Body mass index (BMI) has been linked to heart disease, high blood pressure and Type 2 diabetes and seems to have links with dementia as well--but it's not as straightforward as it seems at first glance.
A study published in Neurology in May indicated that people who were overweight in their middle-aged years were more likely to develop Alzheimer's disease, and other studies have shown that being overweight later in life has a reduced risk for cognitive problems. In a paper in Neurology in November, it seems that people with mild memory or thinking problems (mild cognitive impairment or MCI), often seen as a prequel to Alzheimer's disease, are actually more likely to be underweight.
As part of the Alzheimer's Disease Neuroimaging Initiative, the brains of 506 people with no memory problems, mild cognitive impairment or Alzheimer's disease were imaged and their cerebrospinal fluid checked for Alzheimer's disease biomarkers such as cerebral amyloid and tau. The study showed links between lower BMIs and Alzheimer's biomarkers in people with no memory problems or MCI--for example, around 85% of people with MCI and a BMI less than 25 had signs of beta amyloid plaques, compared with around 48% with MCI and a BMI over 25 (overweight). This could suggest a link between changes in food intake or metabolism, and changes in the brain--people in the early stages of Alzheimer's disease can be seen to lose weight from both fat and muscle.
According to the study's author, Dr. Jeffrey M. Burns of the University of Kansas School of Medicine: "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 unrecognized disease or a long-standing trait that predisposes a person to developing the disease."
It's not yet clear whether the lower weight is a result of the disease processes in MCI and Alzheimer's disease, or whether it's a sign of something that predisposes people to developing cognitive problems--however, it does show that weight changes in elderly people should be watched carefully, and it's another reminder that being overweight in middle age is not good for the physical or mental health.