When the National Institutes of Health convened a panel of independent experts this past April on how to prevent Alzheimer’s disease, the conclusions were pretty grim. The panel determined that “no evidence of even moderate scientific quality” links anything—from herbal or nutritional supplements to prescription medications to social, economic or environmental conditions—with the slightest decrease in the risk of developing Alzheimer’s. Furthermore, the committee argued, there is little credible evidence that you can do anything to delay the kinds of memory problems that are often associated with aging. The researchers’ conclusions made headlines around the world and struck a blow at the many purveyors of “brain boosters,” “memory enhancers” and “cognitive-training software” that advertise their wares on the Web and on television. One of the panel experts later told reporters in a conference call that the group wanted to “dissuade folks from spending extraordinary amounts of money on stuff that doesn’t work.”
But did the panel overstate its case? Some memory and cognition researchers privately grumbled that the conclusions were too negative—particularly with respect to the potential benefits of not smoking, treating high blood pressure and engaging in physical activity. In late September the British Journal of Sports Medicine published a few of these criticisms. As a longtime science journalist, I suspected that this is the kind of instructive controversy—with top-level people taking opposing positions—that often occurs at the leading edge of research. As I spoke with various researchers, I realized that the disagreements signaled newly emerging views of how the brain ages. Investigators are exploring whether they need to look beyond the brain to the heart to understand what happens to nerve cells over the course of decades. In the process, they are uncovering new roles for the cardiovascular system, including ones that go beyond supplying the brain with plenty of oxygen-rich blood. The findings could suggest useful avenues for delaying dementia or less severe memory problems.
Dementia, of course, is a complex biological phenomenon. Although Alzheimer’s is the most common cause of dementia in older adults, it is not the only cause. Other conditions can contribute to dementia as well, says Eric B. Larson, executive director of the Group Health Research Institute in Seattle. For example, physicians have long known that suffering a stroke, in which blood flow to the brain has been interrupted by a clot or a hemorrhage, can lead to dementia. But research over the past few years has documented the importance of very tiny strokes—strokes so small they can be detected only under a microscope after death—as another possible cause for dementia. Studies at autopsy of people who had dementia have detected many of these so-called microvascular infarcts either by themselves or along with the plaques and tangles more typical of Alzheimer’s in the brains of people with dementia. These findings suggest that most dementias, even those caused by Alzheimer’s, are triggered by multiple pathological processes and will require more than one treatment.
Proving that cardiovascular treatment is one of those approaches will take some doing. Just because microinfarcts may make dementia worse does not mean that preventing them will delay the brain’s overall deterioration. Maybe severe dementia makes people more vulnerable to microinfarcts. And just because better control of high blood pressure and increased physical activity seem to decrease a person’s risk of stroke, that does not necessarily mean they are less likely to suffer microinfarcts. Correlation, after all, does not necessarily imply causation. That scientific truism was the problem that kept bothering the panel of outside experts put together by the NIH. Thus, the expert panel concluded, with one exception, that “all existing evidence suggests that antihypertensive treatment results in no cognitive benefit.” Data showing the benefits of boosting physical activity in folks with confirmed memory problems were “preliminary.”