A S WE RATTLE along into old age, it's not heart attacks, strokes or cancer that worries most people. It's the specter of losing our grip through the advent of Alzheimer's, whose sine qua non is memory loss.

But memory loss is not the principal failure of our brains as we grow older, contrary to popular belief, says Dr. Steven C. Masley. It's loss of mental speed and flexibility.

Masley should know. He is medical director of the Carrillon Executive Health Center in St. Petersburg, Fla., where he runs the "Ten Years Younger, Trimmer, Fitter" program.

Masley cited this slightly heretical idea and various others at the recent Scientific Assembly of the American Academy of Family Physicians, in a speech called "Lifestyle Program to Enhance Physiological Age."

He said Americans are about 10 years older, physiologically, than they were in the 1960s--meaning they are feeling the ill effects of age faster than they should be.

As you lose mental speed and flexibility, it takes longer to balance your checkbook, for example, or you have trouble catching on to new ideas. (Masley cites difficulty installing a new software program as a great example.)

The best prevention for cognitive decline is aerobic exercise, Dr. Masley tells us, and it's an integral part of the whole program he runs to get people feeling younger.

Masley advocates 30 minutes of exercise five to six days a week--and for it to be aerobic, you have to get your heart rate between 70 percent and 85 percent of the maximum rate. Maximum heart rate is roughly 220 minus your age. But a ballpark way of knowing if it's aerobic, I tell my patients, is when you're out of breath to the point of being unable to carry on a conversation.

During Masley's description of his program, he shared some of his other slightly heretical ideas.

Heresy No. 2: Low-carb diets are all wrong. "It makes me crazy," he says, when his patients tell him they can't eat fruit and vegetables because they're on a low-carb diet. "Almost all the anti-aging nutrients come from carbohydrates," he says.

Avoid the bad carbs by all means--white flour, sugars and in particular corn syrup, which he describes as "one of the most common toxins." Also avoid hydrogenated fats--which he refers to as "embalming fluid."