Does one's body mass index (BMI) play a role in progressive cognitive decline as adults age? According to a new study, the answer seems to be yes. But rather than higher or lower BMIs being the issue, the main problem was that increases and decreases in BMI over time are what may be linked to speeding up a loss of cognition.
Whether one was normal weight, overweight or obese was not the issue. But keeping one's BMI stable when in old age may be able to provide some means of protection against this process.
This was the result of a joint US-Israeli study, the findings of which were published in the peer-reviewed academic periodical Alzheimer's & Dementia: the Journal of the Alzheimer's Association.
Progressive loss of cognition, also known as dementia, is a major issue and finding ways to prevent and treat it is considered a high priority in the field of public health.
A vast majority of dementia cases are caused by the fatal neurodegenerative condition Alzheimer's disease, which currently has no medication or treatment to cure it or even in any way stop or reverse its progression outside of some temporary fixes.
However, many studies have indicated that obesity is a contributing factor, with findings detailing that midlife obesity can create a 2- to 3-fold higher risk of dementia in old age. However, other studies found no change in risk or even less risk of developing dementia, and other studies have actually shown a link between weight loss and higher dementia risk. This is something that has been known as the obesity paradox, which is the belief that being overweight during old age is better than being skinny, though not all scientists agree on this point.
It should be noted that BMI does not refer specifically to weight from fat, but also takes into account bone and muscle.
But despite these links having been noted in prior studies, and despite how subtle consistent BMI loss tends to occur in old age, actual evidence on simultaneous changes in BMI and cognitive decline are notably lacking.
True, BMI stability has been noted to be otherwise beneficial in a number of ways, such as reduced disability and mortality. Its effects on cognition, however, remained to be explored.
So that's exactly what this study did.
The team, led by Prof. Michal Schnaider Beeri of the Ichan School of Medicine at Mount Sinai, gathered data from approximately 16,000 people who participated in Alzheimer's Disease Centers throughout the US. Each of these participants were at least 60 years old and were initially non-demented, with a Clinical Dementia Rating of 0 or 0.5.
In each visit to Alzheimer's Disease Centers, the participants were always administered a standardized neuropsychology battery, a series of tests meant to evaluate memory, attention, executive function and language.
The participants' BMIs, which are calculated based on weight and height, were organized into three groups for underweight/normal, overweight and obese based on classifications by the World Health Organization (WHO). The BMIs were then examined during follow-up visits to look for any changes.
So what did the study find?
To some extent, all three groups experienced cognitive decline. But there was a clear difference. According to Beeri, participants with non-stable BMIs saw a decline in cognition at a rate of over 60% faster than those with stable BMIs.
Does this disprove the obesity paradox?
According to Beeri, not exactly.
"We didn't contradict it fully, but we did find out that there's a lot more to it," Beeri explained. "The obesity paradox is overly simple and doesn't tell the full story. The reality is a lot more complicated."
There are, admittedly, some limitations to the study. For one thing, 80% of participants were white, and a significant majority of them were women. This latter point is especially noteworthy as prior studies have shown that women and men can have different BMI trajectories.
Also noteworthy are other factors such as diabetes and hypertension, and other variables were likely underreported - especially ones that can impact BMI, like cancer.
"Cancer, in particular, is something we didn't have data on," Beeri said. But overall, the team is confident in their findings, and found other ways to test their data.
"We readjusted to account for different variables and medical conditions, gender, ethnicity and more, and the data held up each time," she said. "While we didn't have data on cancer, we did have data on medications. Generally, the number of medications one takes correlates to a level of morbidity."
What impact will the study have on further research?
These findings can have significant implications for further studies on cognitive decline.
“Disentangling the biological pathways underlying different trajectories of BMI in old age, and their contributions to brain health and disease, is necessary in order to develop potential therapeutics,” Beeri noted. “Our study findings help point us in the direction of that ultimate goal, while also providing important considerations that can be assessed in the clinical management of patients as they age.”