Higher body mass index (BMI) may negatively impact a person's mental wellbeing, according to a study.

The research, published in the BMJ, explored the impact of aspects of physical health, such as body weight, heart health and blood pressure, to see whether a wide age range of individuals with poorer physical health went on to be less happy and less satisfied with their lives.

Using a technique called Mendelian randomisation, researchers from the University of Bristol in the UK asked whether poorer physical health causes lower mental wellbeing, or whether individuals with lower mental wellbeing are more likely to go on to have later problems with their physical health.

This technique provides evidence of the direction of causation by using genetic variants that have been associated with physical health and mental wellbeing.

The research team was able to test 11 measures of physical health including coronary artery disease, heart attack, cholesterol, blood pressure, body fat and BMI, a measure of body fat based on height and weight.

Results suggested a consistent causal effect of higher BMI on lower mental wellbeing. There was little evidence that the other physical health traits were leading to less happiness and life satisfaction.

The same pattern of results was seen in a follow-up analysis using the UK Biobank cohort of over 300,000 individuals aged 40 to 70 years old.

The researchers were able to look at different aspects of life satisfaction and found that the key impact of higher BMI was on lower satisfaction with health.

They were also able to show that the effect is present from age 40 through to age 70, and in both men and women.

When testing whether mental wellbeing caused any of these physical health traits, the researchers found little evidence for a causal impact in that direction.

"Results so far highlight the pressing need to tackle the obesity crisis because higher BMI is causing the population to be less happy and less satisfied with their lives," said Claire Haworth from the University of Bristol.

"This information could be immediately useful to clinicians encouraging patients to maintain a healthy weight," Haworth said.

"Frequently individuals are encouraged to lose weight because this will lead to better physical health, but for many this is not motivating enough," she said.