A hormone that plays a major role in preventing people getting fat may also help guard against Alzheimer’s disease, new research suggests.

Leptin, which was only discovered back in 1994, helps regulate people’s appetite and gives them signals that tell them they are “full”, stopping them from over-eating.

Studies have shown that obese people tend to have lower levels of leptin or are resistant to it in much the same way diabetics are resistant to insulin.

And the new study, which took place in Boston University’s School of Medicine, looked at whether the hormone could have beneficial effects on the brain as well.

Researchers scanned the brains of about 200 men who had their leptin levels measured an average of 7.7 years earlier.

They found that higher leptin levels were associated with not just a lower risk of dementia but a lower risk of all other forms of dementia as well.

Those in the bottom 25% of leptin levels had a 25% chance of developing Alzheimer’s over a 12-year period, while those in the upper quartile were only at 6% risk of developing the disease in this time.

Lower leptin levels were also associated with a greater decrease in total brain size.