More than 250,000 New Yorkers have Alzheimer's, a degenerative disease of the brain.

And although the average age of diagnosis is in the early-70s, it's important to start protecting one's brain much earlier, says Dr. Richard Isaacson, an Alzheimer's specialist and neurologist, who believes that the disease takes root in the brain 20 to 30 years before a diagnosis is ever made.

"You have to make brain healthy choices before it's too late," he said.

Primarily a disease of aging, Alzheimer's affects cognition, memory, behavior and the ability to function independently. Five million Americans -- about one in eight people over 65 and one in three over 85 -- have Alzheimer's.

It's the sixth leading cause of death in the United States, and the only one in the top-10 where there is no cure, says Jed Levine, the executive vice president and director of programs and services at the Alzheimer's Association, New York City Chapter.

"This is major public health crisis," Levine said. "We need to make this a national priority."

November marks National Alzheimer's Disease Awareness and Family Caregiver Month, so it's the perfect time to start taking positive steps to increase one's overall brain health.

The first recommended changes are straightforward.

Don't smoke, Levine says, and protect your head, whether that is by wearing a helmet when riding a bike or wearing a seatbelt when riding in a car.

Then there are the positive lifestyle changes that will impact your weight, blood pressure, cholesterol and blood sugar levels, which in turn, will affect the health of your brain, says Isaacson, who is affiliated with NewYork-Presbyterian Hospital/Weill Cornell Medical Center and the author of two books -- "The Alzheimer's Diet" and "Alzheimer's Treatment Alzheimer's Prevention."

He promotes -- and adheres to himself -- a Mediterranean diet rich in plant-derived foods, fresh fruit, lean fish and poultry, with moderate amounts of olive oil, low-fat yogurt and milk and red wine, avoiding refined carbohydrates like breads, pastas, soda and sweets. He also promotes a healthy amount of exercise.

His patients are asked to track their food intake and to adhere to a nine-week program that slowly has them drop their carbohydrate consumption from their baseline down to 120 grams per day, the level recommended by the USDA, and ideally to 75 grams per day.

While there is no proven way of preventing, delaying the onset or slowing down the progression of the disease, Isaacson says he's seen positive changes from his patients, who range in age from mid-30s to upper-80s, like weight loss and improvements to their engagement, alertness and memory.

Asked to provide small changes New Yorkers of any age can make to their diets to promote brain health, he recommends:

1. Reducing or getting rid of empty calories.

2. Eating berries on a regular basis.

3. Eating lean rather than fatty meats.

4. Drinking coffee in moderation, with some dark cocoa powder.

"We can all do ourselves a favor and our family and society a favor if we just start making more brain healthy choices," Isaacson said.