Was Republican candidate Donald Trump drowning his sorrows in a bottle of Diet Coke?

Donald Trump's repeatedly sounded off about how he believes diet soft drinks make people fat.

But he apparently doesn't practise what he preaches - accidentally outing himself as a secret sipper, after doing a poor job of hiding his Diet Coke while having his photo taken.

The Republican nominee for the US Presidency shared a campaign photo on Facebook on Tuesday, showing him aboard a private jet, with an upbeat message: "On my way to New Hampshire - expecting a big and spirited crowd!"

Eagle-eyed Trump-watchers have spotted what appears to be a small bottle of Diet Coke under his desk, glinting in the light - apparently intended to be tucked away out of the photographer's shot.

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Nothing out of the ordinary?

Not quite.

The keen tweeter has repeatedly railed against Diet Coke, with posts online about how sugar-free soft drinks make people gain weight - including a series of bizarre tweets and Facebook posts over a week-long period.

It began with one post, shared to both social media sites on October 14, 2012: "I have never seen a thin person drinking Diet Coke".

The following day, another tweet appeared: "The more Diet Coke, Diet Pepsi, etc you drink, the more weight you gain?"

A week later, he continued.

"People are going crazy with my comments on Diet Coke (soda). Let's face it--this stuff just doesn't work. It makes you hungry," he tweeted on October 22.

It now appears his message was "do as I say, not as I do".

Perhaps the original tweets were a commentary on his own battle against a sweet tooth?

Or maybe his plane-ride photo caught him drowning his sorrows over his loss to Ted Cruz in the Iowa caucus, just hours earlier.

The billionaire property tycoon hasn't yet explained his secret indulgence - but a rambling series of tweets is anticipated in the near future.

While he may not be quite the average American, the photo goes to show that when it comes to hiding evidence of secret junk food consumption, The Donald is just like the rest of us.