But even the study researchers recommend caution in interpreting the results, emphasizing that the improvement from the supplements was modest for the participants.
"Long-term beta-carotene had the effect of delaying changes in their memory by about a year," says Francine Grodstein, ScD, associate professor of medicine at Harvard Medical School and associate epidemiologist at Brigham & Women's Hospital in Boston, who led the study.
Other experts familiar with the new research say the study is no reason to begin taking supplements of beta-carotene simply to avoid dementia.
Beta-Carotene and Memory
Long-term oxidative stress caused by "free radical" compounds circulating in the blood is thought to be a major factor in many health problems, including a decline in cognitive functioning. Decreased cognitive functioning, in turn, is thought to strongly predict the development of dementia.
So researchers are focusing on beta-carotene and other antioxidants, trying to find out if high intake or high blood levels can prevent the oxidative stress. Findings so far have been mixed.
In the study, Grodstein and her co-researchers evaluated the effects of beta-carotene or placebo on cognitive abilities in nearly 6,000 men, including those who participated long term and short term.
The long-term group included 4,052 men who had been taking either 50 milligrams of beta-carotene or a placebo tablet every other day from 1982 to 1995 as participants in the Physicians' Health Study. The study was originally designed to look at the effect of beta-carotene, aspirin, and placebo on heart disease and cancer, and when the beta-carotene arm ended in 1995, researchers had found no effect on either disease from beta-carotene.
Some of the participants agreed to join the sequel study, Physicians' Health Study II, beginning in 1997, and kept their original assignments to take either beta-carotene or placebo.
An additional 1,904 men, new recruits, were assigned to take either the supplement or placebo, joining between 1998 and 2001.
The men were all in good health when they started the studies. Those who had taken part in the original study were about age 73 when the sequel study began; the new recruits were about 56 on average.
The men were followed up through 2003, completing yearly questionnaires about their health and their compliance with taking the pills. Their cognitive functioning was tested by telephone at least once between 1998 and 2002.
"The focus was on verbal memory," Grodstein tells WebMD. For instance, researchers would read a list of words and ask the participants to read them back immediately and again 20 minutes later.
"We found [in] the men who had been on beta-carotene long term -- 18 years [on average] -- their memory was significantly better than the placebo group," Grodstein says. In the 18-month interval between studies, she says, some of the men in the original study may have stopped the beta-carotene temporarily.
Grodstein says she would categorize that improvement as modest. Even so, she says, modest improvement may lead to substantial differences in a person's eventual risk of getting dementia. Taking the supplements short term had no effect, she says.
The study is published in the Nov. 12 issue of Archives of Internal Medicine. The research was supported by grants from the National Institutes of Health, BASF Corp. (which makes a beta-carotene supplement), and others.
The unusual study design, in which some participants were carried over from the other study and kept their original assignments, may have affected the results, says Peifeng Hu, MD, staff geriatrician at Santa Monica-UCLA Medical Center & Orthopaedic Hospital, Calif. Hu is also assistant professor of geriatric medicine at the University of California Los Angeles David Geffen School of Medicine.
The findings don't warrant recommending supplements of beta-carotene, he says. "I would not change my practice based on this study," says Hu, who found in his own research that high blood levels of beta-carotene (not necessarily from supplements) reduced the risk of cognitive decline in people genetically predisposed to get Alzheimer's. His study findings were published in 2006 in the Journal of Gerontology.
Hu says the area is worthy of more study but points out that the evidence so far is conflicting: "There have been randomized clinical trials that have demonstrated the potential harm resulting from beta-carotene supplementation in pill form."
In an editorial accompanying the study, Kristine Yaffe, MD, a researcher at the San Francisco VA Medical Center and University of California, San Francisco, also calls for caution. She references several other studies in which beta-carotene and other antioxidants have had little or no effect on cognitive function.
What to Do Next?
"It's good to be cautious," says Grodstein.
Although the effect of the supplements was modest, she notes that the benefits she found were greater than what was observed in another study in which Aricept, a drug commonly used to treat the symptoms of Alzheimer's, was given to healthy people to test its effect on cognitive function.