Houlihan's announcement on her Instagram account Monday arrives days before the start of US Olympic track and field trials in Eugene, Oregon, where the top three in each event earn a spot to the postponed Tokyo Games.

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

The 28-year-old Houlihan said she received an email from the Athletics Integrity Unit (AIU) on January 14, notifying her a drug testing sample returned a finding for the anabolic steroid nandrolone.

She said she's since learned 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. Pig organ meat (offal) has the highest levels of nandrolone."

Houlihan made a list of all the food she ate leading up to a December 15 test that produced the adverse analytical finding for the anabolic steroid.

"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," Houlihan wrote. "I notified the AIU that I believed this was the source."

An email and text were left with her agent.

Houlihan added "although my levels were consistent with those of subjects in studies who were tested 10 hours after eating this source and WADA technical guidelines require the lab to consider it when analysing nandrolone, the lab never accounted for this possibility.

They could have reported this as an atypical finding and followed up with further testing. The anti-doping experts I have reached out to say they should have. I did everything I could to prove my innocence."

Houlihan said she was informed last Friday the Court of Arbitration for Sport, "did not accept my explanation of what had occurred and has subsequently banned me from the sport for four years."

"I feel completely devastated, lost, broken, angry, confused and betrayed by the very sport that I've loved and poured myself into just to see how good I was," Houlihan wrote.

She set the American 1,500-metre record of 3 minutes, 54.99 seconds at the world championships in Doha, Qatar, on October 5, 2019.

Last July, Houlihan broke the US 5,000-metre mark with her time of 14:23.92 in Portland, Oregon.

"I want to be very clear. I have never taken any performance-enhancing substances. I'm not interested in cheating," Houlihan wrote.

Australian Associated Press