Avocado oil is booming. Most of it is rancid.

Scientists analyzed 22 samples of commercially available avocado oil: 15 had gone rancid before their best-by date, while six were likely cut with large quantities of other, cheaper oils.

The waxing popularity of the avocado in the past decade—a boom driven by the appeal of its health benefits, as touted by highly effective industry marketing efforts—has led American eaters to smear the buttery fruit onto every surface of their lives. Avocados can be found on our faces and in our hair, in our ice cream and on our toast. (Some would say at the cost of our access to financial stability.)

Avocado oil, too, has grown in popularity, as a “heart healthy” alternative to other cooking oils, and an increasingly common substitute for butter in baked goods and for vegetable oils in mayonnaise and margarine. Packaged products featuring avocado oil saw more than a 30 percent growth in sales between 2016 and 2018, according to market research firm Nielsen.

If something smells off about avocado oil’s success, it could be that some products sold on grocery store shelves are, in fact, rancid. In a new study conducted at the University of California, Davis, researchers tested 22 commercially available samples of extra virgin, virgin, and refined avocado oil for quality and purity. The authors also extracted their own avocado oil as a baseline for the pure product. They discovered that 82 percent had gone rancid before their expiration date, or had other, cheaper oils mixed into the bottle. In three instances, bottles labeled as avocado oil actually contained a different oil entirely.