US middle-distance runner Shelby Houlihan announced Tuesday that she had been suspended for four years after testing positive for anabolic steroid nandrolone, something she blamed on a pre-test pork burrito.

"I want to be very clear. I have never taken any performance enhancing substances," said Houlihan, the US record holder in the 1500m and 5,000m, adding that she had "never even heard" of nandrolone.

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"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. Pig organ meat (offal) has the highest levels of nandrolone," she said on social media.

"In the following five days after being notified, I put together a food log of everything that I consumed the week of that December 15th test.

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"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."

The AIU, track and field's independent anti-doping watchdog, notified Houlihan of the positive test in January, triggering a provisional suspension.

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The Switzerland-based Court of Arbitration for Sport (CAS) informed the 28-year-old on June 11 of the decision to uphold the suspension.

The four-year ban means Houlihan will now miss both the Tokyo Olympics this summer and those in Paris in 2024, as well as the 2022 world championships on home soil in Oregon.

"I'm going to continue fighting to prove my innocence," vowed Houlihan, who finished 11th in the 5000m at the 2016 Rio Olympics.

"I will not sit down and accept a four-year ban for something that I did not and would never do. I absolutely respect and whole-heartedly support the fight to catch athletes who disrespect the sport by cheating and doping. But I am not one of them."