Dietary Fiber Tied to Lower Dementia Risk

Batya Swift Yasgur, MA, LSW

March 07, 2022

Eating a high-fiber diet, especially one rich in soluble fiber, is linked to a lower risk of incident disabling dementia, new research shows.

Investigators administered a dietary survey to 3700 healthy adults at midlife and then followed them for up to 20 years. They found that participants who consumed the most fiber had approximately a 25% lower risk of developing dementia in later life.

"This study showed that people with a high intake of dietary fiber, especially soluble fiber, have a lower risk of dementia," study investigator Kazumasa Yamagishi, MD, PhD, professor, Department of Public Health Medicine, Faculty of Medicine and Health, Services Research and Development Center, University of Tsukuba, Japan, told Medscape Medical News.

"There are still many unknowns about the causes of dementia, and it is not appropriate to determine causality based on the results of a single cohort study. However, the results of this study can be said to be one of the findings that will lead to the prevention of dementia," Yamagishi said.

The study was published online February 6 in Nutritional Neuroscience.

Brain-Gut Interaction

Brain-gut interaction has recently received attention for its potential involvement in the development of dementia. "The concept of brain-gut interaction emerged from the idea that the central nervous system communicates bidirectionally with the gastrointestinal tract, suggesting that the gut microbiome may influence brain plasticity and cognitive function," the authors write.

A diet high in soluble fiber attenuates neuroinflammation in mouse models. Other animal studies have suggested that insoluble fiber might also have a beneficial effect on the microbiome.

The researchers wanted to see whether dietary fiber intake — especially soluble fiber — is associated with a reduced risk of dementia. They also investigated whether there was any difference between dementia in patients with vs without a history of stroke.

In a previous study, these same researchers reported an inverse association between eating beans, which are high in fiber, and risk of disabling dementia. In the current study, the researchers extended the analyses to dietary fiber intake of total, soluble, and insoluble fibers, as well as other fiber-containing foods, such potatoes, vegetables, and fruits. However, they distinguished potatoes from other vegetables because the composition of starch in potatoes differs.

"Dietary fiber is a nutrient found in grains, potatoes, vegetables, and fruits and is known to affect intestinal bacteria," Yamagishi said. "Recently, some experimental studies have shown that intestinal bacteria may be involved in cognitive functions as well as diseases of the digestive tract. However, there have been no studies that have actually examined the relationship between dietary fiber intake and the subsequent risk of dementia in large numbers of general people."

The researchers turned to participants in the Circulatory Risk in Communities Study (CIRCS), an ongoing dynamic community cohort study involving five communities in Japan. The current study focused on communities where disabling dementia surveillance is conducted.

Participants (n = 3739) ranged in age from 40 – 64 years (mean age, 51 years) at the time they completed the 24-hour dietary recall survey, and they participated in annual health checkups from 1985 – 1999. Potential risk factors for disabling dementia were measured at the time the dietary surveys were conducted. Participants were then followed for a median of 19.7 years (1999 – 2020) to confirm incident, disabling dementia.

"Disabling dementia" was defined as dementia that required care under the National Long-Term Care Insurance System and was further categorized on the basis of having a history or not having a history of stroke.

The researchers divided participants into quartiles, based on the amount of total, soluble, and insoluble intake reported in their surveys. They found that men tended to consume less total fiber compared to women.

Unclear Mechanism

During follow-up, 670 participants developed disabling dementia.

Total fiber intake was "inversely and linearly" associated with risk of incident dementia, the authors report, with each successive quartile associated with a lower risk compared to the lowest quartile (P for trend = .03).

Quartile Multivariate HR (95% CI)
Second .83 (.67 – 1.04)
Third .81 (.65 – 1.02)
Fourth (highest) .74 (.57 – .96)

 

The association remained after adjusting for potential factors that might affect dementia onset, such as body mass index, systolic blood pressure, antihypertensive medication use, serum total cholesterol, cholesterol-lowering medication, and diabetes (P for trend = .05).

"The inverse association was more evident for soluble fiber intake and was confined to dementia without a history of stroke," the authors report. Moreover, potatoes, not vegetables or fruits, showed a similar association.

"The mechanisms are currently unknown but might involve the interactions that take place between the gut and the brain," Yamagishi said in a release.

"One possibility is that soluble fiber regulates the composition of gut bacteria. This composition may affect neuroinflammation, which plays a role in the onset of dementia," he suggested. "It's also possible that dietary fiber may reduce other risk factors for dementia, such as body weight, blood pressure, lipids, and glucose levels."

The authors note several limitations. For example, they did not distinguish between Alzheimer's and non-Alzheimer's dementia. Moreover, they classified dietary habits on the basis of a single survey, and participants' dietary patterns might have changed over the study period.

Additionally, Yamagishi noted, it is "important to confirm the association in other populations."

Balance Is Key

Commenting for Medscape Medical News, Uma Naidoo, MD, director of nutritional and lifestyle psychiatry, Massachusetts General Hospital, Boston, and nutrition educator at Harvard Medical School, said the study "adds to the growing pool of evidence suggesting that a diet rich in colorful, plant-based foods can benefit our neurological and psychiatric health, especially as we age."

Naidoo, also a chef and the author of This Is Your Brain on Food, who was not involved in the study, continued, "In nutritional psychiatry, balance is key and therefore consuming a well-rounded diet including ample amounts of fiber — particularly from sources like steel-cut oats, beans, lentils, and numerous other fruits and vegetables — can be part of a healthy lifestyle and prevention against cognitive decline in later years.

"While the study authors admit to limitations within the study, in my opinion, eating healthier has so many mental and physical health benefits that it's a nutritional psychiatry no-brainer," she added.

The study was partly supported by Health and Labour Science Research Grants for Dementia from the Ministry of Health, Labour and Welfare of Japan; JSPS Kakenhi; FULLHAP; and the Osaka University International Joint Research Promotion Programme with University College London. The authors and Naidoo report no relevant financial relationships.

Nutr Neurosci. Published online February 6, 2022. Full text

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