Elderly patients who are underweight experience more cognitive decline over time than peers with higher body masses, according to Chicago scientists.
Furthermore, being obese does not increase one's risk of developing memory problems, though remains medically inadvisable in light of its link to heart disease and stroke.
The six-year study involved 3,885 people aged over 65.
Study author Dr Maureen Sturman commented: "We do not know yet why being overweight or obese does not increase the risk for cognitive decline in old age, however being underweight may be a correlate of the initial stages of Alzheimer's disease.
"While past studies have found obesity in middle age increases a person's risk for dementia or Alzheimer's disease, our findings show obesity in old age has no effect on a person’s memory. These findings are consistent with previous studies showing that weight loss or low BMI in old age may be a precursor of cognitive decline or Alzheimer's disease."
The findings can be viewed on the online edition of Neurology.