Having low body mass index (BMI) does not increase people's risk of developing Alzheimer's disease, scientists have said. Instead, people may go on to lose weight once they are diagnosed, as their behaviours, appetite and sense of smell are modified.

The study, published in the Journal of Clinical Endocrinology & Metabolism, looks at blood and DNA samples from 95,578 participants in the Copenhagen General Population Study (CGPS) – 645 of whom developed Alzheimer's disease.

What is the Body Mass Index?

The Body Mass Index (BMI) is used to quickly and simply assess a person's weight in regard to their height.

It is calculated using the following formula: BMI (kg/m<sup>2) = mass (kg) / height (m)<sup>2

Previous research using data recorded from more than two million people in the UK had established that low BMI is associated with a heightened risk of Alzheimer's disease. Here, the researchers wanted to put these findings to a test.

Instead of looking merely for associations, they went a step further and conducted what is known as "Mendelian randomisation analysis". This involved examining the participants' DNA for the presence of five genetic variants that are known to have a strong association with BMI.

A public health matter

On top of the Copenhagen General Population Study, the scientists also used data from up to 249,796 individuals participating in the Genetic Investigation of Anthropometric Traits (GIANT) consortium.

Based on how many variants they had, all the participants were divided into four groups reflecting the likelihood of low BMI. The scientists then assessed the risk of developing Alzheimer's disease for each of these groups.

"When we simply look at observational associations, we find the same as other previous studies. A low BMI is statistically linked to a higher risk of Alzheimer's. However, association does not mean causality", the study's senior author, Ruth Frikke-Schmidt, Chief Physician at Rigshospitalet in Copenhagen, told IBTimes UK.

"Because genetic variants associated with low BMI are life-long and acquired totally randomly, it is a clean measurement of low BMI. It's good to use as a test of causality for body mass index".

The researchers found that having genetic variants associated with low BMI did not increase the risk of Alzheimer's disease.

The scientists looked for variants associated with a low BMI. Christoph Bock Max Planck Institute for Informatics

Alzheimer's Society Research Communications Officer Dr Louise Walker commented:"Researching the links between BMI, weight and Alzheimer's disease is often complicated and findings are often contradictory. This research is a first step to untangle this complexity".

"We need more large studies like this, as they can tell us what patterns we need to investigate. The main limitation here however is that the scientists do not take into account in their analysis people who might have a low BMI for reasons other than genetic predispositions".

The authors nevertheless come up with an explanation for the link observed between low BMI and the disease. They propose that patients' low BMI is due to behavioural changes, and appetite and weight loss they may suffer from in the early stages of the disease.

Increasing people's BMI would probably have no impact on their risk of developing the disease. "The present research is important since it concludes that the association between low BMI and Alzheimer's disease is not of a causal nature. Changing public health recommendations based on observational data alone would cause more harm than benefits", Frikke-Schmidt concluded.