A study of nearly 2 million people in Britain suggests an unexpected protective effect against dementia: obesity in midlife. 

The research, published Friday in the journal The Lancet Diabetes & Endocrinology, linked a 34 percent increased risk of dementia among people who had a BMI of less than 20 kg/m2. A BMI of 18.5 is considered to be underweight. Meanwhile, the study findings linked very obese people— those with a BMI greater than 40— to having a 29 percent decreased risk of dementia.

Study authors, from the London School of Hygiene & Tropical Medicine, did not examine the reasoning behind the link, but in a news release, they noted their findings contradict previous research that suggests obesity increases dementia risk.

Researchers collaborated with global data research firm OXON Epidemiology to analyze the study participants’ medical records from the Clinical Practice Research Datalink, a database comprising about 9 percent of the United Kingdom population. Study participants had a median age of 55 and an average BMI of 26.5, which is considered overweight, at the beginning of the study. During an average nine years of follow-up, nearly 50,000 people were diagnosed with dementia.

The authors noted a gradual declining trend of dementia risk above a BMI of 25, which is considered healthy, up to 35 and higher. The participants’ birth year, age of diagnosis, as well as potentially confounding factors believed to increase dementia risk— like alcohol use and smoking— didn’t impact the results significantly, the news release said.

Lead study author Nawab Qizilbash, of OXON Epidemiology, and an honorary senior lecturer at the London School of Hygiene & Tropical Medicine, said further research is needed to identify the reason behind the link.

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

Study author Stuart Pocock, medical statistics professor at the London School of Hygiene & Tropical Medicine, said his team’s initial findings may hold promise for future dementia treatment.

“Our results suggest that doctors, public health scientists, and policy makers need to rethink how to best identify who is at high risk of dementia,” Pocock said. “We also need to pay attention to the causes and public health consequences of the link between underweight and increased dementia risk, which our research has established. However, our results also open up an intriguing new avenue in the search for protective factors for dementia.”

Researchers noted in the release that their threshold for “underweight” in the study was 20 kg/m2, which is slightly higher than the BMI usually considered to be underweight, 18.5 kg/m2, to allow for more direct comparison with earlier dementia and BMI studies.