John likes to eat real meals, Sherlock likes to eat snacks or not eat at all. This used to not really be an issue. When they first met, the first night they were colleagues, Sherlock didn't eat. He didn't eat on the next case, either, or the one after. He wouldn't eat for days, up to a week, at a time, and at first John wasn't bothered.
Nor did he want to push a grown man to eat when he didn't want to until they were at that level.
Sex came before that level.
No, Sherlock wouldn't often eat. But then he would get to the point that hunger was unbearable and he'd have to, against his own wishes. He would lay around lazily for a while, not like that was different, until John came in the room. Then, he'd whine.
"Jooooooooooooooohn."
"Yes, Sherlock?"
"I'm hungry."
"What would you like, my darling?" John's voice was almost mocking.
"One of those little cakes your sister brought from America, the chocolate one."
"You ate them all, Sherlock."
"No, no. I saved a few."
"Where did you put them?"
"I don't know."
John sighed.
You know when there's someone you care about more than yourself, and you'd do anything to keep that person happy and healthy, even if that meant destroying your entire flat just to find a measly Little Debby Snack Cake from America, all because this beloved hardly ate, so when he did eat, you let him have whatever he wanted? No? Well, this is how John felt. His heart jumped when Sherlock said he was hungry. This was one thing John could do for Sherlock that would make Sherlock so very happy. Sherlock could have asked John to hop on a plane to America to get more cakes and he would.
An hour later he found the secret stash of cakes, all kinds of cakes and cookies and candies that were hidden in a compartment in the book shelf.
"Geez, Sherlock. What else have you got hidden in this flat?"
"Nevermind." Sherlock's tone was distant and distracted. He took the bag of snacks and wrapped himself up on the couch.
Now, Sherlock didn't throw up often. Actually, the last time he can remember throwing up was on a plane back from Switzerland six summers ago. He didn't really like motion, plane and car and train rides often made him ill. But this moment was worse than throwing up on the plane, at this moment he felt like he was going to die and there was nothing he could do to stop it.
"How many did you eat, Sherlock."
Silence.
"Sherlock?"
"A lot, ok? I don't know."
John looked at the bag that was once full of snacks and was now down 50%.
"Jesus, Sherlock. I could have made you something healthy and less…lethal."
"Shut up, John. I was hungry. Now I don't think I'll ever eat again, problem solved."
Sherlock threw up again. When he finished, John pulled him close and laid Sherlock's head on his own chest.
"Do you feel better?"
"A little."
"Would you like some tea?"
"No."
"Would you like anything particular?"
"Toothpaste."
John chuckled and handed Sherlock his toothbrush.
John remembered that this wasn't the first time Sherlock did anything like this. Sherlock actually did it often, mainly because he hated healthy food of any sort. No, no. He liked fruit, when they weren't sticky. So, he liked bananas and sometimes grapes and sometimes carefully eaten blueberries. And canned fruit, which John thought was rubbish. And he liked yogurt, but only plain yogurt with a little bit of flavoring. He hated the chunks. And he liked jam when he got to eat it off John's fingers. But he hated bread, meat, cheese, peanut butter, and spaghetti. So, really, a lot of the time Sherlock ate junk until he made himself sick to the stomach. But he never threw up from it.
John didn't like Sherlock's eating habits, but he knew Sherlock would have to fix this on his own, John couldn't push him into or out of anything. John left the bag of junk on the table, secretly and shamefully hoping that Sherlock would 'overdose' himself again into sickness.
Four days later, John returned home from work to find Sherlock asleep on the bathroom floor. Sherlock looked pale and very flushed, more than usual. His hair was slightly sticking to his head, probably from sweat.
John poked Sherlock's shoulder, "Sherlock, sweetie, wake up."
He didn't.
"Sherlock, come on." John shook him lightly.
"No, no. Let me die here."
"What happened?"
Sherlock opened his eyes and looked at John. He looked so pathetic, John couldn't help but laugh.
"It's not funny, John! I ate the rest of the bag, ok? And I threw it all back up."
John laughed again.
"Stop it!" Sherlock exclaimed and stormed out of the bathroom, huffing and puffing and stomping and shaking all the way to his bed.
"Come on, Sherlock. It's funny. You're like a child." John laid next to Sherlock and wrapped his arms around him, lightly stroking his stomach.
"How am I like a child?"
"They just want the taste, they don't look for consequences."
"Well, I never want the taste again. Ever."
"You're going to start eating healthy?"
"Or I won't eat at all."
"No, no. You're a grown man. You can eat healthy."
Silence.
More silence.
"Fine," Sherlock pouted, "As long as I never have to feel this way again."
John smiled.
He succeeded.