Maintaining a healthy body mass index at mid-life could have a long-lasting impact in delaying the onset of AD.

Alzheimer’s is likely to occur earlier in people who are overweight in middle age, a study has found.

Being overweight is also linked to more damage to the brain caused by the disease, the research shows.

Scientists studied 1,394 people whose mental functioning was assessed every two years for around 14 years.

In the 142 individuals who went on to develop Alzheimer’s, earlier onset was closely linked to Body Mass Index (BMI) at 50 years of age.

BMI is a measurement based on a person’s weight and height. The study found that every unit increase in BMI at 50 was associated with the appearance of Alzheimer’s symptoms 6.7 months earlier.

The research, led by Madhav Thambisetty, from the National Institute on Ageing in the US, is published in the journal Molecular Psychiatry.

The authors wrote: “We found that greater mid-life adiposity was associated with earlier AAO (age at onset) of AD (Alzheimer’s disease) and greater severity of Alzheimer’s neuropathology.

“Our findings suggest that maintaining a healthy body mass index at mid-life could have a long-lasting impact in delaying the onset of AD. Our findings raise the possibility that inexpensive, non-invasive interventions targeting mid-life obesity and overweight could substantially alter the trajectory of AD, reducing its global public health and economic impact.”

A BMI of 25 to 29.9 is considered “overweight” and 30 and above “obese”. The men and women studied had an average BMI of 25.8 at the age of 50.

The research showed that higher mid-life BMI was associated with larger clumps of sticky amyloid protein in the brain, one of the hallmarks of Alzheimer’s.

The study did not show that being overweight in middle age increases the risk of Alzheimer’s, only that it raises the likelihood of earlier onset in those who do develop the disease.