Obesity in middle age is a significant risk factor for developing dementia in later life, but the relationship may be more complicated than we thought.
Researchers from the US and Canada have now turned up evidence that extra body weight is associated with a longer life once a dementia diagnosis has been given.
Considering how many health problems are linked to obesity – from high blood pressure to several cancers – it's a surprising finding.
But the discovery may give experts some useful insight into how dementia develops and how it might be slowed down.
The research team analyzed health records from 4,523 US adults aged 51 and over who didn't have dementia at the beginning of the study period but went on to develop it.
Based on their body mass index (BMI) at the start, they were put into four groups: normal weight, overweight, class 1 obesity, and class 2/3 obesity.
As reported in the Journal of Alzheimer's Disease, over a follow-up period of up to 20 years, those who were at a normal weight were significantly more likely to die earlier.
"This doesn't mean that patients with dementia should attempt to become overweight or obese, as our study was not designed to test that question," says behavioral scientist Virginia Chang from the New York University School of Global Public Health.
"But our findings do suggest that weight status may serve as a useful prognostic signal for survival at the time of dementia diagnosis. They also support the notion that nutrition is a key factor in the health and survival of people with dementia."
Compared to people of normal weight, the overweight group had a 16 percent lower risk of death over the study period.
For those with class 1 obesity it was 27 percent lower, and for those with class 2/3 obesity it was a 22 percent lower risk.
That was after the researchers factored in other important influences on mortality rate, including sex, educational level, race and ethnicity, smoking status, and other chronic health conditions.
The researchers actively checked for any bias that might have crept in. For example, they looked at the weight history of the participants to see if those in the normal weight bracket might have previously been overweight and lost weight due to ill health, making them more vulnerable – but the associations still held.
"Because this finding is somewhat counterintuitive, an important question is whether it could simply be an artifact of bias," says gerontologist Yuan Zhang from the Columbia University Mailman School of Public Health.
"Our analyses suggest that the major sources of bias we examined do not explain the survival advantage associated with higher weight."
The next question is why this counterintuitive connection exists.
For now, it's not clear, but the researchers speculate that dementia may lead to people skipping meals and forgetting to eat. For those who have more body fat or muscle in reserve, that's potentially less of an issue.
The researchers think that excess weight may have benefits in terms of catabolic reserves for the body. (Alexas_Fotos/Unsplash)"Nutrition can be a big problem for people with dementia," says Chang.
"They may experience wasting and an inability to eat, which can be due to changes in the brain's central drive, leading to loss of appetite and initiative, increased energy expenditure, and difficulties feeding themselves."
This isn't a completely new idea. The same 'obesity paradox has been seen in heart and kidney disease as well.
Further research will be needed, specifically tracking weight and eating habits over time, to understand why the association is there.
For now, it's not evidence of direct cause and effect, so it's not certain that weight is what's keeping people alive for longer. However, there's a strong case that obesity status is a signal worth paying attention to for people with dementia.
"While the underlying mechanisms are not fully understood, these findings suggest that weight status at diagnosis may serve as a useful prognostic indicator alongside other factors," the researchers explain.
The research has been published in The Journal of Alzheimer's Disease.
This article was fact-checked by Rachel Garner and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
