People with poor literacy face more mental health problems such as loneliness, depression and anxiety, according to a review of studies which used data from nine countries, including India.

The research, published in the journal Mental Health and Social Inclusion, is the first to look at the global picture of literacy and mental health.

The researchers from the University of East Anglia (UEA) in the UK found that little or no literacy disproportionately affects women, who account for two-thirds of the world's illiterate.

"Despite rising literacy rates over the past 50 years, there are still an estimated 773 million adults globally who can't read or write," said Bonnie Teague from UEA's Norwich Medical School.

"Literacy rates are lower in developing countries and those with a history of conflict, and women are disproportionately affected," Teague said.

The researchers noted that people with more literacy tend to have better social outcomes in terms of things like finding employment, being paid well, and being able to afford better food and housing.

Not being able to read or write holds a person back throughout their life and they often become trapped in poverty or more likely to commit crime, they said.

"We also know that lower literacy is related to poorer health, chronic diseases and shorter life expectancy," Teague said.

"There has been some research examining the potential association between literacy and mental health but this is the first study looking at the issue on a global scale," he added.

The team reviewed data from 19 studies that measured both literacy and mental health. These studies took place across nine different countries -- US, China, Nepal, Thailand, Iran, India, Ghana, Pakistan, and Brazil, and involved almost two million participants.

The researchers used information relating to mental health and literacy to assess the globally reported relationship between these two factors. They found a significant association between literacy and mental health outcomes across multiple countries.

"People with lower literacy had greater mental health difficulties such as anxiety and depression," said Lucy Hunn, who completed the review as part of her Doctorate in clinical psychology training at UEA.

"We can't say for sure that poor literacy causes poor mental health, but there is a strong association," Hunn said.

The researchers noted that there may be multiple factors impacting mental health which also impact literacy such as poverty or living in an area with a history of conflict.

However, the data suggests that even in these places, worse mental health is seen for those without literacy skills, they added.