When will we learn not to give credence to celebrities — movie stars, pro athletes, presidential candidates — when they start talking about subjects outside of whatever it is they do that made them famous?

Generally speaking, they need to stay away from making pronouncements on such issues as foreign policy, the tax code, or in this case, the science involved in livestock production.

Because when they do, they sound like idiots.

Ladies and gentlemen of the jury, allow me to introduce Exhibit A: An MMA fighter by the name of Frank Mir.

For those who don’t follow the cult-like sport of mixed martial arts, it’s basically what you get when you cross-breed boxing, wrestling, karate and the kind of street fighting in which smashing your elbow into a guy’s face while he’s lying on his back is considered to be a “strategic” move.

Don’t get me wrong: I’ll watch it like anyone else, just to experience vicariously the human destruction that typically accompanies most matches — especially when it involves heavyweights like Mir, a Las Vegas-born, 240-pound beast who’s beaten some of the best MMA fighters in his career.

Until he got knocked out in the first round by one Mark “Super Samoan” Hunt at what was billed as UFC Fight Night 85 in Brisbane, Australia, back in March.

It’s bad enough to lose just minutes into a fight, but in the aftermath of his defeat, Mir failed a drug test administered by the U.S. Anti-Doping Agency (USADA), the same folks who spent years investigating and finally proving that Lance Armstrong was involved in an elaborate blood-doping scheme.

Of course, suspicion about using performance-enhancing drugs usually swirls around the superstars of whatever sport is under scrutiny, but in this case, the 36-year-old Mir was previously linked to legal testosterone-replacement therapy, and truth is, when athletes are on the downside of their careers, the temptation to use banned substances to boost their training sometimes becomes overwhelming.

One Barry Lamar Bonds comes to mind.

A statement gone awry

Observers at Fight Night 85 noted that Mir didn’t look like he was “hocked up on performance enhancers,” as the colorful Aussie website “The Bloody Elbow” phrased it. But there’s no disputing the failed drug test administered by USADA — unless you can find a convenient scapegoat to take the blame.

In his case, Mir pointed to kangaroos.

Seriously? Yeah, seriously.

You’re fighting Down Under. Hey, why not implicate Australia’s most iconic species?

In a recent online post, Mir attempted to clear his name with the following statement:

“I have recently been notified by USADA that the test I took on the night of the fight came back positive for a substance that I did not take. I don’t know how it is possible, as I did not take any performance-enhancing drugs to compete. I ask you to hold judgment against me until all the facts have been revealed.”

In legal circles, taking the witness stand to claim “I didn’t do it!” doesn’t really qualify as a defense.

But it gets worse.

Mir went on to finger kangaroo meat as a potential suspect, and this is where his explanation, as told to The Bloody Elbow, ends up in the same place he did: down and out.

“Well, you know, kangaroos are wild, and [suppose] this guy wanted to beef ’em up. So he bought something that was very abundant, oral turinabol, [which] could be bought in powdered form. You put it on the food and you bulk up your livestock, and you could sell it into the stores because now you get more bang for your buck.

“An animal that takes two years to reach maturity, you know, muscle weight, now in four months he’s bigger than he’s ever gonna be and you slaughter him. It’s a common practice.”

Like I said, they sound like idiots.

What’s most disturbing isn’t Mir’s ridiculous attempt to blame kangaroo meat for his failed drug test — nobody’s “beefing up” kangaroos; they’re not raised commercially — but rather the apparent belief he has that dumping steroids into livestock feed is “a common practice.”

Or that he believes his equally clueless fan base might actually buy his “somebody drugged my kangaroo steak” defense.

In either case, he needs to keep his mouth shut.

And not just when he’s taking a punch in the face, but more importantly, when he’s not inside the ocatagon. 

Dan Murphy is a food-industry journalist and commentator