A nationwide study has found that more Americans than ever before now report suffering from serious mental health problems - but the country's healthcare system is struggling to meet the demand.

The survey spanned eight years and involved more than 200,000 people living in the US aged between 18 and 64. It concluded that 3.4 percent of the population now suffer from what researchers call 'severe psychological distress' (SPD) - mental health issues so serious that they affect someone's physical wellbeing.

Extrapolated, that suggests more than 8.3 million adult Americans suffer from mental health issues, far higher than researchers had previously assumed.

And yet the study also found that access to mental health care services actually deteriorated between 2006 and 2014 for people suffering from SPD, when compared to those who didn't report mental distress. In other words, the gap is getting bigger.

"Although our analysis does not give concrete reasons why mental health services are diminishing, it could be from shortages in professional help, increased costs of care not covered by insurance, the great recession, and other reasons worthy of further investigation," said lead researcher Judith Weissman from NYU Langone Medical Center.

To get this insight, the researchers looked at data from the 2006 to 2014 US CDC's (Centres of Disease Control and Prevention) annual National Health Interview Survey, which has been conducted for more than 60 years in about 35,000 US households around the country.

Participants are asked how often over the past month they felt certain feelings, such as being so sad that nothing could cheer them up, or that everything they did was worthless.

Together, the frequency of these symptoms allows researchers to get an idea about whether someone is suffering from SPD or not.

While SPD itself isn't a formal diagnosis, previous research has linked a high SPD score with mental health issues such as depression and anxiety, as well as chronic disease, lower socio-economic status, and a reduced lifespan.

"Based on our data, we estimate that millions of Americans have a level of emotional functioning that leads to lower quality of life and life expectancy," said Weissman.

"Our study may also help explain why the US suicide rate is up to 43,000 people each year."

For perspective, that's the highest it's been in the US in 30 years.

In total, around 3.4 percent of the population showed signs of suffering from SPD - previous estimates had put that number at less than 3 percent.

But despite the increase in people needing support, people with SPD specifically are also finding it harder to get support than before.

Interestingly, the report suggests that the mental health care situation has gotten worse despite the introduction of the 2008 Mental Health Parity and Addiction Equity Act and the 2010 Affordable Care Act (ACA). Both included provisions that were meant to make it easier for people without insurance to access mental health support.

The team found that, in 2014, 9.5 percent of Americans who reported SPD still didn't have health insurance that would give them access to a psychiatrist or counsellor - a slight increase from 9 percent in 2006.

About 10.5 percent of people who reported SPD in 2014 experienced delays in getting professional help due to insufficient mental health coverage, while only 9.5 percent of people said they experienced such delays in 2006.

And, in 2014, 9.9 percent of people with SPD couldn't afford to pay for their psychiatric medications, up from 8.7 percent in 2006.

There are some limitations to this research - primarily the fact that the classification of SPD was based on self-reported symptoms, rather than being objectively determined by a healthcare professional.

The team was also specifically looking at the access people with SPD had had to health care, but didn't investigate whether there were other patterns in demographics that would make it harder for them to get help.

While the results of this latest study paint a pretty grim picture about US health care, the team is now looking at ways the system could be improved, and suggest that getting primary care physicians more involved in mental health support could help.

"Our study supports health policies designed to incorporate mental health services and screenings into every physician's practice through the use of electronic medical records, and by providing training for all health care professionals, as well as the right resources for patients," said one of the researchers, Cheryl Pegus.

The research has been published in Psychiatric Services.