I can relate to your calorie counting obsession. Been there, done that. Before I give my suggestion, I want to congratulate you on your weight loss! Although a size 0 may mean that you're underweight now, considering your height. I'm 5'0" and weigh 105lbs, and I can't even fit in a size 0!!! A size 0 won't fit over my thighs and hips! I'm more like a 2-4.
I would suggest you throw your food journal out the window! Quit cold turkey. You are obsessed with consuming 1200 calories, which is NOT ENOUGH calories for your height and weight. Like I said, I'm 5'0" and 105lbs, and I eat about 1600-1700 calories a day to maintain my weight. You're 5'5"! You can easily go to 1600-1700 and maintain your weight perfectly fine.
You have to increase your calories NOW, or else your body will go into starvation mode, your metabolic rate will slow down so much that you are stuck eating 1200 cals worth of food to maintain your weight.
1200 calories a day for life is no way to live and enjoy life, and enjoy food. I LOST 10 or so pounds by consuming between 1300-1600 calories from food per day. I made sure those calories came from whole, unprocessed foods, lots of fresh fruit and veggies, whole grains, and nuts. Your dietician is right. You want to choose lean sources of protein, high-fiber foods like fruit and veggies, and good fats from nuts and avocados.
Like I said earlier, I was like you for a while, counting each and every calorie until it made me crazy! I quit cold turkey for a while and just started to listen to my body and make smart food choices. Now, I pretty much know the general calorie count in the foods that I eat. I don't eat like a 100 different foods, just maybe 30 or so different foods, possibly a new food every once in a while. The point is, now, I am able to quickly and generally count calories in my head or on paper to make sure I am in the ballpark range of 1500-1700 calories by the end of each day. If I go over that one day, I don't freak out--it's not going to make a huge difference.
So to answer your questions at the end. If you're worried about your pants not fitting one day, I reccommend that you weight yourself bi-monthly or monthly (if you weigh yourself daily, that can become an obsession). If you notice the numbers creeping up, then you know you have to exercise more and shave just a couple hundred calories off each day until your pants fit again! Then, consume the calories you need to maintain that new weight. Don't obsess, but be vigilant about maintaining a healthy weight. You can treat yourself to something once a week, or once every two weeks, or once a month, whatever you decide, ENJOY it! Don't worry that it's going to make you fat! It won't. Unless you have it every day and it's highly caloric! One treat every now and then will not make you fat and can even "trick" your metabolism into firing up (changing up calories every day does this, for example 1300 one day, then 1800 the next, then 1550 the next, etc). Yes and no as far as counting for the rest of your life: Yes, it's okay to get a ballpark figure on most days, but no, you can't obsessively count each and every single calorie every single day of your life unless you want to go crazy!
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