An estimated 35 million people worldwide currently have Alzheimer's disease, and that number will probably double by 2030, according to Alzheimer's Disease International, as better medical treatment enables more people to live long enough to get it.
That means the costs of treatment and caretaking will rise, too, which causes economists to warn of an approaching tsunami of expense that could swamp the health care system.
But the tsunami may be losing power. For reasons that remain obscure, the incidence of Alzheimer's appears to be dropping around the world. That means that even if older people continue to make up a larger portion of the population, the percentage of them who develop dementia might be considerably smaller.
Alzheimer's rarely strikes anyone before age 60, but about 5 percent of people 70 to 80 develop the disease, and the rate increases rapidly until 85, when nearly half of people that age have Alzheimer's or its precursor, mild cognitive impairment.
If this rate of Alzheimer's were to continue, the world would indeed be facing a crisis, but a study that just appeared in the journal Neurology found that the presence in the brain of beta-amyloid — the type of protein believed to trigger Alzheimer's — has declined significantly since 1972. People 85 and older in 2006 had less beta-amyloid in their brains than people ages 75 to 84 did in 1972.
A recent study in the Lancet found that the rate of dementia among people 65 and older in England and Wales has dropped more than 25 percent in the past two decades, from 8.3 percent to 6.2 percent.
What's going on?
The most persuasive explanation involves better cardiovascular health. What's good for the heart is good for the brain, neurologists say, and apparently exercise, healthy diet, declines in smoking, better control of blood pressure, and the use of statins to discourage accumulation of plaque in the arteries have combined to help aging hearts continue to provide a generous supply of oxygen to the brain.
Another factor appears to involve an increase in a hypothetical condition known as cognitive reserve. For some reason, the incidence of Alzheimer's is lower among people with a higher education. Perhaps people who seek out more education have greater brain power to begin with, which allows them to lose more brain cells before the symptoms of dementia appear. Or perhaps education itself somehow builds cognitive reserve.
Yaakov Stern, who has studied cognitive reserve for more than two decades, suspects that both are true. People benefit from what he calls "passive" and "active" cognitive reserve, he says. Passive reserve involves the size of the brain and the number of brain cells it contains — factors that can't be changed. Active cognitive reserve, in contrast, seems to involve larger numbers of synapses, or connections among brain cells — a product of learning and other challenging mental activities. The larger number of synapses creates stronger brain networks better able to resist degeneration.
"I used to believe that the number of synapses we have was pretty much fixed, and all we could do was lose them," said Stern, a professor of neurology, psychiatry, and psychology at Columbia University in New York. "But animals that are more physically active, and live in a stimulating environment, have higher rates of synaptogenesis," which is the creation of new synapses.
Also, studies have shown that placing demands on the human brain increases the density of connections in various regions. London taxi drivers, for example, who spend many years memorizing maps depicting the maze of the city's streets, usually have a larger-than-average hippocampus — the part of the brain that records new memories.
So the decline in the incidence of Alzheimer's seems to support the standard advice for keeping the disease at bay — exercise, don't smoke, control your blood pressure and take statins to keep your cholesterol low, but above all, place demands on your brain by reading, solving puzzles and socializing. Like the body itself, the brain appears to thrive on exercise.