The brain doesn’t like obesity
Excess weight has a negative impact on various aspects of cognitive function and increases the risk of developing dementia.
The impact of excess body fat on the ageing process
Obesity can induce pathophysiological processes in brain structures similar to those associated with the ageing of the body – as pointed out by researchers from Virginia Polytechnic Institute and State University in the article ‘Can obesity make the brain age faster?’¹, published in ‘Virginia Tech News’.
“We knew that obesity had a negative impact on memory. Ageing had a similar effect. However, it was not clear whether the same cellular process lay at the root of both these phenomena,” explained Professor Timothy Jarome, the author of the discovery.
In studies using rodent models, his team observed that in rats fed a high-fat diet, there was an increase in the level of a specific molecule – K63 – which directly resulted in memory impairments. An identical accumulation of this molecule was noted in the brains of elderly individuals.
– We were surprised to find that in young, obese animals we observed neurobiological changes typical of much older brains, but developing over a much shorter period of time. This suggested that memory impairments caused by obesity and those associated with ageing might be directly linked via the same signalling pathway,” stated Prof. Jarome.
Risks associated with abdominal obesity
In the pathogenesis of many diseases, so-called visceral fat – which accumulates deep within the abdominal cavity, surrounding the internal organs – plays a particularly significant role.
Researchers at Ben-Gurion University of the Negev investigated the impact of this type of adipose tissue on brain atrophy and cognitive function. They presented the results of their analysis in the paper ‘Sustained visceral fat loss is associated with attenuated brain atrophy and improved cognitive function in late midlife’², published in the journal *Nature Communications*. The researchers analysed data from 533 adults who had previously taken part in lifestyle intervention programmes lasting between 18 and 24 months. Between 5 and 16 years later, the participants underwent MRI (magnetic resonance imaging) scans of the abdomen and brain, and their cognitive function was assessed.
It was shown that individuals with lower, long-term exposure to visceral fat achieved higher scores in cognitive tests. Furthermore, a greater reduction in this tissue during the intervention correlated with better preservation of brain volume years later. Crucially, these associations remained statistically significant regardless of overall changes in body weight. No similar protective effect was observed in the case of subcutaneous fat. The authors suggested that a key mechanism might have been improved glycaemic control (blood glucose levels) achieved through the reduction of visceral fat deposits.
Dementia: a link to obesity and underweight
Researchers at Qingdao University in China also investigated the link between BMI values at different stages of life and the risk of cognitive impairment and dementia in old age.
They published their findings in the article ‘Association of late-life body mass index with the risk of Alzheimer’s disease: a 10-year nationwide population-based cohort study’³ in *Scientific Reports*. Pooled analyses showed that both underweight and obesity were associated with an increased risk of developing dementia – by 39 per cent and 31 per cent, respectively. A particularly strong association was observed for being underweight in old age, which correlated with as much as a 64 per cent increase in risk.
Conversely, being overweight and moderately obese in later life were associated with a lower risk of cognitive impairment (by 21 per cent and 25 per cent, respectively). However, this phenomenon had its limitations – in cases of severe obesity, the protective effect disappeared, particularly in relation to vascular dementia.
The negative impact of excess body fat on the nervous system was explained, amongst other things, by its role in inducing hypertension, insulin resistance, type 2 diabetes, dyslipidaemia, as well as chronic inflammation and vascular diseases. Conversely, the apparent protective effect of a higher BMI in older adults was most often due to the fact that low body weight in this age group was often a secondary consequence of other conditions: malnutrition, frailty, sarcopenia (loss of muscle mass), chronic conditions, or was already developing in the preclinical, as yet undetected, stage of dementia.
Some experts also pointed to the potential role of leptin – a hormone secreted by adipose tissue – which may limit the aggregation of pathological proteins (beta-amyloid and tau protein), constituting the diagnostic triad of Alzheimer’s disease.
However, it is worth comparing these findings with the results of another meta-analysis, carried out by a team from Sheffield Hallam University and published in the *Journal of Alzheimer’s Disease* under the title ‘Impacts of Overweight and Obesity in Older Age on the Risk of Dementia: A Systematic Literature Review and a Meta-Analysis’⁴. The authors of this study grouped the analysed data according to the duration of patient follow-up. It turned out that a higher BMI in older adults was associated with a slightly lower risk of dementia, but only in the short term (less than nine years). In long-term follow-ups, this association disappeared entirely.
Obesity impairs cognitive function as early as middle age
Experts from Capital Medical University in Beijing investigated the links between obesity and cognitive function in middle-aged people. They analysed cohort data from over 70,000 participants in the UK Biobank project. In the article “Associations between obesity indices and cognitive function: Evidence from a middle-aged observational and Mendelian randomisation study”⁵ published in *Diabetes, Obesity and Metabolism*, they demonstrated that high body fat indices were consistently associated with poorer results in neurocognitive tests.
After adjusting the data for confounding variables, it was found that individuals with a higher BMI scored lower than those of normal weight, particularly in tests assessing memory and fluid intelligence (the ability to cope with new situations and solve problems in unconventional ways). An even stronger association was found for the WHR (waist-to-hip ratio). Individuals with a high WHR showed significantly reduced performance in terms of fluid intelligence, memory and information processing speed.
Summary
The conclusions from the cited studies were unequivocal. Modern neurology has provided further strong evidence that maintaining a healthy body weight is one of the key elements in the prevention of neurodegenerative diseases. Maintaining a healthy metabolic profile was a fundamental prerequisite for preserving intellectual function throughout one’s lifetime.
Footnotes:
- www.news.vt.edu/202606.
- www.nature.com/71141-4.
- www.nature.com/sr41598-022-19696-2.
- www.shura.shu.ac.uk/30418.
- www.ukbiobank.ac.uk/3465.
Read also: ‘Trauma-induced obesity’ and ‘Out of sight, out of mind’.
