A fundamental shift is underway in how scientists look for ways to ward off the devastation of Alzheimer’s disease — by testing possible therapies in people who don’t show many symptoms, before too much of the brain is destroyed.

The most ambitious attempt is an international study announced Tuesday that will track whether an experimental drug can stall the disease in people who appear healthy but are genetically destined to get a type of Alzheimer’s that runs in the family. If so, it would be promising evidence that maybe regular Alzheimer’s is preventable, too.

A second study will test whether a nasal spray that sends insulin to the brain helps people with very early memory problems, based on separate research linking diabetes to an increased risk of Alzheimer’s.

The new focus emerges as the Obama administration adopts the first national strategy to fight the growing Alzheimer’s epidemic — a plan to have effective treatments by 2025.

“We are at an exceptional moment,” with more important discoveries about Alzheimer’s in the last few months than in recent years, Francis Collins, director of the National Institutes of Health, said Tuesday.

But a meeting of the world’s top Alzheimer’s scientists this week made clear that meeting the 2025 deadline will require developing a mix of treatments to attack the different ways that Alzheimer’s damages the brain — much like it can take a cocktail of drugs to treat high blood pressure or the AIDS virus.

Perhaps more important, it will require testing possible drugs before full-blown Alzheimer’s sets in, when it may be too late to do much. After all, Alzheimer’s starts ravaging the brain at least a decade before memory problems occur. And doctors don’t wait until the worst symptoms appear before treating heart disease, cancer or diabetes, noted Reisa Sperling of Harvard Medical School.

“Once the train leaves the station of degeneration, it might be too late to stop it,” Sperling said. “We need to define the critical window for intervention.”

Already, 5.4 million Americans have Alzheimer’s or related dementia. Barring a research breakthrough, those numbers will rise by 2050, when up to 16 million Americans are projected to have Alzheimer’s.

There is no cure for the disease, and the five medications available to treat it only temporarily ease some symptoms. Finding better ones has been a disappointing slog: Over the past decade, 10 drugs that initially seemed promising failed in late-stage testing, Sperling said.

Moreover, scientists still don’t know exactly what causes Alzheimer’s. The chief suspects are a sticky gunk called beta-amalyoid, which makes up the disease’s hallmark brain plaques, and tangles of a protein named tau that clogs dying brain cells. One theory: Amyloid may start the disease while tau speeds up the brain destruction.

Previous studies of anti-amyloid drugs have failed, but the new international study will test a different one, in a different way. About 300 people from a huge extended family in Colombia who share a gene mutation that triggers Alzheimer’s in their 40s will test an experimental drug, Genentech’s crenezumab, to find out whether it delays the onset of symptoms. The study will include some Americans who have inherited Alzheimer’s-causing gene mutations.

The first National Alzheimer’s Plan also promises to support overwhelmed families along the way.

“A lot more needs to be done and it needs to be done right now, because people with Alzheimer’s disease and their loved ones and caregivers need help right now,” Health and Human Services Secretary Kathleen Sebelius said in announcing the plan.

A new Web site — Alzheimers.gov — that Sebelius called a one-stop shop for families offers easy-to-understand information about dementia and links to resources. The government will offer free training to doctors and other health-care providers on how to spot the early signs of Alzheimer’s and care for those patients. This summer, a campaign will begin to improve public awareness of the disease.

Patient advocates applauded the move, and country music legend Glen Campbell, who has Alzheimer’s, appeared on Capitol Hill to urge more research.