CHICAGO — A milder type of mental decline that often precedes Alzheimer’s disease is alarmingly more common than has been believed, and in men more than women, doctors reported Monday.

Nearly a million older Americans slide from normal memory into mild impairment each year, researchers estimate, based on a Mayo Clinic study of Minnesota residents.

That’s on top of the half-­million Americans who develop full-blown Alzheimer’s or other forms of dementia — a problem sure to grow as baby boomers age. The oldest boomers turn 62 this year.

“We’re seeing that in fact there’s a much larger burgeoning problem out there” of people at risk of developing dementia, said Dr. Ronald Petersen, the Mayo scientist who led the study.

The mild impairment rate is two to three times larger than many researchers had expected, he said.

Petersen is the scientist who defined mild cognitive impairment, or MCI, as a transition phase between healthy aging and dementia. It is more than “senior moments,” like forgetting where you parked the car, but not as severe as having dementia, where you forget what a car is for.

People with it have impaired memory but not other problems such as confusion, inattention or trouble putting thoughts into words.

The Alzheimer’s Association says more than 5 million Americans have Alzheimer’s, but no estimate for this “pre-dementia” has been available until now.

Petersen’s federally funded study involved roughly 1,600 people, ages 70-89. All tested normal when they were enrolled in the study, but more than 5 percent had developed mild impairment when evaluated a year later.

Men were nearly twice as likely as women to develop it. That’s a surprise, because some studies have found more women with Alzheimer’s than men. But while more men may be impaired, women outlive them and therefore have more time to develop full-blown dementia.

There are no treatments now to prevent the mental slide of dementia or reverse it once it starts.