Having a high BMI at the age of 50 has been linked with an earlier onset of Alzheimer's disease. Researchers found that in adults who would later go on to develop the disease, each unit increase in midlife BMI predicted an earlier onset of Alzheimer's by 6.7 months.

The study, published in the journal Molecular Psychiatry, used data on 1,394 cognitively normal individuals. They went through neurophysiological assessments about every two years for 14 years. In this time, 142 people developed Alzheimer's. Of these, the people with higher BMIs had an earlier onset of the disease.

Previously, studies have shown changes to diet and exercise habits delay the onset of Alzheimer's, but what effect being obese or overweight in midlife has on the disease was unknown. Study author Madhav Thambisetty, from National Institute on Ageing of the US health department's National Institutes of Health, said: "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.

"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."

Thambisetty said the findings are important because they add to our current understanding of how obesity affects Alzheimer's, while showing if people can maintain a healthy BMI it could have "long-lasting protective effects" in delaying the onset of the disease decades later.

The researchers say that if Alzheimer's could be delayed by just two years, the prevalence of the disease could drop by as much as 22.8 million cases worldwide by 2050. However, they also note the current study does not examine the mechanisms that underlie the effect, so more research is needed to better understand the link.