More than 26 million people worldwide have Alzheimer's disease and a new forecast says the number will quadruple by 2050, with the biggest part of that expected in Asia.

At that rate, one in 85 people will have the brain- destroying disease in 40 years, researchers from Johns Hopkins University conclude.

The new estimates, presented on Sunday at an Alzheimer's Association conference in Washington, are not very different from previous projections of the looming global dementia epidemic with the greying of the world's population.

But they serve as a sobering reminder of the toll to come if scientists cannot find better ways to battle Alzheimer's and protect aging brains.

"If we can make even modest advances in preventing Alzheimer's disease, or delay its progression, we could have a huge global public health impact," said Johns Hopkins public health specialist Ron Brookmeyer, who led the new study.

The biggest jump is projected for densely populated Asia, home of almost half of today's Alzheimer's cases, 12.6 million. By 2050, Asia will have 62.8 million of the world's 106 million Alzheimer's patients, the study projects.

A recent US study estimated that the Alzheimer's toll in America will reach 16 million by 2050, compared to more than 5 million today. The new estimate is significantly lower, suggesting only 3.1 million North American cases today and 8.8 million by 2050.

Among the estimates for other regions are Africa, 1.3 million today and 6.3 million in 2050; Europe, 7.2 million and 16.5 million; Latin America and the Caribbean, 2 million and 10.8 million; Oceania, 200,00 and 800,000.