London, April 10 (IANS): In a surprising finding, a large study shows that middle-aged obese people have a significant -- nearly 30 percent -- lower risk of developing dementia than people of a healthy weight.

The findings based on medical records of nearly two million people contradicts results from some previous research, which suggested that obesity leads to an increased risk of getting diagnosed with the disorder.

"Our results also open up an intriguing new avenue in the search for protective factors for dementia," said professor Stuart Pocock from the London School of Hygiene & Tropical Medicine.

"If we can understand why people with a high BMI have a reduced risk of dementia, it's possible that further down the line, researchers might be able to use these insights to develop new treatments for dementia," Pocock pointed out.

For the study, the researchers analysed the medical records of nearly two million (1,958,191) people with an average (median) age of 55 years at the start of the study period, and an average (median) BMI (body mass index) of 26.5 kg/m2 (kilograms/square metre) -- just within the range usually classed as overweight.

During an average (median) of nine years follow-up, nearly fifty thousand (45,507) people were diagnosed with dementia.

People who were underweight in middle age were a third (34 percent) more likely to be diagnosed with dementia than those of a healthy weight, and this increased risk of dementia persisted even 15 years after the underweight was recorded.

As participants' BMI at middle age increased, the risk of dementia reduced, with very obese people (BMI greater than 40 kg/m2) 29 percent less likely to get dementia than people in the normal weight range, the researchers noted.

"The reasons why a high BMI might be associated with a reduced risk of dementia are not clear, and further work is needed to understand why this might be the case," the study's lead author Nawab Qizilbash from OXON Epidemiology, a London/Madrid-based clinical research organisation, noted.

"If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present. Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part," Qizilbash said.

The research was published in the journal The Lancet Diabetes and Endocrinology.