Heart patients in Norway — where unlike many countries foods are not enriched with folic acid — were more likely to die from cancer if they took folic acid and vitamin B12 supplements compared with those who did not take them, Norwegian researchers said on Tuesday.
The team found lung cancer rates were 25 percent higher among those who took the supplements compared with the general population, but overall cancer deaths and deaths from other causes were also higher in the supplement group.
They said folic acid given over a period of more than three years may feed the growth of cancers that were too small to be detected otherwise, and raises new questions about the benefits of fortifying foods with folic acid.
"Our results need confirmation in other populations and underline the call for safety monitoring following the widespread consumption of folic acid from dietary supplements and fortified foods," Dr. Marta Ebbing of Haukeland University Hospital in Bergen, Norway, and colleagues wrote in the Journal of the American Medical Association.
Folic acid, a B vitamin, helps the body make healthy new cells, and getting enough of it is crucial for women before pregnancy to prevent serious birth defects like spina bifida.
For this reason, many countries, including the United States, fortify flour and grains with folic acid. But recent studies have raised concern that folic acid may raise the risk of cancer.
Because there is no folic acid fortification of foods in Norway, the study population offered a good way to test for the effects of folic acid on cancer risk.
Ebbing and colleagues analyzed data from two large trials involving people with heart disease who took folic acid and vitamin B12 supplements to try to lower levels of the amino acid homocysteine, which has been linked with heart attacks and strokes.
The three-year studies failed to show a heart benefit, but the team continued to follow patients for over three years to see if the supplements had any effect on cancer risk.
When they combined the findings from the two trials for a total of more than 6,800 patients, they found those who got folic acid and vitamin B12 supplements had a higher risk of being diagnosed with cancer, of dying from cancer and of dying from any other cause.
"These findings were mainly driven by increased lung cancer incidence," the team wrote.
They said since the study took place in a population that is normally not exposed to folic acid, the findings raise concerns about the long-term effects of folic acid supplements.
The study is not the first to suggest folic acid may increase a person's risk for cancer. A study in March suggested that folic acid supplements raise the risk of prostate cancer. And a study in April suggested that fortifying foods with folic acid raised the risk of colon cancer.
In animal studies, the effects of folic acid have been inconsistent, with some studies suggesting the supplements protect normal colon tissue while enhancing the growth of abnormal tissue.
The team found no link between colon cancer risk and folic acid and said this may suggest the dual effects of the supplements on this cancer cancel each other out.