The oft controversial animal-rights group PETA has beef with the ostensibly animal-friendly Impossible Burger.

PETA, short for People for the Ethical Treatment of Animals, is fired up over the fact that the vegetarian burger’s maker, Impossible Foods, tested the safety of its faux patty on animals. Impossible Foods has openly noted that it conducted animal experiments—involving a total of 188 rats—to convince the Food and Drug Administration that the burger’s key, blood-like ingredient qualified as a safe food additive. The company was after a controversial FDA designation called “generally recognized as safe” or GRAS. The animals were sacrificed after the testing.

In a blistering blog post, PETA claimed the testing was “voluntary” and that Impossible Foods conducted the test after “disregarding advice from a PETA scientist who said that there’s no need to hurt and kill animals to test its burger.” To further scorch the burger’s name, PETA made the dubious suggestion that the burger could increase risks of cancer in consumers.

Impossible Foods fired back with its own juicy blog post, calling the PETA’s post “malicious, defamatory, erroneous and mendacious.” The burger maker claimed that the rat studies were necessary to convince the FDA that its unique burger ingredient, called leghemoglobin, fell into the GRAS category and could safely be marketed and consumed. Leghemoglobin is an iron-containing protein pulled from soybean roots that’s similar to hemoglobin, an oxygen-carrying protein in red blood cells. In the plant-based patty, it helps mimic the red, juiciness of raw beef. But it had never been used in foods before, hence the safety testing.

Impossible Foods argued further—as it always has—that if consumers choose meatless meat options, such as its burger, it could potentially spare the lives of millions of cows.

Last, the chic food start-up revealed that there’s been an ongoing food fight with PETA; the recent blogging was just the latest shots fired. Impossible Burger reported:

[L]ast August, a few PETA activists launched a misguided attack against Impossible Foods. They mobilized trolls in an attempt to intimidate Impossible Foods and its supporters on social media, bombarded our employees with thousands of auto-generated complaint emails, and tried to disrupt and damage our business.

In an email to Ars, PETA disputed this characterization saying its activists were conveying valid concerns and were not "trolls" out to "intimidate" the company.

Testing regulations

The question of whether Impossible Foods was required to conduct the animal tests is a little murky, but the truth largely falls on the side of the burger. The FDA allows food makers to determine GRAS designations on their own, generally with the help of consultants using internal testing. In fact, the FDA doesn’t even require food companies to notify the agency when they make a GRAS designation and start using a new additive. This is why the designation is controversial.

That said, the FDA does require that companies conduct such testing and be able to prove that their additives are safe after the fact. The FDA can step in at any time after an additive is put on the market and safety questions arise.

With its rising profile and high-stakes investors, Impossible Foods did voluntarily seek out the FDA’s opinion on whether leghemoglobin met the qualifications for a GRAS designation based on the company’s internal safety data. But it would have needed to win over the FDA regardless if there were ever any questions raised about the burger.

It just so happened that in 2015, the FDA determined that leghemoglobin did not meet the agency’s standards for a GRAS designation. The FDA wanted more clinical data showing safety. Impossible Foods resubmitted its application last year with more expert analyses—but not additional animal testing or clinical data—and got a “no questions” response from the FDA. That is effectively a nod of approval from the FDA for the GRAS designation.

That cancer claim

As for Impossible Burger’s link to cancer, PETA is on even shakier ground. The animal-rights group noted:

Impossible Foods’ big claim to fame is that there’s heme in its burgers. Well, here’s a shocker: This heme comes from soy leghemoglobin and contains more iron than that found in the heme of a similar serving of red meat. Having too much iron in your blood can mean a greater risk of developing cancer, especially for men and postmenopausal women.

Researchers have indeed linked excessive iron (aka iron overloads) to risks of cancer. But it seems rather implausible to achieve such levels by simply eating an Impossible Burger, or a hundred. For one thing, healthy people typically don't accumulate excessive levels of iron. As the National Institutes of Health’s Office of Dietary Supplements notes, “Adults with normal intestinal function have very little risk of iron overload from dietary sources of iron.”

Iron overload is generally defined as a total body iron load in excess of 5 grams. Acute intakes of amounts of 20 milligrams per kilogram can cause other, less dire problems such as vomiting and constipation. For a 68kg (150 pound) adult, that would require 1,360 mgs (1.36 grams) of iron.

A 3-ounce serving of Impossible Burger meatless meat contains 3 mgs of iron. That is around 15 percent of the daily recommended allowance of iron for adults. An eight-ounce Impossible Burger patty would thus have 8 mgs of iron. So, a 68-kg adult would need to eat 170 of those to get an acute dose of iron and 625 to achieve a 5-gram overload.

This post has been updated to include a response from PETA regarding Impossible Foods' blog post.