Abdominal Fat Linked to Lower Brain Volume in Midlife

New research provides strong evidence of an association between abdominal fat and reduced brain volumes, particularly those involved with cognitive function.

In large study of healthy middle-aged adults, greater visceral and subcutaneous abdominal fat on abdominal MRI predicted brain atrophy on imaging, especially in women.

"The study shows that excess fat is bad for the brain and worse in women, including in Alzheimer's disease risk regions," lead author Cyrus Raji, MD, PhD, with the Mallinckrodt Institute of Radiology, Washington University, St. Louis, Missouri, told Medscape Medical News.

The study was published online August 28 in the journal Aging and Disease.

Modifiable Risk Factor

Multiple studies have suggested a connection between body fat accumulation and increased dementia risk. But few have examined the relationship between types of fat (visceral and subcutaneous) and brain volume.

For the new study, 10,000 healthy adults aged 20-80 years (mean age, 52.9 years; 53% men) underwent a short whole-body MRI protocol. Regression analyses of abdominal fat types and normalized brain volumes were evaluated, controlling for age and sex.

The research team found that higher amounts of both visceral and subcutaneous abdominal fat predicted lower total gray and white matter volume, as well as lower volume in the hippocampus, frontal cortex, and temporal, parietal, and occipital lobes.

"The findings are quite dramatic," Raji told Medscape Medical News. "Overall, we found that both subcutaneous and visceral fat has similar levels of negative relationships with brain volumes."

Women had a higher burden of brain atrophy with increased visceral fat than men. However, it's difficult to place the sex differences in context due to the lack of prior work specifically investigating visceral fat, brain volume loss, and sex differences, the researchers caution.

They also note that while statistically significant relationships were observed between visceral fat levels and gray matter volume changes, their effect sizes were generally small.

"Thus, the statistical significance of this work is influenced by the large sample size and less so by large effect size in any given set of regions," the investigators write.

Other limitations include the cross-sectional nature of the study, which precludes conclusions about causality. The analysis also did not account for other lifestyle factors such as physical activity, diet, and genetic variables.

The researchers call for further investigation "to better elucidate underlying mechanisms and discover possible interventions targeting abdominal fat reduction as a strategy to maintain brain health."

"Helpful Addition to the Literature"

Commenting on this research for Medscape Medical News, Claire Sexton, DPhil, Alzheimer's Association senior director of scientific programs and outreach, noted that "previous studies have linked obesity with cognitive decline and increased risk of dementia. Rather than using BMI as a proxy for body fat, the current study examined visceral and subcutaneous fat directly using imaging techniques."

Sexton, who was not associated with this study, said the finding that increased body fat was associated with reduced brain volumes suggests "a possible mechanism to explain the previously reported associations between obesity and cognition."

"Though some degree of atrophy and brain shrinkage is common with old age, awareness of this association is important because reduced brain volume may be associated with problems with thinking, memory, and performing everyday tasks, and because rates of obesity continue to rise in the United States, along with obesity-related conditions including heart disease, stroke, type 2 diabetes and certain types of cancer," she added.

"While a helpful addition to the literature, the study does have important limitations. As an observational study, it cannot establish whether higher levels of body fat directly causes reduced brain volumes," Sexton cautioned.

In addition, the study did not take into account important related factors like physical activity and diet, which may influence any relationship between body fat and brain volumes, she noted. "Overall, it is not just one factor that is important to consider when considering risk for cognitive decline and dementia, but multiple factors.

"Obesity and the location of body fat must be considered in combination with one's total lived experience and habits, including physical activity, education, head injury, sleep, mental health, and the health of your heart/cardiovascular system and other key bodily systems," Sexton said.

The Alzheimer's Association is leading a 2-year clinical trial known U.S. POINTER to see whether combining physical activity, healthy nutrition, social and intellectual challenges, and improved self-management of medical conditions can protect cognitive function in older adults who are at increased risk for cognitive decline.

This work was supported in part by Providence St. Joseph Health, Seattle, Washington; Saint John's Health Center Foundation; Pacific Neuroscience Institute and Foundation; Will and Cary Singleton; and the McLoughlin family. Raji is a consultant for Brainreader ApS, Apollo Health, Voxelwise LLC, Neurevolution LLC, Pacific Neuroscience Institute Foundation, and Icometrix. Sexton reports no relevant financial relationships.

Aging Dis. Published online August 28, 2023. Full text

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