ST. LOUIS, May 14--For decades, Sister Mary Timothy Ryan taught history to students at Rosati-Kain High School and at the old Notre Dame College in south St. Louis County. Still active at 87, she is no longer standing in front of a blackboard, but she is still teaching.

Her classes are much bigger: doctors and thousands of medical professionals and average readers around the world interested in old age dementia and Alzheimer's disease. Her doctorate is in history--not medical science--but her brain will be yielding results for decades to come.

By looking at the nun's youthful writings, he could predict with 85 percent to 90 percent accuracy which ones would show brain damage 60 years later.

Ryan is part of a study that shows that traits in early, middle, and late life have a strong relationship with the risk of Alzheimer's disease. According to the study, people who have positive attitudes about life and are full of ideas in their late teens seem to have a lower risk for mental and cognitive disabilities in old age.

Ryan is one of 110 School Sisters of Notre Dame of St. Louis who volunteered to participate in a national study of nuns over age 75. The study included 678 sisters from the order nationwide.

David Snowdon's 15-year-old "Nuns' Study" at the Sanders-Brown Center on Aging at the University of Kentucky Medical Center forms the basis for his book, "Aging with Grace: What the Nun Study Teaches Us About Leading Longer, Healthier and More Meaningful Lives" (Bantam Books), which was published May 8. Snowdon, a former Southern California Roman Catholic altar boy, has donated 50 percent of his book royalties to the order.

He has a doctorate in epidemiology and is a professor at the University of Kentucky. His study, funded by the National Institute on Aging, began in Minnesota in 1986 and expanded to include the sisters in St. Louis and five other cities in 1991. Snowdon hopes the research will continue for decades.

Nuns are a great study group, he said. Their order mandates that all of their medical records, work records and the autobiographies they must write before taking their vows and on retirement are securely filed. The nuns opened these files to Snowdon and his research team. And each nun signed a form giving her brain to science at her death.

Of the 110 St. Louis sisters who originally signed up for the study, 44 are alive. The others have been buried, but their brains are carefully preserved and tested in a medical lab at the University of Kentucky.

"Too many people think that old people are incapable and will all lose their minds," Ryan said at her residence in her order's red brick St. Louis Province. "This study shows not everyone gets dementia, that you can protect life, respect life and improve the life of the elderly. Maybe (the study) will help improve life for other elderly people."

With lab brain tests in one hand and autobiographies that the young novices (aspiring nuns) had to write when they were 18 or so, Snowdon found that elderly nuns who showed signs of Alzheimer's had consistently written simpler autobiographies that had fewer ideas per sentence.

Complicated grammar indicated how well their memory was functioning, he said. By looking at the nun's youthful writings, he could predict with 85 percent to 90 percent accuracy which ones would show brain damage 60 years later, he said.

"We learned that early education, linguistic ability and the way we look at life seems to indicate what we will be like in the end of life," he said in a phone interview from his Kentucky center Tuesday afternoon.

Other aging experts are dazzled by the study.

"It is a fantastic study," said Dr. John Morris, 53, director of the Alzheimer's Disease Research Center at the Washington University Medical School. He has used many of Snowdon's 12 papers on the study in his classes.

"Until this study, everyone was only looking at elderly people. Now, because of these sisters' records and writings, we can look for evidence of the disease early in life."

Morris is impressed that those early autobiographies show that not just the nuns' intellectual abilities--85 percent of the study participants have bachelor's degrees, 45 percent have master's degrees--but their attitudes about life seem to predict their brain function in old age.

"If we wait to look only at patients late in life, we may miss early risk factors," he said. "We don't know if biologically our brains are wired in such a way from early childhood that makes us high risk for the Alzheimer's disease, or if there are things we can do to modify risk for the disease."

The risk of death in any given year after the age of 65 is about 25 percent lower for the School Sisters of Notre Dame, said Snowdon. These mentally active women can teach others to keep on reading and contributing to society, he said.

Since 1991, professional testers annually have given Ryan and all "Nuns' Study" participants an hour-long test to assess their memories, concentration, language, and visual-spatial abilities and orientation about time and place.

In the tests, Ryan read flash cards with simple words on them--leg, cheese, forest, menu- -and several minutes later was asked to recite those words on the flash cards from memory. She's taken the study's mandated psychological and physical exams and blood tests annually.

"The nuns may live longer than other groups because they don't smoke, they drink little, they exercise, and are surrounded by love."

Ryan, just as many nuns in the study, doesn't believe in retiring. Since her mid-70s, she has just changed jobs to suit her energy. After 50 years of teaching, she became a parish pastoral administrator in Illinois until she was 82. Now, she goes every Wednesday to St. Anthony's Parish Food Pantry in the city's Dutchtown neighborhood, where she serves as hospitality coordinator. Mostly, she listens to the low-income patrons and encourages them to find hope and be good parents.

Other days she serves bread, often an extra couple of slices, at lunch at St. Patrick Center downtown to homeless people.

The nuns may live longer than other groups because they don't smoke, they drink little, they exercise, and are surrounded by love, Snowdon said. They eat nourishing meals with good friends. They are valued. They connect with their work, he said. They gather for daily Mass, prayer, and meditation.

The study suggests that being mentally and physically active can stave off dementia.

About a dozen members of the motherhouse "mail team" spend six or more hours a day voluntarily stuffing envelopes for charities they want to help, such as a farm workers group and the Kidney Foundation.

One of the champion envelope stuffers is getting a lot of attention. She is the only St. Louis participant mentioned in Snowdon's book, which arrived at the motherhouse Tuesday.

"David (Snowdon) sent me a note to look on page 60," said Sister Loran Roche. He mentions a 1994 Post-Dispatch article about the study in which she said: "We joke about all these Notre Dame nuns running around heaven without their brains."