The genes encoded by bacteria within the human gastrointestinal tract actually outnumber human genes by more than 100 times (2)!
Bacteria species – which are taxonomically classified by genus, family, order, and phyla – can play a variety of roles.
While every gut microbiome is different due to an individual’s exercise and diet habits – among other factors – a healthy microbiota composition enables metabolic and immune functions to perform optimally. An imbalance of gut microbiota, on the other hand, has been linked to intestinal diseases like irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD).
Pesticides have become a key part of agricultural production because their use can lead to greater crop yields. But pesticides intended to control weeds and pests can ultimately reach the soil, water, air, and food, negatively affecting humans. The gut microbiome metabolizes chemicals, but pesticides can impair its activity and composition. The gastrointestinal tract — which is a barrier against pathogenic microorganisms and toxins — is a key site for exposure.
“Many factors influence the gut microbiome, including diet, such as ultra-processed food, as well as medications, illness, environmental chemicals such as pesticides, and hormones,” said Karen Costenbader, a physician specializing in rheumatology at Brigham and Women’s Hospital.
Costenbader and others have determined that in addition to added chemicals in one’s diet, pesticides are another exposure that research shows may increase the risk of developing an autoimmune disease. However, it is still not completely clear how pesticides interact with the microbiome and what effects the interaction has on human health.
Pesticides and insecticides have been found to influence autoimmune hepatitis (AIH), systemic sclerosis (SSc), inflammatory bowel disease (which includes Crohn’s disease and ulcerative colitis), systemic lupus erythematosus (SLE), and rheumatoid arthritis (RA), (3) among other ADs. Multiple Sclerosis (MS), Crohn’s Disease and thyroid diseases have also been associated with pesticide exposure.
A multitude of studies have examined the role that environmental factors play in the pathogenesis of autoimmune diseases. In patients with SLE, for example, exposure to pesticides in rural areas resulted in more than 3.5 times more oxidative DNA damage compared to exposure among those living in a city (4). Researchers have also found an association between insecticide use and RA and SLE among groups of post-menopausal women and farmers — with more exposure associated with a higher risk of developing disease. Lupus may have origins in abnormal lymphocyte functions.
“It is plausible to think that pesticides could damage genes that are important to lymphocyte function,”
said Costenbader. But research can be complicated because the research available is often based on data using small population sizes. It’s also difficult to isolate one agricultural exposure or to look specifically at a single pesticide as the cause of an autoimmune disease. “It can be challenging to rule out biases and prove causality in observational settings,” said Christine Parks, a staff scientist at the National Institute of Environmental Health Sciences. “There’s a lot of work to be done.”
Pesticides include a wide variety of chemicals such as herbicides, insecticides, and fumigants that are used in the home and in farming. Some, like DDT, are no longer in use but may still be circulating in people’s blood systems. Others are easy to purchase and less acutely toxic.
“You have herbicides to treat the weeds, and then insecticides, and then among each one of those, you have dozens of different types of chemicals and products that are used,” said Parks. Although pesticide use is associated with autoimmune diseases, the role of specific pesticides in the development of systemic autoimmunity is not entirely established. Certain organochlorine insecticides are associated with an increased risk for ADs (5). Exposure to chlordecone, an organochlorine that was previously used widely to control rust mites, wireworms, crickets, and other bugs, for example, has been noted to accelerate the appearance of SLE. Crystalline silica has also been linked with an increased incidence of systemic autoimmune diseases, such as SLE, rheumatoid arthritis, and scleroderma. Chemicals kill insects by impacting their nervous system.
How would an herbicide promote the development of an autoimmune disease?
“A pesticide that disturbs the immune response may help it go off balance in the context of another exposure which reveals an autoantigen. And the cycle develops into a clinically apparent disease,” explained Parks.
There’s wide interest in the relationship between autoimmune diseases and composition of the microbiome. While much remains unknown, every person has a distinct gut microbiome, and diet plays a key role. Some methods of improving the microbiome may include:
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