LOS ANGELES – Strokes are rising dramatically among young and middle-aged Americans while dropping in older people, a sign that the obesity epidemic may be starting to shift the age burden of the disease.

The numbers, reported Wednesday at an American Stroke Association conference, come from the first large nationwide study of stroke hospitalizations by age. Government researchers compared hospitalizations in 1994 and 1995 with ones in 2006 and 2007.

The sharpest increase — 51 percent — was among men 15 through 34. Strokes rose among women in this age group, too, but not as fast — 17 percent.

“It’s definitely alarming,” said Dr. Ralph Sacco, American Heart Association president and a neurologist at the University of Miami. “We have worried for a while that the increased prevalence of obesity in children and young adults may take its toll in cardiovascular disease and stroke,” and that appears to be happening, he said.

Stroke still takes its highest toll on older people. For those over 65, there were nearly 300 stroke cases among 10,000 hospitalizations in the more recent period studied. For males 15 to 34, there were about 15 stroke cases per 10,000, and for females in that age group there were about 4 per 10,000.

The researchers examined federal records from a sample of hospitals in 41 states, covering about 8 million cases each year. They looked at the percentage of all hospitalizations for stroke by gender and in six age groups.

For every 10,000 hospitalizations in 1994-95 compared with 2006-07, strokes also rose: 47 percent, from 36 to 52.9, in males 35 to 44; and 36 percent, from 21.9 to 30, in females 35 to 44.

Trends went the opposite way in older people.

Strokes dropped 25 percent among men 65 and older (from 404 to 303 per 10,000 hospitalizations), and 28 percent among women in this age group (from 379 to 274).