, in the largest study of this kind, experts say that having genetic variants associated with high body mass index (BMI) can lead to depression, with a stronger effect in women than in men. What's more, they say that research suggests that the effect may be caused by factors such as body image.
"People who are more overweight in the population are more depressed, and this will probably be at least partially [a] causal influence of BMI [on] depression "- said Prof. Tim Frayling, co-author of the report from the medical school of the University of Exeter.
Writing in the International Journal of Epidemiology, researchers from the United Kingdom and Australia described how they used data from a British biobank, a research sample involving 500,000 participants aged 37-73 who were recruited in 2006-2010.
Researchers analyzed 73 genetic variants associated with high BMI that are also associated with a higher risk of diseases such as diabetes and heart disease. They also looked at 14 genetic variants associated with a high percentage of body fat, but which were associated with a lower risk of such health problems.
The team then analyzed the participants' hospital data and the responses from multiple questionnaires – including reports on the GP visit or psychiatrist's visit due to anxiety or depression. The team identified around 49,000 participants with whom they were sure they were depressed.
In general, the team found that people with higher BMI are more likely to be depressed.
The researchers also found that the genetic predisposition to a higher BMI is associated with depression, and this effect is stronger in women than in men. The results were maintained even after additional tests, such as the exclusion of persons with familial depression, and when the analysis was repeated on data from a large international project called Consortium of Psychiatric Genomics.
Focusing on 73 genetic variations and taking into account factors including age and sex, they found that for every 4.7-point increase in BMI, the chances of depression were increased by 18% in total, and by 23% among women.
When the team collected data from various sources, they found that 14 genetic variants that increase fat, but are not related to metabolic disease, were also associated with an increased chance of depression.
"It suggests that the psychological component is just as strong as any physiological component, if [the latter] it is at all, "said Frayling, suggesting that a weak body image can be one of the mechanisms in the game.
The study has some limitations: it mainly concerned people with white European roots and contained some of their own data.
While the study did not show that increasing weight for other reasons would increase the risk of depression, Frayling said it was likely. "It allows [one] to conclude that greater effects in BMI will have a greater impact on depression, "he said.
Naveed Sattar, professor of metabolic medicine at the University of Glasgow, welcomed the study. "These new discoveries are probably the strongest to date, suggesting that higher weights can contribute to depression," he said. "Of course, many other factors can cause depression, but even weight loss can be helpful in improving mental health in some people, while keeping a slimmer one should help reduce the chances of depression."