By Michael Kahn

LONDON (Reuters) - Children born to fathers older than 30 are more likely to develop bipolar disorder, a common condition sometimes known as manic depression, researchers reported on Monday.

The paternal risk also grows with the age of a father, rising to 37 percent by the time a man is 55 years, said Emma Frans, an epidemiologist at the Karolinska Institute in Sweden, who led the study.

The brain disorder causes extreme shifts in mood, energy and ability to function. It is marked by high periods of elation or irritability and low periods of sadness and hopelessness that can last months.

The findings published in the Archives of General Psychiatry bolster evidence that children of older fathers are at higher risk of psychological conditions such as bipolar disorder, autism and schizophrenia, the researchers said.

"Advanced paternal age is a risk factor for bipolar disorder in the offspring," Frans and colleagues wrote.

One explanation could be that a man's degraded sperm quality as he ages could increase the likelihood of genetic mutations that may lead to bipolar disorder, Frans said.

"Despite the robust evidence supporting the association between paternal age and severe mental disorders, the association between advanced paternal age and bipolar disorder has not been investigated," the team added.

The findings are another step toward unraveling the mystery of how the condition affecting an estimated 1 percent to 3 percent of adults worldwide arises, the researchers said.

Last month, an international research team linked two genetic variants to an increased risk for the disease. The condition often runs in families.

The Swedish researchers used a national medical registry to identify nearly 14,000 men and women diagnosed with bipolar disorder. For each person, they also randomly selected five people of the same sex and age without the condition.

After factoring for maternal age, the researchers found that children born to fathers older than 30 had an 11 percent higher risk of developing bipolar disorder compared to younger fathers. Children whose fathers were older than 55 had a 37 percent increased risk.

Frans said the findings did not mean that older men should not father children because the overall risk is still low, she added.

"The study sheds light on the negative effect of older fathers but most older men will still have healthy children," she said in a telephone interview.

Related MedlinePlus Pages:

• Bipolar Disorder
• Child Mental Health