We have known for decades that mental health plays an important role in one's quality of life, but a study released this week suggests it is also an important factor in one's quantity of life.

A new University of Toronto study that followed 12,424 adult Canadians from the mid-1990s until 2011 found that those who were in suboptimal mental health at the beginning of the study died, on average, 4.7 months earlier than their peers who were in excellent mental health. 

The study took into account the 'usual suspects for premature mortality' including the respondents' functional limitations; health behaviors, such as smoking, heavy drinking and physical activity level; physical diseases, such as cancer, heart disease, diabetes, and chronic pain; and social support at baseline. 

"Ever after fully adjusting for these risk factors, we still found that those with suboptimal mental health at the beginning of the study had a 14% higher risk of all-cause mortality over the 18 years of the study," says Esme Fuller-Thomson, lead author of the study. Fuller-Thomson is Director of the University of Toronto's Institute for Life Course and Aging and Professor at the Factor-Inwentash Faculty of Social Work (FIFSW) and the Department of Family and Community Medicine.

A dichotomous flourishing indicator was created to identify people who were happy and satisfied with life and had good psychological functioning at baseline (in 1994 and 1995), in contrast to those with suboptimal mental health.

"The flourishing scale we developed set a very high bar," reports Yu Lung, a doctoral student at the FIFSW. 

In 1994 and 1995, 81% of the sample were flourishing and 19% were in suboptimal mental health.

"The findings of this Canadian study are in keeping with my previous research in the US, where I found that baseline suboptimal mental health was associated with higher 10-year mortality," says co-author Corey L.M. Keyes, Professor of Sociology at Emory University in Atlanta.

Unfortunately, the study's secondary data analysis did not have sufficient information to understand why excellent mental health is associated with longer life. 

"We have several hypotheses that we would like to investigate in future research," says co-author Keri J. West, a doctoral candidate at the FIFSW. "Previous research has found that positive affect is associated with lower levels of cortisol, reduced inflammation, and better cardiovascular activity. Furthermore, individuals with high levels of mental well-being are more likely to consume nutritious foods, adhere to treatment regimens, maintain strong social ties, and have better sleep quality, which may contribute to longevity." 

In addition to mental health, the researchers looked at other factors at baseline that were associated with premature mortality. 

"As expected, modifiable risk factors, including smoking, heavy drinking, and infrequent physical activity, were associated with a higher probability of all-cause mortality," reports co-author Philip Baiden, Assistant Professor of Social Work at the University of Texas at Arlington. "Cancer, heart disease, diabetes, chronic obstructive pulmonary disease, and high blood pressure were associated with a higher probability of death over the follow-up period."

The researchers examined data from a nationally representative sample of 12,424 respondents aged 18 years and older in the Canadian National Population Health Survey. Participants were first interviewed in 1994 and 1995 (wave 1) and then were followed until 2010 and 2011 (wave 9). Mortality data was ascertained by the Canadian Vital Statistics-Death Database in wave 9. By the end of the study period, 2,317 of the participants had died. This research was published online ahead of press this week in the Journal of Psychosomatic Research.

"The association between suboptimal mental health and premature death is a robust relationship that is independent of health conditions, pain, functional limitations, and negative health behaviors at baseline," says Fuller-Thomson. "Our findings underline the importance of considering the mind and body as a true continuum."