High levels of a hormone involved in appetite control seem to be linked to a lower risk of developing Alzheimer's.

The findings build on previous studies showing people who are overweight and obese at midlife tend to be at increased risk of dementia compared with the general population.

Animal studies also suggest the hormone leptin helps the hippocampus, a region of the brain that controls temperature, hunger and thirst.

Over 12 years, Dr. Sudha Seshadri, a professor of neurology at Boston University School of Medicine, and her colleagues gave regular brain scans to 198 volunteers whose average age was 79.

A quarter of those with the lowest levels of leptin went on to development Alzheimer's compared with six per cent of those with the highest levels, the team reported in Wednesday's issue of the Journal of the American Medical Association.

During an average followup of 8.3 years, 111 participants developed dementia, including 89 who were diagnosed with Alzheimer's.

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

More research is needed to understand the possible risks and benefits of taking leptin, as well as to test its effect on non-Caucasians and people in their 50s and 60s.

Preventing Alzheimer's and dementia

"If our findings our confirmed by others, leptin levels in older adults may serve as one of several possible biomarkers for healthy brain aging and, more importantly, may open new pathways for possible preventive and therapeutic intervention," the study's authors concluded.

The magnitude and strength of the association is "striking," said Dr. Eric Larson, executive director of Group Health Research Institute, who wrote a commentary accompanying the study.

If it's confirmed, it could lead to research on how factors like exercise help in the prevention and treatment of age-related decline and diseases such as Alzheimer's, he said.

Given increases in life expectancy, it's "imperative for research to find solutions that prevent, delay, slow, and treat Alzheimer's disease and related dementias," the editorial said. "It would be difficult to overstate the urgency of this need."

Susanne Sorensen, head of research at the Alzheimer's Society, called the research "important," adding it could help in understanding the causes of the disease and developing drugs for it.