Gut bacteria, sleep and brain health

This paper is part of the 2018 NMJ Special Issue on Cognition and Mental Health. Download the full issue here.

Relation

Anderson JR, Carroll I, Azcarate-Peril MA, et al. A preliminary investigation of gut microbiota, sleep and cognitive flexibility in healthy older adults.Sleep medicine. 2017;38:104-107.

Study objective

Assessing relationships between gut microbiota, sleep quality, and cognitive flexibility in healthy older adults.

Draft

A preliminary observational study

Participant

Data from 37 participants aged 50 to 85 years (73% female, 92% Caucasian) were analyzed. Individuals with significant neurological or gastrointestinal diseases were excluded, as were those who had taken probiotics or antibiotics within 30 days prior to the study. The presence of hypertension, diabetes, and sleep apnea were tracked as covariates, as was dietary macronutrient intake per participant report (EPIC-Norfolk Food Frequency Questionnaire).

Target parameters

Self-reported sleep quality was assessed using the Pittsburg Sleep Quality Index (PSQI), which tracked sleep latency and duration. Higher scores on the PSQI indicate poorer sleep quality. Cognitive flexibility was assessed by Stroop color word tests. During the Stroop test, participants read color words (“Stroop word”) aloud, identified the ink color of rows of Xs (“Stroop color”), and identified the ink color of incongruent color words (“Stroop color word”) as quickly as possible. Higher values ​​indicate better identified elements. Gut microbiome samples were analyzed by uBiome using 16S ribosomal RNA amplicon sequencing and reported as percentage of phyla present.

Key insights

Controlling for covariates, reduced sleep quality correlated with worse Stroop test scores (Stroop Word,P=0.018; Stroop color word,P=0.010 and Stroop color,P=0.053). Again, controlling for covariates, the study found participants with a higher percentage ofVerrucomicrobiahad improved cognitive flexibility as measured with Stroop Word (P=0.034) and Stroop color (P=0.071) tests. TheLentisphaeraeStrain did not show a similar correlation; However, it showed a stronger relationship with Stroop Color-Word performance (P=0.015), regardless of sleep quality.

Practice implications

As the rise in microbiota research suggests, the potential impact of the gut microbiota on our understanding of human health and our approach to treating various health conditions is enormous. Although this particular study is slightly confounded by the number of variables discussed, along with its admitted limitations (e.g., results based more on observations than causal relationships, lack of detailed screening measures for sleep, and superficial exploration of the mechanism of action), it does provide some interesting food for thought: Could gut microbiota be a factor affecting sleep quality and cognitive health connects?

Observational studies such as the one discussed here are the first steps in a direction that may ultimately help us harness manipulation of the microbiome for therapeutic purposes.

Given that more than 30% of Americans report lack of sleep at night and an estimated 50 to 70 million adults report a sleep disorder,¹continued research into modifiable factors is a necessity. The gut microbiome is a largely unexplored area, and our journey to understanding its impact on health needs is just beginning. The influence of the gut microbiome on human health needs to be understood more deeply. Observational studies such as the one discussed here are the first steps in a direction that may ultimately help us harness manipulation of the microbiome for therapeutic purposes.

The researchers suggest that their results should be taken together with those from other related studies^2-4suggest that sleep quality is directly proportional to the presence of specific gut microbiota phyla, which then correlates with cognitive function. Both phyla discussed in this article have been linked to sleep disorders. TheVerrucomicrobiawas then directly correlated with cognition duringLentisphaeraewas possibly indirectly correlated. The question that wasn't so clearly asked was whether the cognitive impairment was a direct result of impaired gut flora or whether it was secondary to the chronic, non-communicable health conditions also linked to both, such as obesity, diabetes and heart disease.

The researchers failed to discuss the possible inverse correlation, where the microbiota proportions of the specified phyla are poorLentisphaeraeand Verrocumicrobia could be more directly causative of reduced sleep quality. We know that sleep quality is directly linked to chronic illness and cognitive decline; The Centers for Disease Control and Prevention shows that adults who sleep less than 7 hours a night are more likely to report chronic health problems than those who get enough sleep.⁵Due to the observational design of this study, it is not possible to draw direct cause-and-effect conclusions. However, as the gut-brain connection becomes more widely recognized in the community, perhaps the relationship between these two variables should be viewed as dynamic and bidirectional.

The strong correlation shown in this study between theVerrucomicrobiaandLentisphaeraePhyla and sleep quality contrasts with a 2016 study by Benedict and colleagues that showed a correlation with phyla other than these 2.²Additionally, upon further investigation of the 2 phyla above, their associated genera and species are not part of the more commonly seen and prescribed probiotics today. All of this is to say that while this information is certainly fascinating, we are at the very beginning of understanding how the gut microbiota may (or may not) influence sleep and cognition.