A large study has found that infrequent bowel movements are associated with a startling 73% increased risk of cognitive decline, including deficits in memory and attention, adding to growing evidence that gut health plays a key role in dementia and illnesses like Alzheimer’s disease.
To understand how constipation relates to brain health, Chaoran Ma at the University of Massachusetts Amherst and her colleagues analysed data from 112 753 women and men from the Nurses’ Health Study (aged 30- 55), the Nurses' Health Study II (aged 25-42), and the Health Professionals Follow-up Study (aged 40-75), all of whom reported the frequency of their bowel movements between 2012 and 2013.
They also assessed changes in their own memory, attention and other aspects of cognition over a two- to four-year period with a survey.
The researchers found that those with chronic constipation – having bowel movements every three days or more – had worse cognitive function than those with daily bowel movements, equivalent to three additional years of cognitive ageing.
New Scientist reports that while chronic constipation was linked to a 73% increased risk of cognitive decline, having more than one bowel movement daily was associated with just a 37% increased risk.
Genetic analysis of stool samples collected from 515 participants revealed that those with worse cognition and chronic constipation had fewer gut bacteria for digesting dietary fibre. They also had more gut bacteria known to cause inflammation compared with other participants.
These differences in gut bacteria may explain why chronic constipation is linked to declining brain health, says Ma, who presented these findings at the Alzheimer’s Association International Conference in the Netherlands on 19 July. For instance, persistent inflammation is known to damage neurons in people with Alzheimer’s disease.
“However, our study was not designed to test the causal relationships between bowel movements, the gut microbiome and cognition,” she said. Even so, these findings further support the notion that gut health is closely related to brain health.
She told CNN that they were surprised “at how strong the associations were, especially for those with very infrequent bowel movements”.
About 16% of the worldwide adult population experiences constipation, but it’s even more common among older adults due to age-related factors like lack of exercise and dietary fibre, and the use of medicines that can cause constipation as a side effect.
Few studies have investigated variations in intestinal motility and cognitive function.
“The more we learn about the gut-brain access, the more we understand that it’s just so important to ensure that (preventing or addressing cognitive decline) is a system approach,” said Maria Carrillo, chief science officer of the Alzheimer’s Association, who wasn’t involved in the research.
“The brain is not completely isolated from what’s happening in your blood flow.”
Chronic constipation has long been linked with inflammation and mental disorders such as anxiety and depression, but there have been many unanswered questions about the relationship between digestive health and long-term cognitive function.
“Further studies are needed to identify the microbes involved, and their function,” Ma said of her research.
Study details
Bowel movement frequency, the gut microbiome, and cognitive function in women and men
Presented at the Alzheimer’s Association International Conference on 19 July 2023
Background
Bowel movement frequency and the gut microbiome may be associated with the risk of dementia.
Method
We examined the association between bowel movement frequency and cognitive function in 112,753 women and men from Nurses’ Health Study (NHS), Nurses’ Health Study II (NHSII), and Health Professionals Follow-Up Study (HPFS) and explored the role of the gut microbiome in explaining these associations in a sub-cohort of 515 NHSII and HPFS participants. We collected data on bowel movement frequency in 2012/13 and subjective cognitive function from 2014 to 2017 in all participants and objective cognitive function using a neuropsychological battery between 2014 and 2018 in 12,696 NHSII participants. We profiled the gut microbiome using shotgun metagenomics.
Result
Bowel movement frequency was associated with overall objective cognitive function and learning and working memory in an inverse J-shape dose-response manner both Pnon-linearity<0.05). Compared to those with once daily bowel movements, participants with bowel movement frequency of every 3+ days had significantly worse cognition, equivalent to 3.0 (95% confidence interval (CI),1.2, 4.7) years of additional ageing. We observed similar J-shape dose-response relationships of bowel movement frequency with the odds of subjective cognitive decline and the likelihood of having more subjective cognitive complaints over time. Bowel movement frequencies of every 3+ days and ≥twice /day, compared to once daily bowel movement, were associated with odds ratios of subjective cognitive decline of 1.73 (95% CI, 1.60, 1.86) and 1.37 (95% CI, 1.33, 1.44), respectively. These relationships were generally consistent across three cohorts and subgroups. Bowel movement frequency and subjective cognition were significantly associated with the overall variation of the gut microbiome (both P<0.005) and specific microbial species. Butyrate producers were depleted in those with less frequent bowel movements and worse cognitive function, while a higher abundance of dysbiosis-related, pro-inflammatory species was associated with bowel movement frequency of ≥twice/day and worse cognitive function.
Conclusion
Less frequent bowel movements were associated with worse cognitive function. The gut microbiome may be a mechanistic link underlying the association between intestinal motility patterns and cognitive function.
New Scientist article – Chronic constipation is associated with cognitive decline (Open access)
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