Depressed senior citizens in the United States require the equivalent of $9 billion worth of unpaid help each year to do daily activities.

The burden of that assistance is carried by the depressed seniors' spouses, children and friends and represents a huge, unrecognized cost of depression, says a University of Michigan and VA Ann Arbor Healthcare System study.

Even moderately depressed seniors require many more hours of help doing everyday activities than seniors with no depression, regardless of other health problems they may have.

The researchers estimated that it would cost society $9 billion a year if the family and friends who take care of depressed seniors were paid the wages (about $8.23 an hour) of a home health aide worker. That total means depression is second only to dementia in the national annual cost for informal caregiving.

For this study, researchers analyzed data from 6,651 people over the age of 70 from across the country and found 18 percent of them reported four to eight depressive symptoms within the previous week. Another 44 percent reported one to three depressive symptoms.

Of the seniors who had many depressive symptoms, 38 percent reported receiving informal care from family or friends. That figure was 23 percent among seniors with few depressive symptoms and 11 percent among seniors with no depressive symptoms.

"People with depressive symptoms also had a significantly higher likelihood than others of needing help with tasks such as dressing, bathing, eating, grocery shopping, taking medicines, paying bills and using the telephone," study author Dr. Ken Langa, an assistant professor of general medicine, said in a prepared statement.

"Even those with just a few depressive symptoms were more likely to need help with these everyday activities than those without signs of depression," Langa said.

The study appears in the May issue of the American Journal of Psychiatry.

The U.S. National Institute on Aging has more information about seniors and depression