xercise your brain. Nourish it well. The earlier you start, the better.

This is the best advice doctors can offer to ward off Alzheimer's disease.

There is no guarantee. But more and more research shows that some fairly simple steps can truly lower your risk of the
deadly dementia.

If Alzheimer's strikes anyway, people who have followed this advice tend to do better -- their brains withstand the attack longer before symptoms become obvious.

The goal: build up what's called a 'cognitive reserve'.

"Cognitive reserve is not something you are born with," says Dr Yaakov Stern, Columbia University, USA. "It's something that changes, and can be modified over time."

In fact, the Alzheimer's Association is offering free classes around USA to teach people of any age, but especially baby boomers, just how to do it. They call it 'maintain your brain'.

"There is tremendous interest in making sure that by the time you are 80, your brain is there with you," explains California psychologist Elizabeth Edgerly, who leads the programme.

A healthy brain weighs about .9 kilograms, the size of a cauliflower. Networks of blood vessels keep oxygen flowing
to 100 billion braincells.

Branch-like tentacles extend from the ends of those cells, the brain's own specialized wiring, to communicate. Under a microscope, they look like bushy hairs.

A healthy brain can continue to grow new neurons, rewire and adapt itself throughout old age. And you want your
brain to be as bushy as possible.

This growth starts in childhood, when parents read to tots. This also depends heavily on getting education. The less educated have double the risk of getting Alzheimer's decades later than people with a college education.

Likewise, people who are less educated and have a not-so-chllenging job have three to four times the risk of getting Alzheimer's, says Stern.

If you are already 40, don't despair. Your brain is like a muscle. Use it or lose it.

Brain scans show that when people use their brains in unusual ways, more blood flows into different nerual regions and new connections form.

Try to challenge your brain daily, says Edgerly. Here's how you can do it:

1. Do a new type of puzzle.

2. Learn to play chess.

3. Take a foreign language class

4. Solve a vexing problem at work.

5. A healthy brain isn't just an intellectual one. Social stimulation is crucial, too.

Don't just sit in front of the television. People who are part of a group, whether it is a church or a book club, age healthier.

Declining social interaction predicts declining cognitive function, new research shows.

So do stress and anxiety. People who have what's called chronic distress -- extreme worriers -- are twice as likely to develop some form of dementia, reports Dr Robert Wilson of Rush University Medical Centre.

Why? Autopsies show these people actually had fewer bush-like tentacles, or dendrites, linking their brain cells, meaning their brains were more vulnerable when disease struck.