Scientists discovered that higher levels of leptin are associated with a reduced risk of Alzheimer's.

Leptin is produced by fat cells and sends a ''feeling full'' signal to the brain that reduces appetite.

But there is growing evidence that the hormone also benefits brain development and function, and memory.

Earlier research has shown that it reduces levels of beta-amyloid protein in the brain, a major component of the sticky deposits that are a key hallmark of Alzheimer's.

In the latest study, scientists carried out regular brain scans on 198 older volunteers after measuring their leptin levels.

Over a 12-year follow-up period, a quarter of those with the lowest levels of leptin developed Alzheimer's compared with six per cent of those with the highest levels.

Higher leptin concentrations were also associated with greater total brain volume.

The findings are published in the Journal of the American Medical Association.

Study leader Dr Sudha Seshadri, from Boston University Medical Center in the US, said: ''If our findings are confirmed by others, leptin levels in older adults may serve as one of several possible biomarkers for healthy brain ageing and, more importantly, may open new pathways for possible preventive and therapeutic intervention.''