Rat Feeding Study Suggests the Impossible Burger May Not Be Safe to Eat

Rats fed the genetically modified yeast-derived protein soy leghemoglobin – the burger’s key ingredient – developed unexplained weight gain and signs of toxicity. Report by Claire Robinson and Michael Antoniou, PhD

At-a-glance

  • The Impossible Burger is a plant-based burger, the key ingredient of which is a protein called soy leghemoglobin (SLH), derived from genetically modified (GM) yeast
  • A rat feeding study commissioned by the manufacturer Impossible Foods found that rats fed SLH developed unexplained weight gain and changes in the blood that can indicate the onset of inflammation or kidney disease, as well as possible signs of anemia
  • Impossible Foods dismissed these statistically significant effects as “non-adverse” or as having “no toxicological relevance”
  • The company’s conclusion of safety is unsound, due to the short duration of the feeding study and the small number of animals used. Only a longer-term study with a larger number of animals can clarify the significance of the worrying effects seen
  • A nonprofit group is collecting data from people who believe they have had an adverse reaction to the burger.

The Impossible Burger is a plant-based burger, the key ingredient of which is a protein called soy leghemoglobin, derived from genetically modified (GM) yeast. The burger arrived in New York City’s restaurants with much fanfare – but now it is almost impossible to find, according to an article in the New York Post.1

Possible reasons put forward by the Post’s reporter include that the burger is expensive and can’t compete with cheaper options; that the company that makes it, Impossible Foods, is having manufacturing problems that mean it can’t keep up with demand; and that people don’t see any reason to buy it when plant-based veggie burgers with more everyday ingredients are commonly available.

But it’s also possible that NYC restaurant owners and their customers are becoming aware – and wary – of the GMO (genetically modified organism) status of the product and are choosing to avoid it. The results of a rat feeding study commissioned by Impossible Foods and carried out with soy leghemoglobin (SLH) suggest that they may have good reason.

SLH is the substance that gives the burger its meaty taste and makes it appear to bleed like meat when cut. The US Food and Drug Administration (FDA) initially refused to sign off on the safety of SLH when first approached by the company. The rat feeding study results suggest that the agency’s concerns were justified. Rats fed the genetically modified (GM) yeast-derived SLH developed unexplained weight gain and changes in the blood that can indicate the onset of inflammation or kidney disease, as well as possible signs of anemia.

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