Alec does have control – for a time. He increases his training beyond his already punishing schedule. He not only tests but redefines his own limits, seeing how far he can go, and on how little. He struggles to sleep, because a full-body pain hits at night and wakes him early, but even when it hurts, he enjoys it: he feels clean and safely removed from at least part of his anxiety. This is fortunate, because it turns out, when you stop eating, and give up most forms of social interaction to disguise this fact, you have a lot of time to think. Alec doesn't want to think, so he assigns himself extra patrols, runs unofficial reconnaissance trips, and, when he has nowhere else to go, simply runs, jogging mindless laps of the city until his legs give out. At times he feels especially depleted, and he debates utilizing a stamina or nourishment rune, but dismisses the idea. That would feel like cheating, somehow. It would remove the pain and fatigue that means he's accomplishing something, and, if anyone caught on, would invite awkward questions.
So he just… moves. Keeps going. Tries in every literal and figurative way to outpace what he can't articulate.
It's after one of these runs, perhaps four weeks after 'the switch', as he's come to think of it, that he starts to question how much control he has. He's staying at the loft, partly because he knows Magnus's client list that day will take him late into the evening, and he'll likely have the place to himself. He knows there'll be no-one there to question or scrutinize him, so he doesn't try to disguise his harsh breathing as he lets himself in and heads to the kitchen. He pours himself a glass of water and takes a large sip before he even thinks about it. The cold hits his stomach immediately, radiating to tighten his chest, and he shivers. He can practically hear the water slosh inside him, and he's appalled by his harsh breathing, his gulping, his desperate need. He looks at the glass in his hand.
It's big. Is it big? He's never really considered the volume, the density, of liquid before. He's thirsty enough to down the whole glass, but looking at it again, the thought seems obscene. It might ease his thirst, but what else? It would leave him queasy, waterlogged, bloated. The glass seems to grow bigger as he studies its dimensions, comparing the weight and volume of the liquid with the space inside him. His throat and chest constrict, his stomach cramping tighter as if in protest.
Alec scolds himself. This is ridiculous. It's water. There's literally nothing to it. You don't have to finish it all now. He shakes his head a little and brings the glass back to his lips.
But.
But.
Sick. Swollen. Undisciplined. If there's nothing to it, why does he need it?
He can't. He tells his hand to tilt the glass, tells his eyes to close, and his mouth to drink, but none of this happens. Mechanically, he empties the glass into the sink and cleans it out before putting it away. It looks better empty. Less daunting. His stomach gurgles loudly, the gulp of water settling, but it doesn't register as thirst, or hunger, or even relief, satiety, not then.
It just makes him uneasy.
Alec needs to leave, suddenly unable to stand being in this space. He decides to go and shower, taking time to brush his teeth beforehand. He wants to cleanse his body, because he can't cleanse his mind of warring emotions, his pride and fear and scorn.
He does drink, eventually, to prove (to who?) that he can, carrying a small water bottle around the Institute from which he takes slow, measured sips, at perfectly timed intervals over the rest of the day. He resents every part of the exercise, and it's with relief and no small amount of spite that he crumples the empty plastic into the recycling on his way back to Brooklyn. He reminds himself of the run, the fact that nothing else has passed his lips today. It reassures him. He wonders if it should.
That's the first time he questions his control.

The next time comes about a fortnight later. He's spent another night at the loft, because it was closest to his patrol route, and, to be completely honest, because he's tired. He's exhausted. Every muscle aches like he has the flu, and he simply sleeps better here, breathing in silk and burnt sugar and sandalwood: all things magic and Magnus. Part of him scorned his own weakness, and he feels guilty now for allowing himself this respite, this security and indulgence, but last night his fatigue was louder – so loud that when Magnus had come home, detained long past midnight by a new vampire client, he'd only stirred long enough to murmur some hopefully-sweet nothing and reach clumsily for Magnus's hand.
Now, he's woken too early again, eyes, head, limbs sore, and with no sense of having rested. He leaves Magnus sleeping and showers, toweling his hair dry and trying to limit the time spent in front of the mirror in which his reflection always seems alarmingly close and large, something leering and unwieldy. He's half dressed, choosing a shirt, when Magnus wakes.
'Early start?' he asks, frowning and blinking sleep from bleary cats eyes (it sometimes takes a few minutes after he wakes to restore the glamour, and Alec is secretly glad, always sorry to see the gold shine hidden).
'Yeah, I never filed my report from last night, and I'm leading weapon drills today. Figured I'd get on top of things.'
'Oh, but you look so good beneath them.' Magnus sighs, stretching.
'It's too early for you to start objectifying me.' Alec says, hiding his flush as he pulls on a t-shirt.
'No such thing – maybe I'm just getting a head-start, too.' Magnus says, unapologetic. Alec scrambles for a witty comeback (there's an innuendo there somewhere), but when he turns to look at Magnus he's met not with a smirk, or a mischievous grin, but a frown.
'You okay?' he asks, pushing down instant alarm, and Magnus studies him.
'Yes, it's…' he pauses and continues slowly. 'Are you losing weight?'
'Um. I don't know. Maybe? Am I?' Alec asks, frowning himself now. He's not being coy, or evasive (yet): he truly doesn't know. He doesn't feel smaller. It's getting difficult to pinpoint his shape, his size, when he catches his reflection, but it's hard to believe he looks different to anyone else. He's used to questioning the surface of things, to the distortion created by the glamour between the Mundane and Shadow worlds. He's even, to an extent, grown more discerning of magical illusions. But this discrepancy is… different. Can Magnus really see a change that he can't? If so, which of them is right? Perhaps neither of them. Aside from blissful, floating moments where the emptiness cuts too sharply to be denied, all he ever feels is heavy. It doesn't make sense that he'd be smaller, but he can't think of a reason Magus would lie to him.
That doesn't mean there isn't one.
One logic tells him that, naturally, rationally, reducing your intake or increasing your activity will, eventually, lead to weight loss. Intentional or not, it's biology. Energy in versus energy out. Shadowhunter's have their quirks and their gifts, but their bodies aren't exempt from most human rules. By that token, he shouldn't be surprised. This logic tells him that Magnus, with his keen eye for aesthetics, his proximity to Alec on a near daily basis (never close enough, always too close), his understanding of Alec's sense of honor, his love for Alec (this, Alec tells himself, is beyond question. To challenge what they share would be a form of sacrilege), would notice a change and be honest about it.
Normally, that would be enough – but now, there's something else.
Another logic screams that his bones, his muscles, are so leaden, his eyes so difficult to keep open, that he must be heavier, if anything. When every movement costs so much, requires so much effort, and his brain feels like wet cement, it only makes sense that he would physically weigh more. That his human half increases the effect of gravity. This part of him tells him that Magnus, above everything, is kind, and has no compunction about a white lie in the service of showmanship or protecting other's feelings.
Stop it, he tells himself, aghast. He can question and scrutinize himself, but to doubt Magnus? His boyfriend deserves much better than that.
'Let me rephrase.' Magnus says carefully, standing and padding towards Alec. He rests his hands on Alecs shoulder, thumbs tracing his clavicle. Alec tries not to flinch. He's been avoiding most contact, including physical, and it's starting to feel foreign. He doesn't want Magnus's hands on him right now, not because of anything to do with Magnus, but because Alec hasn't earned that reverence, that gentle touch. Magnus can be so unexpectedly tender, and Alec never thought he could have this, someone touching him like he's precious, and it still feels too good for him. So far, Magnus hasn't noticed (or at least hasn't said anything) but Alec doesn't want to hurt him with any perceived rejection. 'You've lost weight.' Magnus continues. 'It's noticeable. Have you been trying to?'
'No.' Alec says, and it's not a lie. He has no idea how much he weighs; can't remember the last time he would have needed to step on a scale. Childhood medicals, maybe? He hasn't been obsessively checking nutritional labels, or religiously tracking macronutrients. He's not sure how a measurement would sit in his head, how he would interpret or evaluate an arbitrary numerical value. He's never even had to pay much attention to his clothing size: he's allocated new gear when his current supply is damaged or starts looking worn, but the size hasn't altered in years. Izzy does most of the family's personal shopping, having far more interest in their appearances than they do themselves. Trying to lose weight?
Whatever the purpose of his new regimen, it's not that. Robert's voice in his head scoffs: he isn't some Mundie celebrity or insecure tween. Deliberately losing weight is a vanity project, surely – there's far more important things that require their attention.
Still… 'Losing', in this context, doesn't sound like a bad thing. He turns the idea over. Being leaner, sharper, less intrusive somehow. Perhaps there'd be less capacity for his anxiety, his grey moods, to escalate. That seems sound enough. Neat, and logical. That seems… perfect. Everything contained and manageable.
Maybe he should start thinking more deliberately.
His boyfriend calls him back to himself.
'No? Then… Have you been sick? Are you getting sick?' Magnus asks, laying the back of his hand on Alec's brow as though to check for a fever. He's hamming it up, half in jest, but he looks worried, so Alec's gentle as he pushes his hand away.
'No. I'm fine. I've just been busy I guess.' He says as neutrally as possible, as if it means nothing. And really, it is nothing. There's no objective value attached to size or shape. Growing up with Jace meant he had to unlearn his teenage insecurities quickly, and besides, they have responsibilities. There's no merit or purpose in such self-consciousness and scrutiny. So he continues breezily. 'I'll probably put it all back on and then some the next time we have Madzie stay over – I know the two of you have already cooked up some sugary scheme. It'll be death by ice cream, or cake.'
'I confirm and deny nothing.' Magnus says, finally smiling. He squeezes Alec's hand before making his way to the mirror where he fusses with his hair. 'Just take care of yourself. I, for one, think you are wonderful, and I want you to be well, okay?'
'Okay.' Alec says, voice suddenly choked. Sick. Well. When was the last time he felt whole? What did a 'well' version of himself look like? It felt like another impossible standard to live up to considering all the wrongness inside him. How is he ever meant to purge this? He ducks into the closet for his shoes, grateful to hide his face.
'On that note… Dinner tonight?' Magnus asks. 'Or are you staying at the Institute?'
Alec opens his mouth to make his excuses, the 'no' engrained and automatic (he's noticed this lately, that he's forgetting how to say yes), but there's a hopeful expression on Magnus's face, and after the conversation they've just had, he doesn't want to worry Magnus unnecessarily.
And it would be unnecessary. Alec is fine. He'll prove it.
'Dinner sounds great.' Alec says as brightly as he can. 'I'll see you tonight.' He declines the offer of a portal and presses a kiss to Magnus's cheek on his way out.
He can't stop wondering if his steps are lighter, or if his clothes hang any looser. Has anyone else detected a change? Is everyone evaluating him, holding him under a microscope? If so, what do they suspect? How do they explain him now? How did they see him before? He's visited by a creeping claustrophobia, his skin crawling and tight, so he tries to turn his attention elsewhere. The morning air is biting. When he looks up, he's blinded, even though it's overcast. The sun is a silver coin behind the clouds, a thumbprint on a fogged-over mirror.
A spotlight.
Dinner. Conversation. Even an evening with his boyfriend has become a threat. What choices will he be confronted with? It will be a tense balancing act, making silent adjustments and finding a compromise between pleasing Magnus and following his routines. Will reneging on the whole thing just come back to (hah) bite him?
There's no way to win.
There's no right choice.
Something else out of his control.
He wants to scream, or strike out, but that would be childish. Composure is key. He could hideout and curl in on himself, pulling further and further inside until nothing can touch him, but that's no good, either. He would not escape himself.
He picks up his pace. He breathes. He works, and later that day…
He eats fucking dinner.
The world does not end, at least not for anyone else, but Alec implodes. He's distracted, anxious, overwhelmed. It doesn't matter that dinner was the single time he'd eaten that day, the first full, hot meal he'd seen in weeks, that he'd left it (politely, not quite suspiciously) unfinished. All that matters is that he failed. He failed Magnus. He failed himself. He broke the rules, and, in the end, they were both disappointed anyway. He broke the rules for nothing. And something has been broken, he knows that much. Alec can't explain why sitting down to a meal is no longer allowed, but it isn't. Nor does he know precisely where these rules are coming from, but that's beside the point right now. He broke the only one that matters. And worse, he broke it of his own volition.
Do. Not. Eat.
It's the first time he's consciously spelt it out, the first time he has fully confronted what he's doing. He does not know how to feel, can't follow any particular train of thought, but he knows he wants those words branded or tattooed on his skin. He wants to find better ways to avoid the turmoil of compromise and deception.
To win, he must lose. To lose, he must win.
He hates himself and Magnus both.
That night, when he is kept awake by the concrete certainty that he is being swallowed, buried, by his skin, when he wants to split himself apart until he can breathe again, and the only thing stopping him from waking Magnus is the fear of his own anger and despair, is the second time he questions his control.

Alec didn't let himself question anything after that. The only thing that made each day endurable, the only thing that worked, was restricting his intake. He'd tried the alternative, and it had been awful. That was all the convincing he needed to resume and tighten his regime. It was clear, and simple, and came down to structure, to consistency. To following rules, as he'd always done.
No wonder it felt so much better.
At least these were his rules. Alec didn't think he was good for much, but he was good at this. He'd always had a reputation for honesty, so people weren't inclined to question his excuses and evasions too far. He spent most of his time working, or alone, and through sheer stubbornness subsisted on as little as possible for as long as possible. When he kept that hollowness inside, stoked the bright, hot, emptiness, and felt the grounding stab of torn muscles, nothing else could phase him. Nothing could even reach him.
He relished the sense of detachment, but it made things difficult sometimes. When he had to ask people to repeat themselves, or letters refused to resolve into words, he wondered if he was literally starving himself stupid – but it wasn't as if it was all the time, and he couldn't find it in him to worry. As far as he was concerned, he was onto a good thing. He was in the zone. One of the only things that disturbed him were his dreams.
He only ever dreamed about food. Buying it. Hiding it. Preparing and arranging it. Counting and measuring portions. Spoiling it. Worst of all, eating it. He would wake up convinced he could feel the weight of it in his stomach, or feel the residue of it on his hands, in his teeth, unable to be certain of what he had consumed. He didn't trust his body not to sabotage him. He didn't trust anything, or anyone. He dreamed he was forcing vegetables down the bathroom sink, and was skeptical of its pristine sheen when he eventually checked it. Once, he dreamed that Izzy had cooked for him, that they'd sat at a table together and passed bizarre combinations of dishes back and forth, laughing. He had to avoid her for two days before the sting of the imagined betrayal began to fade.
But he still didn't question anything. If he ever had doubts, he knew the way to alleviate them.
Work harder.
There was always, always, scope to do better.
Even he could see a difference in himself now, and that didn't exactly make up for the lies, the secrecy, but it was something tangible to hold onto.
The trouble was, it was not only him that saw the difference. He did feel bad about the secrecy, sometimes. Izzy wasn't the only one he avoided. Magnus clearly didn't understand why Alec was reluctant to spend the night, to commit to most plans, and why he often shrank from physical affection as he had in the early days of their relationship, when he was still learning how to be loved. What was more, after their first conversation about Alec's weight, Magnus had grown vigilant. He studied Alec much more closely, and though he was tactful about what he said, he looked pained or even angered by every measured portion, every declined invitation. If Alec spent any length of time at the loft, they were both on edge, making uneasy compromises without ever saying anything directly, and watching each other with a suspicion that had never been there before. Alec hated it. He didn't want to hurt Magnus, didn't want something so… ugly, and embarrassing to come between him and the man he loved.
But he couldn't stop.
He didn't want to stop.
And it wasn't as though anyone else was any easier to be around. If he slipped and let Jace sense anything through their bond, or flagged during training, he could be sure that the next time he saw his parabatai, he'd be met with a scowl and a snack: fruit, or a granola or candy bar. Once a homemade shake, as if that would be easier. As if that was an option. Alec had shaken the glass, disturbed the unquantifiable liquid, and nearly rolled his eyes. 'Eat' was implied, but never said. Neither was the 'no' when Alec took whatever was offered and waited for the chance to press the gift on Max, return the food to the kitchen, or, more often, just throw it out (at least then he could be sure it was gone). Izzy knew something. She'd stopped him on his way out to patrol once, saying: 'I don't what's going on, but I haven't seen that look on your face since we were kids. Come find me later if you want to talk, okay?'. He hadn't, but the uncharacteristic hesitance in her words was enough to shame him into sitting down to two square meals the next day, even if he only managed a few mouthfuls at each. Looking at the food, being near it, had been challenge enough.
It was best, safest, to be alone.
A season passed this way, the last of winter giving way, reluctantly, to a spring only a fraction milder. Three months, maybe four.
It was not Alec's limit, but it was Magnus's.