Glyphosate—the controversial active ingredient in Monsanto's top-selling weedkiller Roundup and other herbicides—can cause non-alcoholic fatty liver disease in rats at very low, real-world doses, according to a peer-reviewed study published in Nature.

The groundbreaking research is the first to show a "causative link between an environmentally relevant level of Roundup consumption over the long-term and a serious disease," stated lead author Dr. Michael Antoniou of King's College London, who described the findings as "very worrying."

Non-alcoholic fatty liver disease is the accumulation of extra fat in liver cells not caused by alcohol. It's a serious and common condition that affects up to 90 million people in the U.S.

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Activists have been relabelling bottles of Monsanto's Roundup weedkiller in garden centers and DIY shops across the UK. Global Justice Now

For the study, the researchers used cutting-edge molecular profiling methods to examine the livers of female rats who were fed an extremely low dose of Roundup over a two-year period. The rats were administered an ultra-low dose of only 4 nanograms per kilogram of body weight per day, which is 75,000 times below EU and 437,500 below U.S. permitted levels—basically thousands of times below the amount allowed by regulators around the world.

As King's research associate Dr. Robin Mesnage explained to the Daily Mail, "the concentration of glyphosate that was added to the drinking water of the rats corresponds to a concentration found in tap water for human consumption."

"It is also lower than the contamination of some foodstuffs," Mesnage added.

The team found evidence that consumption of low doses of glyphosate over time can cause cell damage, serious fatty liver disease and areas of dead tissue or necrosis in the livers, as the Daily Mail reported from the study.

The researchers concluded:

"The results of the study presented here imply that chronic consumption of extremely low levels of a GBH formulation (Roundup), at admissible glyphosate-equivalent concentrations, are associated with marked alterations of the liver proteome and metabolome. These changes in molecular profile overlap substantially with biomarkers of [non-alcoholic fatty liver disease] and its progression to [non-alcoholic steatohepatitis, a serious liver disease]."

"Our results also suggest that regulators should reconsider the safety evaluation of glyphosate-based herbicides," Dr. Antoniou said.

According to the American Liver Association, non-alcoholic fatty liver disease may cause the liver to swell and cause cirrhosis over time and may even lead to liver cancer or liver failure. People who are overweight, obese, or have diabetes, high cholesterol, high triglycerides are at risk of developing the condition although rapid weight loss and poor eating habits also may lead to the disease.

"Regulators worldwide accept toxicity studies in rats as indicators of human health risks. Therefore, the results of this latest study may have serious consequences for human health," the King's researchers said.

Glyphosate is the most widely applied weedkiller worldwide, especially in the U.S. Glyphosate is used on Monsanto's line of "Roundup Ready" crops such as soy, corn, canola, alfalfa and cotton that are genetically altered to withstand direct applications of the herbicide, as the product kills only the weeds.

A recent analysis revealed that since 1974, when Roundup was first commercially sold, more than 1.6 billion kilograms (or 3.5 billion pounds) of glyphosate has been used in the U.S., making up 19 percent of the 8.6 billion kilograms (or 18.9 billion pounds) of glyphosate used around the world.

The chemical is so ubiquitous that it can now be detected in common food items from breakfast cereals, wine and even childhood vaccines.

In March 2015, the World Health Organization's International Agency for Research on Cancer (IARC) concluded that glyphosate is a "probable human carcinogen," touching off an international row on the health and safety of the widely applied herbicide. However, other regulatory agencies such as the United Nation's Food and Agriculture Organization said the ingredient is "unlikely to pose a carcinogenic risk to humans from exposure through the diet." Monsanto and other chemical companies have maintained the safety of their glyphosate-based products.

The Crop Protection Association, which speaks for Monsanto and other chemical companies, questioned the validity of the new study in a statement to the Daily Mail.

"Glyphosate is amongst the most thoroughly tested herbicides on the market, and those studies by expert regulators have consistently concluded that glyphosate does not pose a risk to public health," the association said. "Glyphosate is a crucial tool in a farmers' armory. To put things in perspective, glyphosate is less toxic than baking soda, table salt, the caffeine in our coffee and many other products we all use or consume regularly."

Monsanto also told Farmers Weekly that the results of the study should be questioned, claiming that its researchers, including Mesnage and Gilles-Eric Seralini, published similar past studies that "have been widely rejected by the international science community due to faulty science."

However, Peter Melchett, policy director at the UK's Soil Association, told Farmers Weekly, "This research is the first evidence of a clear causative link between consumption of Roundup at levels that are found in the real world and a serious disease."

The Health and Environment Alliance, a European non-profit which addresses how the environment affects health in the European Union, has also urged for a ban on glyphosate in response to the study.

"Glyphosate is already classified by IARC as a 'probable carcinogen' … It is also described as a 'potential endocrine disrupting chemical.' This new study adds to evidence about the likely harm to human health from Roundup and other glyphosate based herbicides. Given people's unavoidable exposures from the massive increase in the use of these weed killers over the past 30 years, surely it is time to ban it on precautionary grounds?" said GÃ©non K. Jensen, the alliance's executive director.