The surprising connection between high BMI and poor brain health
20 January 2025
Blood is often described as the river of life. If blood flow to any part of our bodies is reduced, the cells dependent on the oxygen and nutrients that our blood delivers cannot function as well. And if blood supply is cut off entirely for even a few minutes, those cells will begin to die.
While every tissue and organ in the body is dependent on our red river of life flowing freely, every second of the day, our brains are particularly sensitive to changes in blood flow. And this makes the findings of a study published in the Journal of Alzheimer’s Disease in August 2020 incredibly concerning.
The study, titled ‘Patterns of Regional Cerebral Blood Flow as a Function of Obesity in Adults‘, reported an analysis of over 35 000 functional neuroimaging scans from nearly 18 000 individuals aged 18–94, across body mass index (BMI) categories from underweight to morbidly obese. The brain scans used single-photon emission computerised tomography (SPECT) to measure blood flow to 128 distinct regions of the brain, and activity within those regions, both when participants were at rest and when they were engaged in the Connors Continuous Performance Test II, which measures the ability to concentrate and pay attention.
In every brain region, both blood flow and brain activity decreased, in a linear fashion, as BMI increased.
In other words, the higher the individual’s BMI, the more his or her brain was starved of the vital oxygen and nutrients supplied by blood, both when at rest and when attempting to concentrate on a task.
The following graphs depict the effect of BMI on blood flow to five regions of the brain that are of key relevance to the development of Alzheimer’s disease, when participants were at rest. First, the temporal lobes, a brain region involved in processing input from our senses – especially hearing – so that we can understand speech, form memories and process our emotions:
And here’s the effect on the parietal lobes, which are important for processing and interpreting somatosensory input:
And on the hippocampus, which is primarily involved with memory and spatial navigation:
And the posterior cingulate, which plays a role in awareness, memory retrieval and pain recognition, and is believed to mediate interactions between emotion and memory:
And finally, on the precuneus, which is involved in numerous complex functions including memory recall, integration of information relating to perception of the environment, forming mental images, and emotional responses to pain:
The same pattern of diminishing cerebral blood flow with increasing BMI category was observed in the scans taken immediately after the concentration task.
It’s important to note that there were no brain regions in which blood flow was higher in overweight or obese participants; the compromised blood flow patterns in people with excessive body fatness were remarkably consistent.
The researchers were particularly interested in the implications of their findings on the risk of Alzheimer’s disease. Our understanding of Alzheimer’s has changed dramatically in the last decade or so, with the realisation that, far from being simply a distressing but inevitable consequence of aging, Alzheimer’s disease is a lifestyle disease – just like type 2 diabetes and coronary artery disease, with which it shares major risk factors. And one of those risk factors is obesity.
Our brains are not magically protected from the devastating metabolic effects of excess body fat. In fact, they are exquisitely sensitive to these effects. Obesity, and even milder degrees of overweight, unquestionably compromise blood supply to the brain. The cerebral arteries not only become narrower in obese people; they may also be less responsive to the biochemical signals that normally cause them to dilate (enlarge their diameter) in order to allow more blood flow to tissues with a temporarily heightened need – including brain regions that are pivotal to particular cognitive tasks.
In the short run, this compromised blood supply impairs brain function, making it more difficult to concentrate and pay attention to challenging tasks. And in the long run, compromised blood flow shrinks the brain and promotes Alzheimer’s disease, a devastating condition which – quite rightly, in my opinion – the majority of people fear more than death.
Research published in March 2024, which tracked brain health in Chinese adults aged between 25 and 83, found that persistent excessive body fatness damages the brain in multiple ways. This study was a subset of a large, multicentre, long-term follow-up, community-based cohort which was initiated in 2006. 1074 participants underwent magnetic resonance imaging (MRI) scans of their brains between 2020 and 2022. Hence, up to 16 years of follow-up data was available, including repeated BMI measures. Since BMI can change over time, the researchers calculated each participant’s cumulative BMI by dividing the number of follow-up years into the sum of BMI measures over the course of follow-up.
The researchers found that high cumulative BMI is associated with multiple detrimental effects on brain health:
- Decreased overall brain volume. In particular, those with the highest vs lowest cumulative BMI had significant grey matter atrophy, predominantly in the frontal lobe, temporal lobe, and anterior cingulate gyrus. Our brains naturally shrink with age, but young adults with a cumulative BMI of over 26.2 kg/m2 had brain volumes that were equivalent to those of people 12 years old than them1. In other words, being chronically overfat aged their brains by a dozen years.
- Abnormal microstructural integrity. Fractional anisotropy is an MRI biomarker that is used to measure connectivity within the brain. Bear in mind that the number of connections between neurons (brain cells) is a strong predictor of general intelligence. Conversely, the loss of connections between neurons is associated with cognitive decline in Alzheimer’s disease and, rather worryingly, there’s already some decline in functional connectivity evident in the brains of middle-aged (41–60 year old) adults. Again, younger adults with higher cumulative BMI had abnormal microstructural integrity as measured by fractional anisotropy.
- Larger volume of white matter lesions. White matter lesions are abnormalities in the brain’s nerve fibres that appear as visible bright areas or spots on MRI scans. A certain number of white matter lesions is normal, but more of them appear in the aging brain. A high cumulative BMI was associated with larger relative volumes of white matter hyperintensity in participants over 60 years of age.
In summary, having a persistently high BMI was associated with adverse changes to the brain, indicative of accelerated aging and increased risk for cognitive decline. And the brains of chronically overfat younger adults were particularly at risk.
Australia has had a weight problem for many years. Currently, 34 per cent of Australian adults are overweight and 32 per cent obese – and the obesity rate is rising. Worse yet, younger birth cohorts are plagued with higher obesity rates at earlier ages. Back in 2017, the Australian Institute for Health and Welfare, our national agency for health and welfare statistics, reported that 15 per cent of late Millennials (people born in the mid-1990s) were obese at age 18–21, compared to just 8 per cent of Gen Xers born in the mid-1970s. And at ages 2–5, about nine per cent of late Gen Z children (born in the early 2010s) were obese, compared with about four per cent of Millennial kids born twenty years earlier.
There’s no way the fat activists and ‘body positivity’ advocates can spin their way out of it: if you are carrying too much fat on your body, you’re causing premature aging to your brain and increasing your risk of developing cognitive decline and dementia. And the earlier in life you start becoming overfat, the more harm you’re inflicting on your brain. But, as I discussed in Of Bonds, brawn, brains and blubber, not all body fat is created equal. Visceral fat is the most dangerous type when it comes to brain health, and we’ll cover relatively new research on that, as well as the protective effects of muscle, in next week’s post.
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- Note that the BMI cutoffs for Chinese people are 24 for overweight and 28 for obesity – somewhat lower than those recommended for Caucasians (25 for overweight and 30 for obesity). ↩︎
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