Central Obesity Linked to Lower Brain Volumes

January 10, 2019

Carrying extra body fat, especially around the middle, is linked to lower brain volume, new data shows.

"We found a linear inverse association between increasing body mass index [BMI] and lower gray matter volume and also increasing waist-to-hip ratio and lower gray matter volume," lead author Mark Hamer, PhD, of Loughborough University in Leicestershire, England, commented to Medscape Medical News. "But the lowest gray matter volumes were seen in people who were both obese and had high waist-to-hip ratios."

Obesity has broad effects on physiology, he noted. "We know obesity contributes to cardiovascular disease and diabetes, but it is also becoming increasingly likely that it is associated with brain health as well. Our results suggest that if we can control obesity better at a population level then this could be one way of reducing the future dementia epidemic," Hamer said.

He added: "We know neurodegenerative disease can have a cardiovascular origin. We also know that adipose tissue — especially that around the middle — produces inflammatory cytokines. There is also a link between adiposity and diabetes, and diabetes is also linked to degeneration of the brain."

The study was published online January 9 in Neurology.

Hamer noted that previous studies have suggested that obesity is a risk factor for later dementia. "We wanted to try and understand the mechanism for this," he said. "Gray matter atrophy is also a risk factor for dementia — we wanted to investigate whether there is a link between obesity and gray matter volume. This study includes a considerably larger population than previous work that has looked at this issue, and our results suggest there does appear to be such a link."

For the study, the researchers analyzed data from the UK Biobank database, a large population-based observational study of half a million people. For the current analysis, researchers included data on 9652 participants (average age 55 years) who had had an MRI brain scan. Data was also available on BMI, waist-to-hip ratio, and total fat mass as ascertained from bioelectrical impedance.

Results showed that 18.7% of the participants were obese (BMI ≥ 30 kg/m2). After adjustment for a range of covariates, including age, physical activity, smoking and hypertension, higher levels of all obesity measures were related to lower gray matter volume.

The combination of overall obesity (BMI ≥ 30 kg/m2) and central obesity (waist-to-hip ratio > 0.85 for women, > 0.90 for men) was associated with the lowest gray matter compared with that in lean adults.

Obese participants who also had central obesity (present in 72%) had lower gray matter volume compared with those who were not categorized as centrally obese (β = −4496, P = .04).

Specifically, they found that 1291 people who were both obese and had a high waist-to-hip ratio had the lowest average gray matter brain volume of 786 cm3, compared with 3025 people of healthy weight who had an average gray matter brain volume of 798 cm3 and 514 people who were obese but without high waist-to-hip ratio who had an average gray matter brain volume of 793 cm3.

The data suggested that obese participants without central obesity had a gray matter volume similar to that of overweight participants.

"This fits in with previous studies suggesting that subcutaneous fat in the hips and legs may be linked to healthier metabolic profiles, compared to central obesity, which may provide partial support for the concept of metabolically healthy obesity," Hamer said.

The researchers also looked at the role of diabetes, which was present in 2.9% of the sample population. Participants with diabetes mellitus had lower gray matter volume (β = −14,200) compared with those without the condition. But when diabetes was added as a covariate, the associations between BMI and gray matter were only partially attenuated.

"Associations between obesity and gray matter volume were only partly explained by diabetes mellitus in the present study," the authors state.

In another exploratory analysis, obesity was also related to various regional brain volumes, including in the caudate, putamen, pallidum, and nucleus accumbens. No associations between obesity and white matter were apparent.

Hamer cautioned that this is a cross-sectional observational study so a causal link cannot be confirmed from this data. "All we can say is that this is a possibility and it is hypothesis generating."

In the paper the researchers note that: "It is unclear whether structural brain abnormalities drive obesity or whether obesity induces changes in gray matter volume that play a mechanistic role in future risk of neurodegeneration."

Hamer added: "We need further studies to provide prospective evidence as to whether long-term weight loss can bring about structural improvements in the brain. But I would say this is another potential reason not to be obese."

This research was conducted using the UK Biobank Resource. Hamer acknowledges support from the National Institute for Health Research Leicester Biomedical Research Centre. The authors have disclosed no relevant financial relationships.

Neurology. Published online January 9. Abstract

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