American middle-distance runner Shelby Houlihan has blamed a burrito after being banned for four years following a positive drugs test.

The United States' record holder in the women's 1500m and 5,000m was told in January that the anabolic steroid nandrolone had been found in a doping sample.

A hearing at the Court of Arbitration for Sport did not find in her favour and the 28-year-old – who has denied ever doping – believes the positive test could have come from eating a contaminated pork burrito the night before her test.

"We concluded that the most likely explanation was a burrito purchased and consumed approximately 10 hours before that drug test from an authentic Mexican food truck that serves pig offal near my house in Beaverton, Oregon.

"I notified the AIU (Athletics Integrity Unit) that I believed this was the source.

"I did everything I could to prove my innocence. I passed a polygraph test. I had my hair sampled by one of the world's foremost toxicologists.

"Wada agreed that test proved that there was no build-up of this substance in my body, which there would have been if I were taking it regularly."