The Impossible Burger is a plant-based burger, the key ingredient of which is a protein called soy leghemoglobin (SLH for short), derived from genetically modified (GM) yeast. It’s already being sold in restaurants and supermarkets in the US. In 2019 the manufacturing company, Impossible Foods, applied for permission to market the burger in the EU and the UK.

However, the results of a rat feeding study commissioned by Impossible Foods and carried out with SLH suggest that the burger may not be safe to eat.

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 GM yeast-derived SLH developed unexplained changes in weight gain, changes in the blood that can indicate the onset of inflammation or kidney disease, and possible signs of anaemia.