American middle distance runner Shelby Houlihan has been banned from athletics for four years - and has blamed it on eating a burrito.

Te US record holder in the 1500m and 5,000m tested positive for a prohibited substance - with anabolic steroid nandrolone being found in her doping sample.

Houlihan says it may have come from eating a pork burrito the night before the test in January of this year, with pig organ meat containing high levels of the substance.

"I have since learned that it has long been understood by WADA 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 in a statement.

"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."

She's denied doping, and says she feels "devastated", "confused" and "betrayed" by the news that the Court of Arbitration for Sport has upheld the suspension.

Houlihan will now miss both the Tokyo Olympics and the 2024 Games in Paris.