Is undereating going to backfire on me?

There is no known disease associated with destroying your metabolism when eating a low calorie diet but there is something associated with eating more. It's called Diabetes type 2, heart disease, and many more. If you're not hungry, not tired, and you are exercising then there is no reason you should force your body to eat.

The only way this will backfire on you is if you constantly overfeed your body, as in going back to your bad eating habits.

I am pretty muscular, and while I will lose some muscle it will be mostly fat. I was around 20% bodyfat when I started. I'm fine with losing muscle. I was and am "fit" (I run and lift weights) I'm just trying to live life at a thinner size.

Muscle loss can be prevented by adding some resistance or strength training exercises to your routine. You should see really fast results if you include some type of resistance training, even if it's not much.

Edit: Take a daily potassium pill if you are going to stick to 800 calories.

I honestly do not understand why you want to atrophy your lean body mass. I turn into a sad panda when I hear athletic women desire that for themselves. I did that to myself a few years ago when I participated in a calorie restrictive experiment with 1000 calories of exercise doing 85% VO2 max and I lost about 9 lbs of lean body mass in two months. It's your decision, but you could just focus on body composition and reduce your body fat to get leaner.

I don't know how much total exercise you do a day, but if your net is too low for 5 consecutive days, you could experience amenorrhea due to disruption in LH pulsatility and ovarian function. I've run your numbers and a net below this number will expose you to such disruption:

15 cal x 59.1 lean body mass (kg) = 886 calories of daily exercise

30 cal x 59.1 lean body mass (kg) x 5 days = 8863 calories consumed / 5 days = 1773 calories consumed daily

So, if you ate 1773 calories per day and engaged in 886 calories of daily exercise so that your net is 886 for 5 straight days, amenorrhea would not occur. However, if your net is below 886 a day, over 5 days, then you risk experiencing amenorrhea.

Thus, for 5 consecutive days, if you only ate 800 calories a day - even with no activity at all - you'd be crossing the threshold.

Here is the study:
http://jcem.endojournals.org/content/88/1/297.full?sid=3f3b16d8-b819-47c8-9cc9-88c8451b52dd

Reading your profile, you seem to be comparing yourself to thinner friends and even stated you are envious of them. You do not have to resemble them, you know. Nonetheless, if you are engaging in 45 minute jog/runs, then by default you will lose some lean body mass thus it's not necessary for you to restrict dietary intake as drastically as you propose. Just by adhering to a 25% or so deficit below maintenance will get you there in time.

I really recommend you talk to a nutritionist. I think you're going to do more harm than good in the long run. Your on the short path to an eating disorder.

I think you have gotten very clear answers, here!! Matey, you really need to stop eating so little. The less you eat, the more muscle you will burn, and your fat will cling tighter than ever until you start to eat more, and then the food you eat will be immediately converted to fat and stored until this time of famine is past. You can't defeat the body's survival mechanisms. You are eating much less than your BMR, which means you are starving yourself.

Probably you are not getting anywhere near the nutrients we all need each and every day. You may not feel the effects now, but if you maintain this type of diet, as you get older your body will start to show the deficiencies. Think long term. Listen to the incredibly great advice available on this site.

Educate yourself. This is harsh, but true. What you are doing is ignorant and dangerous. Stop it. We all wish you the very best, which is why we are bombarding you with exactly the same advice and opinions. Arguing with the answers to your question and defending your decision just begs the question - do you want honest answers, or are you just going to continue doing this anyway? Hope you do the sensible thing. Not to make us happy, but for your health - both now and in the future.

Will totally backfire.
Slow and steady wins the race!

More than likely, yes it will backfire. The less weight you have to lose, the more fuel you need to provide or your body will catabolize lean body mass. The issue with this is two fold; first, your body will adapt and burn less calories and decrease your metabolic rate and second, you will probably not be happy once you get to your goal weight as you will lose muscle and still be flabby. Why don't you aim for a moderate deficit (20% below TDEE) and start heavy weight training so you cut fat.

Ding, ding, ding!! We have a winner!! This is the correct answer.

Yes it absolutely will. I never listened to all the advise of "don't go under 1200 calories" and all that.. i averaged about 700 calories a day for almost a year. It has completely backfired. Of course I lost the weight fast, but your body and mind will revolt. I can't even imagine going back to that amount now. I developed a binge eating disorder because I wouldn't let myself eat.. so now when I get food I just go overboard and eat everything in reach. I have gained some weight back.

My advise is don't diet for quick results. Lose weight slowly and learn how to keep it off. You cannot survive for long on 800 calories a day. It may seem easy now, but it will be VERY VERY hard later on. Good luck!

Thanks for sharing this Kate! I wish you would post on every thread where someone wants to eat at a very low level!! Most people don't realize that the results of adaptive thermogenesis can be fairly permanent. The downregulation of the metabolism in very difficult if not impossible to reverse.