People in the earliest stages of Alzheimer's disease are more likely to have a lower body mass index (BMI) than those who did not have the condition, according to a new study.

Previous studies have shown that people who are overweight in middle age are more likely to develop Alzheimer's disease decades later than people at normal weight.

The study examined 506 people with advanced brain imaging techniques and analyses of cerebrospinal fluid to look for biomarkers for Alzheimer's disease, which can be present years before the first symptoms begin.

The study found that in people with no memory or thinking problems and in people with mild cognitive impairment, those who had the Alzheimer's biomarkers were also more likely to have a lower BMI than those who did not have the biomarkers.

For example, 85 percent of the people with mild cognitive impairment who had a BMI below 25 had signs of the beta-amyloid plaques in their brains that are a hallmark of the disease, compared to 48 percent of those with mild cognitive impairment who were overweight.

The relationship was also found in people with no memory or thinking problems.

"These results suggest Alzheimer's disease brain changes are associated with systemic metabolic changes in the very earliest phases of the disease," said study author Jeffrey M. Burns, MD, MS, of the University of Kansas School of Medicine in Kansas City.

"This might be due to damage in the area of the brain called the hypothalamus that plays a role in regulating energy metabolism and food intake.

"Further studies should investigate whether this relationship reflects a systemic response to an unrecognised disease or a long-standing trait that predisposes a person to developing the disease," he added.

The findings have been published in the November 22, 2011, print issue of Neurology, the medical journal of the American Academy of Neurology.