Anemia affects as many as 23 percent of seniors, the researchers say.
"We found a 60 percent increased risk of dementia with anemia. After controlling for other factors such as other medical illness, demographics, etcetera, the risk remained elevated 40 to 50 percent," said lead study author Dr. Kristine Yaffe, a professor of psychiatry, neurology and epidemiology at the University of California, San Francisco.
"Given how common both anemia and dementia are in older adults, more attention to the connection between the two is important, and I do think screening older adults for anemia makes sense," said Yaffe.
The study of more than 2,500 men and women in their 70s doesn't actually prove that anemia causes dementia, however.
"Because we studied this prospectively, we do think, as best we can tell, that anemia is causally related to dementia, but with observational studies one can never say for sure. But we did our best to exclude other explanations," Yaffe said.
The job of red blood cells is to carry oxygen throughout the body. When you are anemic, less oxygen is delivered to brain cells, Yaffe explained. "We think the association is about low oxygen being carried to the brain," she said.
Anemia could also indicate poor overall health, the study authors noted. Causes of anemia include iron deficiency and blood loss. Cancer, kidney failure and certain chronic diseases can also lead to anemia.
The study -- published online July 31 in Neurology -- should remind doctors that many conditions can lead to dementia, and treating them might ward off mental decline, one expert said.
"One concern about the increased visibility and prevalence of Alzheimer's disease is that some physicians will be tempted to jump straight to that diagnosis without first having followed the 'rule out reversible causes' rule," said Dr. Sam Gandy, director of the Mount Sinai Center for Cognitive Health in New York City. Alzheimer's disease is the most common form of dementia.
"We must always seek to exclude treatable, reversible causes of dementia such as depression, nutritional deficiencies, endocrine disorders and metabolic disorders before rushing into a diagnosis of Alzheimer's," he said.
During the study, all of the participants were tested for anemia and took memory and thinking tests over 11 years.
Almost 400 participants were anemic at the study's start. Over the course of the study, about 18 percent of participants -- 455 -- developed dementia, the researchers found.
Of participants with anemia, 23 percent developed dementia, compared with 17 percent of those who weren't anemic.
People who were anemic at the study's start had a 41 percent higher risk of developing dementia than those without anemia after the researchers took into account factors such as age, race, sex and education.
Additional research is needed to confirm this association before recommendations are made regarding dementia prevention, the study authors suggested.
For more information on dementia, visit the U.S. National Library of Medicine.