Does the Gut-Brain Axis Play a Role In the Emergence of Psychiatric Disorders?
Post by Lani Cupo
The takeaway
The gut-brain axis undergoes many changes in adolescence, and these can, in turn, impact the nervous system. It is possible that environmental changes impacting the gut may influence the emergence of psychiatric disorders during adolescence, but more research is required on the topic.
What's the science?
Over the past two decades, the gut-brain axis has increasingly been investigated as important to mental, psychiatric, and neurological health. Most research, however, has focused on the first years of life or old age, with few studies investigating the period of adolescence where both the gut microbiome and brain undergo many changes and psychiatric disorders commonly emerge. This week in Biological Psychiatry, McVey and colleagues sought to synthesize the current knowledge base around the impact of the environment on the gut-brain axis and its role in the emergence of psychiatric disorders during adolescence.
What did they review?
The authors performed a review of the literature on adolescent development and plasticity — or changeability — of the gut microbiome and the brain in adolescence in both human and nonhuman animal studies. The gut-brain axis comprises several component parts investigated in this review. The gut microbiome includes the microorganisms that live in the intestines. The enteric nervous system (ENS) comprises neural and glial cells that directly control the digestive tract (from the esophagus to the anus). While it operates largely independently of the central nervous system, the vagal nerves carry signals to and from the brain to the ENS. Finally, the review examined brain development during adolescence in the context of psychiatric development and atypicality.
What did they find?
First, examining the gut microbiome during adolescence, the authors found that the diversity and stability of microorganisms in the gut were altered, with important sex differences. Specifically, they found that during puberty male gut microbiomes became less diverse, whereas female microbiomes remained stable. There was evidence of bidirectional influence of gut microbiome and sex hormones, meaning the microbiome influences sex hormones and sex hormones influence the microbiome. An altered diet during adolescence is known to impact the gut microbiome as well, drawing attention to the impact of the adolescent environment.
While the authors found there was a lack of studies examining the ENS during adolescence, there was some evidence pointing to changes to the neurons and glia of the ENS during adolescence. For example, in the duodenum, the ratio of neurons to glia was reduced, but in the colon, it was increased. Regarding the vagal nerve, there is some association between changes in vagal tone, measured via heart-rate variability (changes in the interval between heartbeats), and poorer psychological and physical outcomes. Specifically low (worse) vagal tone in adolescence has been associated with later life cardiovascular disease risk, adolescent asthma, emotional dysregulation in adolescents with autism spectrum disorders, and clinical depression.
Adolescence is a period defined by many brain changes, including in the frontal lobes, which continue to develop well after puberty. Two major areas of brain development include synaptic density (synapses are pruned through childhood and adolescence) and myelination (which increases during early life and adolescence). Both of these processes result in greater efficiency of brain networks from childhood to early adolescence. The authors draw attention to the psychiatric disorders that emerge in adolescence, including major depressive disorder, anxiety disorders, eating disorders, and alcohol use disorders, for all of which there is evidence pointing to the association of alterations in the gut microbiome.
The authors also summarize evidence from nonhuman animal studies linking alterations in communication between the microbiome, gut, and brain and clinical dysfunction. The work they highlight indicates the role of the gut microbiome in modulating the HPA axis activity in response to acute stress, as well as the possibility of moderating the gut microbiome, gene expression in the brain, and behaviors with diet.
What's the impact?
The results of this review first draw attention to the potential role the gut microbiome may play in moderating the emergence of psychiatric disorders in adolescence. Importantly, however, this review identifies a lack of studies that comprehensively investigate this topic, and therefore more research is needed to understand how the gut microbiome influences the development of psychiatric disorders.