Behavioral risk factors, including smoking, obesity, limited physical activity, and a less healthy diet strongly predict the likelihood of depression—and that likelihood increases with each additional risk factor a person possesses. Additionally, the risk factors most strongly linked to depression change with age.

Previous studies had identified behavioral risk factors for depression, but it was unclear how these variables changed across the life span. This study sought to identify how the risk factors varied among three age groups: younger (18-39 years old), middle-aged (40-59), and older (60-99) adults.

The researchers collected data from more than 30,000 survey respondents, who answered questions about their lifestyle, including smoking, weight, physical activity and diet, as well as their history of depression. The team looked for correlations between the risk factors and depression, controlling for variables, such as gender, ethnicity, and socioeconomic status.

Sixteen percent of all participants had a prior diagnosis of depression. Smoking was most strongly associated with depression, especially in younger people: younger smokers had 2.7 times’ greater odds of having had depression, while middle-aged and older smokers had 1.8 times the odds, compared to nonsmokers of the corresponding age. Obesity was the next most important risk factor: younger, middle-aged, and older obese respondents had 65%, 54%, and 67% greater likelihood of depression, respectively, compared to non-obese counterparts.

Participants who had little physical activity were more likely to have depression as they grew older. And a less healthy diet was linked to depression in the middle-aged and older groups only.

Importantly, compared to having no risk factors, having one risk factor increased the odds of having had depression (1.7 times). When a person had two risk factors, the odds of developing depression more than doubled. Having three risk factors increased the odds of developing depression by more than three-fold, and a person with all four risk factors had almost six times the likelihood of depression.

The study is the largest yet to examine the behavioral risk factors for depression across age groups. Given the psychological, social, and economic toll of depression, as well as its growing prevalence, predicting a person’s risk at any age is critical, as are age-specific prevention programs, according to the authors. They said further research about nuanced risk factors, including gender and ethnicity, are warranted.

The study was published in the journal Mental Health & Prevention.