How the Gut Talks (Rapidly! Insistently!) To the Brain
An astonishing array of brain functions demand a well-balanced microbiome.
By Hara Estroff Marano published November 2, 2021 - last reviewed on November 2, 2021
Immigration to the Unites States will have many benefits for the thousands of Afghans fleeing their country in the wake of the Taliban takeover. They will have opportunities for work and access to resources. They will live free of fear of a repressive regime. Their sons and, especially, their daughters will be educated in free public schools.
One other big difference immigration will make is less visible and less talked about but no less consequential: It will shift the composition of their gut microbiome in a way that puts them on a path to developing chronic illness—along with millions of Americans. They will become at high risk for metabolic diseases and obesity, inflammatory bowel disease, psychiatric disorders like depression and anxiety, and neurodegenerative disorders like Alzheimer’s and Parkinson’s diseases.
Studies over the last couple of years demonstrate that immigration to the U.S is accompanied by massive changes to the gut microbiome, particularly changes in composition, with far-reaching effects. Researchers have documented loss of bacterial diversity, obliteration of whole strains of bugs (notably belonging to the Prevotella genus), and loss of functionality, including the ability to fully process carbohydrates and fiber.
What’s more, the changes begin almost immediately upon arrival in the U.S. Animal studies indicate that a high proportion of animal-derived protein relative to carbohydrates can shift the microbiome in one day, putting Prevotella on a path to extinction.
The changes, particularly the loss of bacterial diversity, are thought to be responsible for the mounting obesity rates seen most strikingtly in U.S. immigrants over the last several decades—a period when the American diet moved heavily into industrially processed foods, high-fat red meats, and, especially, away from fiber-laden foods rich in complex carbohydrates, also known as microbiota-accessible carbohydrates (MACs). Some scientists liken the loss of microbial species as a health threat on a par with climate change.
Americanization of the microbiome reflects not only the rise in ultraprocessed foods but also a shift in the ratio between animal-based and plant-based foods. Such dietary shifts have cascading effects, altering the entire microbial ecosystem and making it less receptive to whole populations of beneficial bacteria.
Over the last 20 years, evidence has mounted that such dynamic changes in the gut microbiota can significantly alter brain physiology and behavior. While scientists once thought that cognition was regulated only by the central nervous system, two decades of intensifying research demonstrate that bacteria residing in the gastrointestinal tract influence cognitive function—and dysfunction.
Gut bacteria have direct and indirect effects on brain physiology and function, and ultimately on behavior, through a multitude of complex mechanisms. Microbes produce and release metabolites and other molecules that enter the circulation and affect bodily operations near and far. Some microbial substances, such as the sleep influencer tryptophan and the appetite regulator leptin, are directly neuroactive. Other gut biome derivatives, such as short-chain fatty acids, released through the digestion of MACs, act as signaling compounds in the nervous system and ultimately affect both neurogenesis and neuroplasticity. They also help maintain the integrity of the blood-brain barrier.
Other substances that are molecular signatures of gut bacteria affect brain operations by secreting immunoactive agents that can either inhibit or promote neurogenesis. Some trigger dysfunction of the gut barrier, a condition known as “leaky gut,” thought to play a role in autism. Unauthorized substances that sneak though the gut lining into the bloodstream can set off inflammation in the central nervous system, contributing to the development of pain, migraine headaches, depression, and anxiety in the present and neurodegenerative conditions in the future.
In addition, gut bacteria produce substances that significantly modulate the permeability of the blood-brain barrier, tightening or loosening the junctures between protective cells in cerebral blood vessels and determining whether or not toxic substances have a clear path to infiltrate the brain. There’s evidence that loss of normal intestinal bacteria increases the permeability of the blood-brain barrier to stimulate or further the deposition in brain cells of beta-amyloid peptide, one of the hallmarks of Alzheimer’s disease.
Other bacterial products affect the brain’s overall vascular physiology, and their activity is linked both to increasing the likelihood of stroke and promoting the deposition of atherosclerotic plaques, a significant cause of dementia. Species of gut bacteria act on the choline and carnitine found in red meats to disrupt lipid balance.
Resetting the balance of the microbiome is not only possible but likely to improve many aspects of health throughout the lifespan. Backing away from the overprocessed, fat-rich American diet is considered the most far-reaching route. It can get a big boost by consumption of microbe-rich probiotic supplements and pills. Industry data show that probiotics use, now generating a $50 billion market worldwide, is rising by close to 8 percent a year.
Many probiotics contain an array of bacteria—mixed cultures of live microorganisms—although that is likely to change in the years ahead as studies increasingly show that the benefits of probiotics are strain-specific. A single dose may carry as many as 50 billion colony-forming units. Powerful as they are, probiotics are most effective working in tandem with a plant-rich diet.