WASHINGTON (Reuters) - U.S. suicide rates appear to be on the rise, driven mostly by middle-aged white women, researchers reported on Tuesday.

A woman holds her head in an undated photo. REUTERS/File

They found a disturbing increase in suicides between 1999 and 2005 and said the pattern had changed in an unmistakable way -- although the reasons behind the change are not clear.

The overall suicide rate rose 0.7 percent during this time, but the rate for white men aged 40 to 64 rose 2.7 percent and for middle-aged women 3.9 percent, the team at Johns Hopkins University in Baltimore found.

“The biggest increase that we have seen between 1999 and 2005 was the increase in poisoning suicide in women -- that went up by 57 percent,” said Susan Baker, a professor in injury prevention with a special expertise in suicide.

Writing in the American Journal of Preventive Medicine, Baker, Guoqing Hu and colleagues said they analyzed publicly available death certificate data from the U.S. Centers for Disease Control and Prevention.

“The results underscore a change in the epidemiology of suicide, with middle-aged whites emerging as a new high-risk group,” Baker said in a statement.

“Historically, suicide-prevention programs have focused on groups considered to be at highest risk -- teens and young adults of both genders as well as elderly white men. This research tells us we need to refocus our resources to develop prevention programs for men and women in their middle years.”

Suicide is the 11th leading cause of death in the United States and Baker said the changes are substantial.

“Definitely these are not just little blips,” she said in a telephone interview. “We are looking at a big population change.”

She hopes other researchers will study the reasons behind the shifts. “I certainly think we need research to look at the information that we have on people who have committed suicide,” she said.

“Are these people living alone, with no major responsibility or others to take care of, or are they people overwhelmed with all of the jobs and responsibilities they have? We need to find out more about the conditions under which these people are living.”

The middle-aged women and men used various methods to kill themselves -- poisons, prescription drugs, hanging or suffocation, and firearms, Baker said.

While firearms remain the most common method, the rate of gun suicides decreased while suicide by hanging or suffocation increased by 6.3 percent among men, and 2.3 percent among women.

In September researchers confirmed an 18 percent spike in youth suicides in the United States in 2004 persisted into 2005 after more than a decade of decreases.

And international research published in January found that the young, single, female, poorly educated and mentally ill are all at higher risk of suicide.

According to the World Health Organization, suicide rates have increased by 60 percent in the last 45 years. Depression is the leading cause of suicide.