"Our study is the first to show a relationship between calcium supplementation and increased risk for dementia in older women," said lead author Dr. Silke Kern of the University of Gothenburg in Sweden. Still, the findings from this observational study don't prove calcium supplements directly cause dementia, Kern added by email. Even for women who have had a stroke, it's too soon to say for sure whether it makes sense for them to avoid calcium supplements, Kern noted.
"These findings need to be replicated before any recommendations can be made," Kern said.
Millions of women take calcium supplements to strengthen bones made brittle by osteoporosis, a bone-thinning disorder that typically develops starting during menopause when the body slows production of new bone tissue.
For the current study, Kern and colleagues examined data on 700 women between the ages of 70 and 92 who didn't have dementia.
At the start of the study, and again five years later, women did a variety of psychiatric and cognitive tests including assessments of memory and reasoning skills. A subset of about 450 women also got brain scans. When the study began, 98 women were taking calcium supplements and 54 participants had already experienced a stroke. During the study, 54 more women had strokes, and 59 women developed dementia. Among the women who had brain scans, 71 percent had so-called white matter lesions, which are signs of mini-strokes and other disorders that affect blood flow to the brain. Overall, women who took calcium supplements were twice as likely to develop dementia as their peers who didn't, the study found.
But the increased risk appeared limited to people who had a stroke or other signs of existing cerebrovascular disease.
For women with a history of stroke, the dementia risk was almost seven times higher if they took calcium supplements than if they didn't.
When women had white matter lesions that can be a precursor to strokes, the dementia risk was three times greater when they took calcium supplements. Among women without a stroke history or white matter lesions, however, there wasn't any increased dementia risk associated with calcium supplements. Beyond its small size, other limitations of the study include the lack of follow-up brain scans at the end of the study, which made it impossible for researchers to assess how calcium supplements may have influenced the development of white matter lesions or silent strokes.
In addition, the study didn't look at how much calcium women got in their diets, which can affect the body differently than supplements and is thought to be safe or even protective against blood flow problems, the authors note. "Women and the public need to realize that when we talk about micronutrients -calcium included - and cognitive functioning, we need to consider that the combination of nutrients will be more predictive than one nutrient," said Dr. Neelum Aggarwal, a researcher at Rush University Medical Center in Chicago who wasn't involved in the study.
"For example, calcium, phosphorus and magnesium all are typically looked at for their effects on multiple organs, and cognitive functioning will be affected most likely by a combination of these nutrients," Aggarwal added by email. "To say that only one nutrient increases the risk of dementia is premature and more studies need to look at a combination of nutrients."