Every unit of BMI a middle-aged person is overweight could speed up the onset of Alzheimer's in old age by 6.5 months, a study says.

Being overweight at the age of 50 may speed the onset of Alzheimer's disease in old age, according to a study in the journal Molecular Psychiatry.

A statistical comparison shows that every extra unit in body mass index (BMI, a height-to-weight ratio) in middle age corresponds to earlier onset of Alzheimer's by about six-and-a-half months - what the authors termed a "robust" correlation.

"A healthy BMI at midlife may delay the onset of AD," the study paper said, referring to Alzheimer's disease.

Researchers used the recorded BMI of more than 1300 Americans, all of whom were monitored for an average of 14 years after signing up to be studied.

Of the group, 142 developed Alzheimer's at an average age of 83.

The debilitating disorder is the most common form of dementia, which the World Health Organisation says affects nearly 50 million people worldwide - about 7.7 million new cases a year.

Being obese or overweight in middle age was known to increase the risk for Alzheimer's later, but it was not clear whether it affected the age of disease onset.

The WHO estimated more than 1.9 billion adults, of the world's total population of seven billion, were overweight in 2014. Thirteen per cent were obese.

"We found that for every unit increase in body mass index when these individuals were 50 years of age, they developed Alzheimer's disease on average 6.5 months earlier," the study's lead author, Madhav Thambisetty of the National Institute on Aging of the US health department's National Institutes of Health, said in a video recording on Tuesday.

"We also found in individuals whose brains we could examine after they died that every unit of increase in body mass index was associated with more neurofibrillary tangles in the brain, which is one of the key pathological hallmarks of Alzheimer's."

People with a BMI of 25 and higher are generally classified as overweight, and 30 and over obese. In metric terms, it is calculated as your weight in kilograms divided by the square of your height in metres.

The average BMI of the study participants at 50 years was 25.8, the authors said. A "unit" increase would be one point, say from a BMI of 25 to 26.

"I think these findings are important because they add to a substantial amount of knowledge about how obesity affects Alzheimer's," said Thambisetty.

"But more importantly, it indicates that if we can maintain a healthy body mass index even as early as midlife, it might have long-lasting protective effects towards delaying the onset of Alzheimer's disease decades later."

If one could delay Alzheimer's onset by just two years, the worldwide prevalence of disease would drop by about 22.8 million cases in 2050, the study paper says.

A drawback of the study is that most of the trial subjects were white, highly educated and healthy individuals, the team said, and more research is needed.