American 1,500 and 5,000 meter record holder Shelby Houlihan has been banned for four years following a positive test for what she concluded was a tainted pork burrito.

The Court of Arbitration for Sport confirmed that its panel of judges unanimously determined that Houlihan had failed to prove how the anabolic steroid nandrolone got into her system.

Houlihan posted her reaction on Instagram to getting the results of the drug test in December and the resulting ban.

"I have since learned that it has long been understood by WADA (World Anti-Doping Agency) that eating pork can lead to a false positive for nandrolone, since certain types of pigs produce it naturally in high amounts," she said.

She says she ate a burrito from "an authentic Mexican food truck that serves pig offal near my house" several hours before the drug test.

News of the ban comes days before the start of U.S. Olympic track and field trials in Eugene, Oregon, where the top three in each event earn a spot for the Tokyo Games.

She finished 11th at the 2016 Rio de Janeiro Olympics in the 5,000 meters.

"I have always done it the right way. I’ve put my head down and just worked at being better year after year. I’ve stayed patient and trusted that the work and consistency would show," she posted on social media.

"I still have all of the same dreams I had when I was 5 and I’ve been incredibly fortunate to have been able to have achieved some of them. I still have others that I’m working towards."