Obesity is associated with a huge range of serious health problems, raising your risk of heart disease, multiple cancers, and overall mortality.
But there's one area of the body where obesity's effects have proven more difficult to unpack: the brain.
In a study published in Neurology Open Access, scientists have revealed a new glimpse of what the condition might be doing to the brain, identifying an increased volume of abnormalities in the brain's white matter in people with obesity.
The findings are also important because they make use of newer guidelines for assessing obesity in patients, which could clear up some confusion we've seen across the research landscape before now.
While much scientific evidence links obesity to concerning health effects like accelerated brain aging and brain shrinkage, some research has bucked the trend, suggesting that obesity might actually help protect against dementia and death risk.
"Those studies have been really confusing, with some studies even suggesting that having more obesity might be better for the brain," says neuroscientist Owen Carmichael from Louisiana State University (LSU).
The red herring, in many of these cases, is body mass index (BMI), or rather, BMI's infamous imprecision when used to measure obesity and overweight, given the metric's inability to capture body fat and other health markers.
"One reason for the confusion is that we never did a good job of separating out, among persons with excess body fat, those who did vs. didn't have serious metabolic health consequences associated with it," Carmichael says.
In the new study, researchers tried to overcome some of those limitations, sourcing a long trove of data from the Bogalusa Heart Study, a community-based longitudinal cohort that began studying the health of a cohort of children in the city of Bogalusa, Louisiana, in 1972.
Carmichael's team analyzed data from 190 participants in the Bogalusa Heart Study, who undertook numerous health examinations from 1985 onwards, including MRI scans between 2017 and 2025, at which point they had an average age of 56.5 years.
"We found that people who had more exposure to clinical obesity over decades had worse brain health in midlife." – neuroscientist Owen Carmichael
To classify participants' health and weight, the researchers went beyond BMI measurements, using recent obesity guidelines from a Lancet Commission, which proposes distinguishing between non-obesity, preclinical obesity (metabolically healthy obesity), and clinical obesity (metabolically unhealthy obesity).
"This study is an important example of how that framework can be applied to longitudinal research," says LSU physiologist Eric Ravussin, who wasn't involved with the new study but served on the Lancet Commission.
"By considering the metabolic consequences of excess adiposity over time, researchers can begin to better understand how clinical obesity may contribute to health outcomes later in life, including brain health."
In the Bogalusa cohort, those health outcomes were striking.
The research team found that participants who had spent more of their lives being clinically obese showed greater volume of an abnormality in the brain called white matter hyperintensities (WMHs).
White matter hyperintensities are bright areas seen on MRI scans – a marker of damage to blood vessels and other changes in white matter structure, which have been linked to cognitive decline.
Of the 190 participants, 133 (70 percent) had been clinically obese at some point in their lives, with the proportion of clinical obesity ramping up as people aged – from fewer than one-third of the group in their twenties, to more than a half by their mid-thirties, and over two-thirds by the mid-forties.
The obesity status corresponded to a distinct pattern in the MRI scans in their fifties, with the people who had been clinically obese showing more than twice the median volume of WMHs compared to people in the Bogalusa study who had never been clinically obese.
"Collectively, these findings suggest that obesity is not only widespread but, when accompanied by metabolic dysfunction – defined according to the new Lancet criteria as clinical obesity – it may exert long-term, adverse effects on subclinical brain integrity, which could have implications for cognitive aging," the researchers write in their paper.
The study only shows an association here, and it doesn't demonstrate that people's obesity earlier in life resulted in the increased brain abnormalities seen later. It's also worth noting that WMHs themselves aren't evidence of dementia or cognitive impairment.
But it's certainly a possibility that obesity is behind the changes seen in white matter, the researchers suggest.
In their paper, the team says hypertension, diabetes, and dyslipidemia – all of which are part of the new Lancet diagnostic criteria for clinical obesity – are known accelerants of cerebral small vessel disease, and several other pathways could also potentially explain the WMHs.
"Obesity is characterized by chronic low-grade inflammation and adipokine dysregulation, which impair endothelial function, disrupt blood-brain barrier integrity, and promote white matter injury," the researchers write.
"Insulin resistance, metabolic dysfunction, and associated fatty liver disease, all prevalent among individuals with obesity, have also been implicated in microvascular pathology and WMH progression. Together, these mechanisms suggest that obesity in early adulthood initiates vascular and metabolic processes that culminate in accelerated white-matter injury by midlife."
The researchers acknowledge the small size of the study group analyzed here, and also note that the WMHs were only observed in one set of MRI scans, meaning we can't track the progression of the brain changes over time. The team says future research could examine the temporal dynamics of WMH progression over a series of brain scans.
There's a lot more to learn about what's going on here, but the team says we already know enough to try to stop these preventable health issues from occurring.
"By demonstrating that clinical obesity in early adulthood can leave a detectable imprint on subclinical cerebrovascular health, these findings emphasize the importance of early prevention and intervention in mitigating long-term brain injury," the researchers write.
"When viewed alongside growing population-level evidence linking adiposity to WMH and cognitive decline, these results underscore obesity as a potentially modifiable, midlife target for preserving brain integrity and delaying the trajectory of brain aging."
The findings are reported in Neurology Open Access.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.