Have I ruined my metabolism? If so, how would I get it back?

You are massively under-eating. What happens when you eat at a caloric deficit is that the glycogen stores in your muscles get depleted, along with the water that is stored with it. When you subsequently eat a larger amount of calories, your muscles are basically like OMG AWESOME and the glycogen stores, along with the water, get replenished. That's why you see a jump in weight after eating more calories - you did not gain several pounds of fat overnight, you gained several pounds of water. Water is the most volatile substance in the body and you can see a gain or drop of several pounds simply from eating a salty meal or sweating a bunch at the gym. So basically, no, you did not wreck your metabolism. However, for a male of your age and height, you are eating far too little. Eating too little can result in loss of muscle mass along with loss of fat, which is not what you want to have happen! I have found http://www.younglifenorthdekalb.com/fitness/bmr_meal_calculator.php?age=28&sex=M&height=72&weight=243 to be very helpful in determining how much to eat. (Semi-NSFW, shirtless dude image.) According to that calculator, to lose 1.5 lbs/week you should be eating 2310 calories per day. You're sometimes eating less than half that amount. That's not sustainable for the long term because your body will either compensate by making you have less energy and therefore move less and expend fewer calories, or you will crack from hunger and fall off the wagon. My advice to you is to increase your calories (do it slowly if that is easier for you mentally) and keep up with your exercise. Make sure you are eating enough protein and don't avoid fat. You have made great progress and you can reach your goals without starving yourself.

KathrynT: The weight does go away after a few days, but that does coincide with increased exercise. I guess something I did not take into account is what I may have broken isn't my metabolism but rather my otherwise rational adult brain becoming obsessed with making a number smaller when I've already done so, and very well at that.

Am I reading you right -- you mark a several-pound weight gain within a DAY of when you eat more or eat a lot of carbs? Because if you do, that weight is either in your GI tract rather than your body, or else it's water. Several pounds of fat, let's say three pounds, is almost ten thousand calories' worth -- you're not getting that just by eating pizza, even if you're eating a WHOLE pizza. When this happens, how long does it take you to lose the extra weight?

Breaking my habit of weighing myself every day (and several times throughout day, if my anxiety is flaring up) seems like the best start. This is a self-perpetuating cycle: the more you weigh yourself, the more anxious you will ultimately feel. I am going to be honest, we ANYD but I think if you had said you were female more people in this thread would be concerned about the possibility of an eating disorder, or at least more alarmed about your disordered eating (they are different but related things). I think you should involve a doctor or dietician and probably also a therapist, not because of your weight specifically but more because of the anxiety that you've described here--it is not healthy to be so obsessed with your body's changes that you are constantly monitoring it throughout the day. The internet, being what it is, scared me with horror stories about people with eating disorders who would gain all the weight back and then some while still eating under 2000 calories. Sure, because starvation makes your body hoard. Over time those people's appetites and weights stabilize, but you don't hear horror stories about that part for the same reason you don't hear too many happy stories on CNN. Try not to obsess about the small details--focus on functional capacities (strength, endurance, flexibility) and the long-term trends (not days, but months).

"and on days I have ~2000 calories and/or a lot of non-fiber carbs I tend to gain several pounds." So, gaining // losing a pound requires a surplus // deficit of ~3500 kCal. If you're noticing a bounce of multiple pounds on days you're eating, you're either: A) Being obscenely dishonest about your caloric intake (not likely), or B) Seeing normal fluctuations, that can be caused by a myriad of factors Learn to take a long view of this. Don't worry about the day-to-day, take an average of your weight for a week and use that as your basis point. Learn what your body requires as far as caloric intake on a day-to-day basis for zero gain // loss, and use that to shape your behaviour regarding diet. A BMR calculator may help on this. Don't sweat it too much :D You've made progress. Keep it going!

It occurs to me that if large percentages of the population really could break their metabolisms through variable but basically healthy/non-processed diets, we'd have some evidence from history - various nomad/gatherer populations whose diet varies wildly over the course of the year, for instance; people who have been in non-dangerous food-shortage situations (such as people in Western Europe during and just after WWII); people who do long distance hiking and walking... It seems totally reasonable that some people with particular metabolic problems (or with a particular cascade of physical and mental health problems leading to hormonal issues) could get into an awful cycle of weight gain on really restricted diets. But it seems unlikely that this is a common situation for average humans, particularly when eating basically healthy but just less.

Okay, money is money, I feel you there. I'd start adding in more dairy calories in 100 calorie increments. http://www.npr.org/blogs/thesalt/2014/02/12/275376259/the-full-fat-paradox-whole-milk-may-keep-us-lean So add 100 calories to your daily total, and keep it up for a week, and see how it goes. Then add more. Another thing to do is the http://en.wikipedia.org/wiki/The_Shangri-La_Diet. It was featured on the http://freakonomics.com/tag/shangri-la-diet/ blog. I've been doing it and it does seem to allow for weight loss by resetting your "set point." I bought the book, but like most diet books, the helpful nuggets could be printed on a post card. Basically, between meals, with an hour's buffer on each end, drink 100 calories of no-flavor oil, or some sugar water. Don't associate any flavors with these calories. I'm about 3 weeks into it and it helps with hunger and both Husbunny and I are eating less, but it is strange as FUCK! You might also join Weight Watchers, either the on-line app, or actually go to meetings. Also, don't get too wrapped up in the pounds. If you're going to the gym, muscles weigh more than fat, and if you want to bulk up muscle, you might gain in the moment, but you'll be losing more fat overall. Good luck!

One thing that is happening when you have more calories/carbs - you're retaining water. It is unlikely that you are gaining several pounds from a day or two of a few hundred calories over normal. For me, if I eat a bunch of wheat or salt (say, if I have pizza)...I get all puffy. Not so much noticeable visually, but enough to change my weight a bit. I did almost exactly what you did in my late teens except with more severe (really stupid severe) calorie restrictions. This is all anecdata, but I did not break my metabolism. (I kept the weight off through obsession and misery for about seven years and then regained most of it.) What really sank me in the long run was not addressing why I ate badly in the tweens/teens - loneliness, isolation, lack of control of anything else in my life, no other sources of pleasure. It wasn't my metabolism - it was my braaaaaaiiiiinnnn. What I did right was building good exercise habits, something that has stayed with me ever since. Even now that I'm fatter again, I look different because I'm very active and have actual muscles now. If I were you, I'd add some more exercise. In my own experience, any kind of exercise thins me out if I am consistently pushing myself - walking, as long as I walk enough to tire myself; biking, as long as I feel it in my calves a bit; etc. Everyone always says that only weight training really helps, but I haven't found that to be true. Exercise until you start to feel like you want to stop and then continue a little longer; add your calories as proteins. (For me, it's "anything but wheat" - I can have, say, extra chocolate and not retain water, but anything wheat-ish isn't so great.) I think that contemporary language about diet suffers from too much precision - you must do this or you'll break your metabolism, you must not do that or you'll break your metabolism...when really, throughout human history people have lived on a variety of non-ideal (but not actually starvation) diets and come back from them just fine.

The internet, being what it is, scared me with horror stories about people with eating disorders who would gain all the weight back and then some while still eating under 2000 calories Well, maybe but that's not where you are right now. From your description, you're still losing weight, still feeling good, still eating exactly what you intended to eat (no unhealthy urges to binge, or to starve yourself). The shift that you're thinking about making is from eating a very restrictive diet (1200 calories) for rapid weight loss (40 pounds in 3 months is amazing, as far as I'm concerned!!) shifting over to a slightly restricted diet (1600 calories) for probable continued weight loss, and eventually you'll shift over to a weight-maintenance eating plan. It sounds like you're really good at consistency, eating about the same amount every day, exercising regularly and similarly, etc. and the thing you've been doing for 3 months has worked really well for you, so changing anything is understandably a bit nerve-wracking. Hang in there, make you decisions and follow through with confidence, and don't panic till you've seen longish-term (at least a whole week) trends developing. When you're losing 12 pounds a month, I can see how you'd worry that going 2 days in a row without losing anything means you've got a problem. And yes, instantaneous weight fluctuates several pounds during the average week, but when you're aiming for a significant downward slope, you can reasonably expect to see that slope through the noise. Remember - your goal has changed! You don't actually want to lose more than, say 1.5 pounds a week at this point. That means that maybe you won't see the slope through the noise. That's okay. I can see how you would be worried that you're cultivating borderline-disordered eating behavior, because of your weigh-in anxiety, but I like how aware you are of the way that you're responding emotionally to the situation, and how the scale makes you feel anxious. Keep that awareness, and see if you can talk yourself through the anxiety. I actually kind of disagree with the recommendations to only weigh yourself once a week. You run the risk of having an "up" day right when you do your weekly measurement, and freaking out because you've only lost 0.5lbs this week. If you weigh yourself every day, record the number with no emotion, and convince yourself that you're not allowed to care about anything other than your 5-day running average, then you'll see the trend that you're looking for, in a realistic (and hopefully calming) way. Except you've got 7 times the number of weigh-ins at which to freak out. You know your own mind, and how you feel about this stuff, so make the choice that you think will work for you.