The study adds to the evidence that diet may affect a person's chances of developing the mind-robbing disease that affects 4 million Americans.
Researchers found that people 65 and older who had fish once a week had a 60 percent lower risk of Alzheimer's than those who never or rarely ate fish. The meals included tuna sandwiches, fishsticks and shellfish; the amounts eaten were not specified.
Scientists aren't sure how omega three fatty acids, also found in nuts and vegetable oils protect the brain. But there is some evidence they prevent the formation of the plaques that disrupt normal brain functioning in Alzheimer's disease, reports CBS News Correspondent Elizabeth Kaledin.
Sixty-six year old Maureen McDonnell participated in the study.
"I think I'm okay. I think I'm connecting," said McDonnell. "You can ask my family."
McDonnell says she generally tries to eat fish 2 to 3 times a week.
"This is very promising, but it's very early and really we need to have a lot more studies," said lead researcher Dr. Martha Clare Morris of Chicago's Rush-Presbyterian-St. Luke's Medical Center.
The study involved 815 Chicago residents 65 and older. Follow-up tests nearly four years later found that 131 participants had developed Alzheimer's.
The researchers found an association between eating fish and a reduced risk of Alzheimer's even after adjusting for age, sex, ethnicity and risk factors like heart disease.
The study was published Monday in the Archives of Neurology. It was funded by the National Institutes of Health.
If the finding holds up, it could provide a simple way for people to reduce their risk of Alzheimer's, said Neil Buckholtz, chief of the dementia division at the National Institute on Aging.
Fish is rich in an omega-3 fatty acid that is believed to be important for brain development, Morris said. Studies have shown that animals fed the fatty acids had better learning abilities and memory, she said.
She said some participants in the latest study also saw a decreased risk of Alzheimer's from eating omega-3 fatty acids found in vegetables and nuts.
The same researchers found in an earlier study that people who have diets heavy in saturated fats run double the risk of getting Alzheimer's.
Dr. Rachelle Doody, professor of neurology at Baylor College of Medicine in Houston, questioned the new study's conclusion and warned, "Articles like this raise expectations and confuse people."
Doody said the researchers "can show an association, but they can't show cause and effect" between fish and Alzheimer's.
She said it is not known whether those people who had a reduced risk had eaten fish most of their lives, and whether other dietary habits had an influence. Also, those studied were asked to recall their diets nearly two years later on average, Doody said.
Neurologists, like Dr. Flint Beal believe that eating fish, as long as it's not deep fried, makes sense. But he also says there are some potential hazards.
"You could have some toxins that accumulate in fish, but I think the data that it's beneficial vastly outweighs any potential harmful effects from eating fish," Beal told Kaledin.
Bill Thies, vice president of medical and scientific affairs at the Alzheimer's Association, called the study "an interesting suggestion."
"It's not definitive proof. It points in the direction of benefits," he said.