The World Health Organization's (WHO) 2010 Global Burden of Disease (GBD) project found clinical depression remained one of the foremost leading causes of global disability.

“These findings emphasize the importance of including depressive disorders as a public-health priority and implementing cost-effective interventions to reduce its burden,” concluded the authors of a PLOS Medicine study, which looked at the GBD's assessment of global burden brought on by depressive disorders.

Disability is defined by the WHO as the "years of life lost due to time lived in states of less than full health" and is one of two factors the GBD considers when gauging the extent of global burden that a disease inflicts on a population. The other GBD measure is the number of years of life that have been lost due to premature death on account of a disease. Both of these "time-based measures" are then used to calculate the burden of a disease i.e. disability-adjusted-life-years, or DALY.

In the latest 2010 GBD study, Alize Ferrari from the University of Queensland and the Queensland Centre for Mental Health Research and colleagues found that depressive disorders were the second leading cause of the number of years that people lived with the disability and eleventh in terms of global burden (DALY). When researchers took into account the role that depressive disorders have in suicide and ischemic heart disease, the disorder moved up to eighth place in global burden.

“These results have since made significant contributions to prioritizing depressive disorders, and mental disorders as a group, in global public health agendas; particularly in promoting the addition of mental health interventions to health management plans,” the study authors wrote.

The 2010 GBD project was larger in comparison to the previous GBD studies of 1990 and 2000 by quantifying the burden of 291 diseases and injuries that cause premature death and disability as well as 67 preventable causes of sickness (smoking, high blood pressure, air pollution). A total of 187 countries, 21 world regions, and 20 different age groups of both sexes were analyzed. With respect to depressive disorders, the GBD took into account subtypes for the first time by differentiating major depressive disorder (MDD) from dysthemia, a chronic form of depression that is not as severe as MDD.

MDD was to blame for 8.2 percent of the total nunber of years people lived with disability and 2.5 percent of the years that would have been lived if it weren't for premature death. Dysthymia accounted for a relatively small 1.4 percent and 0.5 percent respectively. “Our findings not only emphasize depressive disorders as a global health priority, but also highlight the importance of understanding the variations both between and within regions when setting global health objectives,” the authors explained, using war-torn regions in the Middle East as an example.

When all causes of disability were taken into account according to country and region, Afghanistan proved to be worst off by ranking highest while Japan ranked lowest. Overall, the GBD 2010 also showed that demographic changes, such as the reduction in child mortality and people living longer, are shifting the primary cause of health problems more towards non-communicable diseases. “Health problems are increasingly defined not by what kills us, but what ails us,” the GBD report concluded. For example, in 1990, childhood malnourishment was at the top of the list of risk factors for ill health. But in 2010, high body mass became a more prominent cause of premature death and disability.