Is Captain Kirk a Stress Eater? and Other Considerations

Played by the bow-slinging, motorcycle-riding Shatner, it's hard to imagine Kirk as anything other than vital. And no matter how much he gets pummeled, or how worried he looks, he ultimately comes across as a person who sleeps well.

I don't dispute Kirk's essential vitality. But I think it's also important to remember: Kirk suffers from anxiety-induced  nausea, headaches, and sleeplessness.
And why shouldn't he, with all his job entails?

Here are some examples:

Stomach Aches

In "The Man Trap", Kirk delivers this memorably film noir-ish line:
"I don't like mysteries. They give me a bellyache, and I got a beauty right now."

Considering the almost constantly freakish adventures of the Enterprise, Kirk must feel queasy often, like the naval hero he's based on, Horatio Hornblower. Of course, it's not mysteries themselves that give Kirk bellyaches, but the anxiety these mysteries cause him.

My guess is that McCoy has a pill for this when it's severe. 

Headaches

If mysteries give Kirk stomach aches, diplomatic tangles give him headaches. In "The Trouble with Tribbles", Kirk does something unheard of for him, and visits sickbay voluntarily, without any prompting. He does so for a headache. On several other occasions throughout TOS, he's seen rubbing his head in tense situations. 

In "Tribbles", McCoy administers a pill for this.

Sleeplessness

Kirk also suffers from sleeplessness, and, you guessed it: McCoy has a pill for that too.
But even in the 23rd century, they haven't managed to make a pill that puts you to sleep without knocking you out.
"Try taking one of those red pills you gave me last week," Kirk tells McCoy sharply, "You'll sleep." ("The Corbomite Maneuver")

Given this pill's apparently unpleasant side-affects, Kirk probably only takes them when tossing and turning doesn't do the trick.

Muscle Kinks

Other aches and pains? How about "a kink in my back"?
This scene has become so famous among Kirk/Spock shippers for its sexual overtones, I worry the implications of Kirk's muscle knot is lost. Between sitting in his straight-backed captain's chair on slow days and throwing punches on fast days, the source of his back kink is no mystery. And if he has back trouble at the beginning of "Shore Leave", he's in for a real treat by the end of the episode.

But another cause of muscle tension is stress, which he also gets his fair share of.

Stress Eating

Kirk's weight visibly fluctuates, especially towards the end of the 5 year mission. In "The Corbomite Manuever", McCoy orders a "diet card change" for Kirk because he's "put on a couple of pounds".

This indicates that everyone aboard the Enterprise has a diet card for their specific nutritional needs. I imagine a diet card allows for variety and the pleasure of choice (Rand offers to bring Kirk something else when he fusses about his "green leaves"), but ultimately makes it difficult to become overweight, especially when combined with a demanding job.

We see Kirk eating on the bridge. A colored cube here and there is likely what sustains him on the most chaotic days. As Rand brings him his meal in "The Corbomite Manuever", she says, "It's past time you had something to eat, sir." No doubt it is.

So how does Kirk gain weight?
In his off-duty time. I bet he likes caramel milkshakes, roast chicken, and wedge fries.

In "The Wrath of Khan", Kirk says he's starving when they're in the underground lab, shortly after a fellow officer has committed suicide, his own son has tried to kill him, and Genesis has been stolen by Khan. "How can you think of food at a time like this?" McCoy says.
Kirk's appetite isn't inhibited by stress, and is perhaps even increased by it.

There's a fic that posits Kirk's eating "quirks" are a result of living through a famine on Tarsus IV, and that his habits intensify after the incidents of "The Conscience of the King".