Often called the “second brain” because it houses nerve cells and produces neurotransmitters, the gut-brain axis may be the most important locus where microbes and pesticides meet.
Pesticides interfere with biological processes. This is their purpose. Unfortunately, they nearly always have unintended consequences, many of which have been ignored by their manufacturers.
A new review article by Irish and Dutch researchers in the ISME Journal adds to the emerging scientific literature examining how pesticides affect the relationship between the human gut and the human brain (the “gut-brain axis”).
Often called the “second brain” because it houses nerve cells and produces neurotransmitters, the gut-brain axis may be the most important locus where microbes and pesticides meet.
The human gut plays host to a variety of microorganisms, ranging from bacteria and archaea to fungi, viruses and yeasts. In a healthy person, these microbes remain in balance and often cooperate both with each other and with human cells.
The gut and the brain are deeply integrated through the vagus nerve and the neuroendocrine system.
The vagus nerve is a treelike bundle of fibers extending from the lower part of the brain to nearly every body organ, but particularly the heart, lungs and digestive tract.
The neuroendocrine system comprises specialized cells inhabiting almost all the organs of the body that respond to signals from the brain and gut to produce hormones that regulate digestive enzymes, the pace of digestion, air and blood flow in the lungs, blood pressure, heart rate, blood glucose levels and other functions.
Pesticides may exert influence over any or all of these processes. They may also affect the immune system, and some, such as glyphosate, can cross the blood-brain barrier.
Pesticides can affect the production of many chemicals by gut bacteria, including serotonin and gamma-aminobutyric acid or GABA, both important neurotransmitters.
They are also notorious for disrupting the endocrine system, including reproductive hormones.
A 2020 review by Spanish scientists proposed that xenobiotics such as pesticides should be termed “microbiota disrupting chemicals” as they can interfere with microbes’ role in metabolizing steroid hormones such as estradiol, cortisol and testosterone.
Beyond Pesticides has previously reported numerous studies elucidating the deleterious effects of pesticides on disease risks involving the gut-brain axis.
These include the close association between digestive disruption and Type 1 diabetes in children and Type 2 diabetes in adults, and the ability of azoxystrobin (AZO) fungicide to impair the function of the colonic barrier in nutrient absorption and protection from harmful substances.
The digestive problems associated with Type I diabetes have been linked to exposure to antibiotics and some pesticides. Such exposures reduce the number of certain bacteria in the gut that can help protect against the inflammation triggered by these chemicals.
Pesticides’ effects on gut microbes have also been linked to autism spectrum disorder, as has digestive dysfunction.
Adult-onset neurological diseases also involve digestive disruption, which in turn may be related to disruption of the gut microbe balance.
In 2022 Beyond Pesticides reported on a study showing that the gastrointestinal disruptions, including damage to enteric glial cells that lead to Parkinson’s disease (the second most common neurodegenerative disease after Alzheimer’s), are associated with exposure to rotenone, chlorpyrifos and herbicides 2,4-D, glyphosate and paraquat.
The Irish and Dutch researchers also reviewed a study showing that glyphosate can enter the brain and raise inflammation levels, a process that has been linked to Alzheimer’s.
A 2022 study suggested that chronic exposure to dietary pesticides can affect gut microbes and trigger a cascade of changes leading to these neurodegenerative diseases.
Pesticides’ effects on host-microbe processes are not confined to humans. Importantly, pesticides affect the microbes associated with plants and nontarget insects, often changing the proportions of various species.
For example, French researchers in 2022 identified glyphosate’s changes to honey bees’ immune systems and gut microbiota, demonstrating a plausible mechanism for the bees’ susceptibility to certain diseases.
Sometimes pesticides have a seemingly perverse — but predictable—Darwinian effect.
In 2018, Beyond Pesticides reported on research detailing how insect pests’ gut microbiota contribute to the skyrocketing incidence of pesticide resistance. Microbes are nothing if not adaptable.
One common bacterial genus, Lactobacillus, which lives in the digestive tract and the female reproductive tract, as well as in fermented foods such as yogurt and kefir, demonstrates abilities that could point toward protection from pesticides’ damage to the gut-brain axis.
Lactobacillus species are adversely affected by herbicides, fungicides and insecticides, according to the authors of the current study.
They are known to enhance mood and reduce anxiety and depression, and they also provide vital services in the gut, where they produce mucus that lines the intestinal walls and enhance signaling among different types of immune cells. Thus their reduced presence in the gut caused by pesticides may contribute to many if not all diseases affecting the brain-gut axis.
However, they may also come to the rescue after pesticide exposure. Interestingly, Lactobacillus and other bacterial genera actually degrade pesticides in the foods they ferment.
A combination of L. acidophilus and Bif. animalis synergistically reduced levels of “up to 48.6% (heptachlor) and 54.7% (pp’DDE) in goat milk bio-yogurts after 14 days of cold storage when both cultures were used,” according to a recent Bulgarian study.
A remarkable Chinese study reported in Cell in 2022 exposed human volunteers to high doses of organophosphorus and organochlorine compounds.
These triggered inflammatory responses and increased numbers of pathogenic bacteria in the gut. The researchers then dosed a subset of the exposed group with a proprietary version of a lactobacillus strain called Lactiplantibacillus plantarum.
Lactobacilli are already present in many probiotic supplements and are used to improve symptoms of eczema, high cholesterol and bowel inflammation.
In the Chinese study’s probiotic group, microbial diversity was reestablished, inflammatory markers decreased — including two factors associated with kidney disease — and the bacteria promoted the breakdown of the pesticides and excretion of their metabolites.
Microbes are everywhere — even in the rocks deep below the seafloor.
They are certainly everywhere in the human body, not only the gastrointestinal tract — one study found pesticides reduced the flora in the human mouth — and it appears that pesticides may affect microbes wherever they are.
Estimates of the total number of microbial cells in a typical human — about 39 trillion-exceed the number of actual human cells — about 30 trillion.
This has led many scientists to adopt pioneering microbiologist Lynn Margulis’s proposal that humans and most other multicellular organisms should be viewed as “holobonts,” that is a single organism comprising a host and one or more symbionts — generally microbes.
It would encourage a paradigm shift away from the pesticide industry’s assumption that its products’ effects are siloed and target only specific agricultural pests. Not even a monoculture field is free of trillions of microbes on its plants, in its soil, and in its water. Many of these are beneficial and may have their own ability to control pests.
The pesticide industry has turned a blind eye to the effects its products have on thousands of non-target organisms, and microbes may be the most significant of all.
Although as usual more research is needed, the trajectory of scientific understanding regarding humans and the microbial world curves toward reduction of pesticide use in favor of striking a balance within the biosphere rather than willful disregard of the interrelationship of all life.
Thus the ability of microbes to protect against pesticide exposure offers remarkable potential for mitigating the harms caused by the indiscriminate effects of pesticides.
But we do not have to wait until research can offer specific means of using microbes in this way. We can reduce our exposure now.
Beyond Pesticides encourages the consumption of organic foods to decrease pesticide levels in one’s body, both to reduce the risk of chronic metabolic diseases and to ingest a wider variety of beneficial microbes.
Supporting organic agriculture helps farmers and other consumers to make the transition away from pesticide-driven agriculture. Regenerative organic agriculture can restore microbial health to soils, which may ameliorate problems caused by pesticides’ damage to host-microbe relationships in plants, insects, aquatic organisms and others.