Glyphosate is the active chemical ingredient in many widely used broad-spectrum herbicides, designed to control weeds in agriculture and non-agricultural settings. Its extensive use globally means that residues are often present in the environment and food supply. The human gut microbiome is the complex community of trillions of microorganisms residing in the digestive tract, playing roles in metabolism, nutrient absorption, and immune system development. Given that glyphosate is engineered to target biological processes found in microorganisms, a significant question is whether exposure to this chemical interacts with and alters the composition and function of the gut microbiome.
The Specific Biological Target of Glyphosate
Glyphosate’s herbicidal action is attributed to its ability to interfere with a metabolic route known as the Shikimate pathway. This pathway is a seven-step sequence that plants, fungi, and most bacteria use to synthesize aromatic amino acids: tryptophan, tyrosine, and phenylalanine. The chemical specifically inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which catalyzes a step late in the pathway.
By effectively blocking the EPSPS enzyme, glyphosate starves the target organism of the necessary aromatic amino acids, leading to plant death. This mechanism was historically considered safe for humans and animals because they do not possess the Shikimate pathway. However, the human gut is populated by a vast number of bacteria, many of which rely on this pathway for their survival. Therefore, the safety profile of glyphosate must consider its antimicrobial effect on the gut flora.
Documented Effects on the Human Gut Microbiome
The potential for glyphosate to impact the gut microbiome stems directly from its ability to inhibit the EPSPS enzyme in bacteria. Research, including in vitro studies and animal models, suggests that glyphosate exposure can alter the delicate balance of the microbial community, a condition termed dysbiosis. These alterations include both compositional shifts, where the relative numbers of different bacterial species change, and functional changes, where the overall metabolic output is modified.
Compositional Changes
Studies have indicated that glyphosate can suppress the growth of certain beneficial bacteria while potentially allowing more resistant or opportunistic species to flourish. Specifically, some research has shown that bacteria commonly associated with a healthy gut, such as members of the Lactobacillus and Bifidobacterium genera, are particularly susceptible to glyphosate exposure. The reduction of these beneficial commensal bacteria can disrupt the microbial ecosystem’s stability and resilience.
In contrast, other bacterial species may exhibit a degree of natural resistance to glyphosate. For example, some opportunistic pathogens like Salmonella and Clostridium may be less sensitive to the herbicide. This selective pressure can lead to a shift in the overall gut profile, potentially favoring the colonization or dominance of less desirable microorganisms.
Functional Disruption
The impact of glyphosate extends beyond the simple number of bacteria, affecting the metabolic functions that benefit the host. The inhibition of bacterial EPSPS reduces the production of aromatic amino acids within the gut, which can limit the availability of these compounds for the human host, who relies on them for various biological processes. Tryptophan, for instance, is a precursor to neurotransmitters and regulatory molecules.
Furthermore, glyphosate exposure has been linked to a reduction in the microbial biosynthesis pathways responsible for producing short-chain fatty acids (SCFAs). SCFAs, such as butyrate, are important metabolites that serve as the primary energy source for colon cells and help maintain the integrity of the intestinal lining. A decrease in SCFA production can result in undesirable outcomes, including the elevation of markers associated with inflammation within the gut environment.
Sources of Human Exposure and Safety Standards
Human exposure to glyphosate occurs through several primary routes, with the most common being dietary intake of food containing herbicide residues. Glyphosate is widely applied to crops, including corn, soy, sugar beets, and oats, often leading to detectable levels of the chemical in finished food products. Non-dietary exposure can also occur, particularly for agricultural workers, through dermal contact, inhalation of spray mist, or via contaminated water sources.
Regulatory bodies such as the U.S. Environmental Protection Agency (EPA) and the European Food Safety Authority (EFSA) set safety standards based on extensive toxicological data. These standards include the Acceptable Daily Intake (ADI), which is the amount of a substance that can be ingested daily over a lifetime without posing an appreciable health risk. The EU ADI for glyphosate is currently set at 0.5 milligrams per kilogram of body weight per day, while the EPA’s ADI is 1.0 mg/kg/day.
Regulators also set Maximum Residue Limits (MRLs), which are the highest allowable concentrations of a pesticide residue in or on food products. While regulatory assessments generally conclude that current dietary exposure to glyphosate is well below the established ADI, the ongoing public and scientific debate centers on whether these standards fully account for the subtle, long-term effects of glyphosate on the sensitive ecosystem of the gut microbiome. The MRLs permit residues up to 10 mg/kg in wheat and even higher in other crops.