Is undereating going to backfire on me?

Yes. Feed your body. The end.

This

To answer your question, yes under eating will backfire on you. I've been there and done that and several of my friends as well. You see just because your not feeling hungry, or just because you feel satisfied doesn't mean that your body is getting all the food it needs. So I just say eat, that's it. Maybe see an expert who can help you.

From my experience, it's really not good. Nobody can really stop you, but just do some more research and try out different ways of eating like lower carbs, higher protein, or whatever. Just exhaust all options before resorting to something drastic, and maybe even risky.

Btw, I know how you feel about the muscle, but after much research, muscle is what helps you burn throughout the day, and it's what makes you leaner and tighter. Maybe do some stretching types of work out, it helped me, and if your lifting weights take it easy, like what was written earlier resistance is good. Do what you have to do, but be smart about it. Take care and good luck!

Everyone has given you great advice here and I think they are right. Just remember that what we are doing here is not just another diet but a change of lifestyle, learning how to eat properly instead of indulging our every whim, which is exactly what I did before I joined MFP.

Yes. Feed your body. The end.

This!

You must have a large amount of bone mass and/or be extremely tall to weigh 163 and be 20% bf - that's a lean body mass of 130 lbs.

One of the worst things about chronic calorie restriction is decreased serum leptin concentration levels. Leptin is the hormone responsible for satiety and drops when you restrict food intake or net too little. Basically, your body will want and need more energy in the form of calories but, because your satiety signaling is messed up, you won't get the message. This will continue the cycle of chronic calorie restriction and potentially other health issues.

Got some great and knowledgeable posters in this thread!! Great job on this one!!

Well, I have been rather solid and athletic my entire life. I did track and field as a thrower, and I never "bulked up" but I felt large and rather masculine. I'm tired of feeling that way.

I think the problem here involves your self image. You need to stop saying "I'm large and rather masculine". I looked at your picture and I certainly did not see that - I see a young thin woman. I think the majority here are giving you great advice, but you don't want to hear it. That's something we can't fix.

We've all seen so many images of famous people like Twiggy that we have chosen to believe that this is the image of beauty. I suspect most of the men here have not noticed any sort of masculinity in your picture. Frankly, some of the women here (like me) are waiting to hit the point of your skinniness and would be thrilled to be at 15% body fat. You definitely need to get over that and stop saying you're big and masculine - you are not!

If you want to drop the fat and become more athletic, you're going to have to eat more and you'll probably need to go get a fitness expert to help you train. Olympian athletes tend to have low body fat BUT I suspect they eat like mad and exercise like crazy (weight lifting, aerobics and more). If that's your goat, get a trainer whose used to training people to become triathletes and who might have knowledge on how to lower your body fat. Unfortunately (there may be some people here who do the triathletes route who could jump in here), I suspect you're going to be totally surprised at the food levels you'll need to eat to change that fat to lean muscle. I also suspect this process will take a while - it's not something that is a quick fix.

I have been in your shoes and have to warn you, it will backfire. In high school I was really overweight, did Atkins and lost 60-70lbs. I became a vegetarian for 2 years and a runner and got thin and muscular. Im 5'10 and weighed about 160lbs when I was 21-22. I kept comparing myself with people who were thinner than me and I felt very self-concious about how I looked. I started eating a lot less and dropped another 10lbs. With my frame (large one) I looked super thin with protruding hip bones but very muscular arms and legs. I still felt fat so I ran more, excercised at the gym longer and slowly developed an eating disorder. I was bulimic for a year or so, my teeth became ruined and still I pushed myself. I'm 26 now, gained a lot of the weight back (thanks to pregnancy) but my objective for weight loss has changed. I'm no longer comparing myself with others who are thinner and smaller than me. I want to be thinner yes but I want to feel healthy and strong and set a good example for my kids. There is times where I struggle with the weight and feel self-concious but that's life.
Long story short ask yourself why you want to be thinner. Is it for yourself? Yes eating less will allow you to lose muscle as well as fat, but what happens when you get there? Will you be satisfied? There are healthy alternatives to your goal.

Your body WILL rebel, by holding onto fat and eating away your muscle mass for energy, OR by sending out overpowering urges to binge on dense calorie foods.

Eat a sensible diet. Change your lifestyle. Its not a race!

I have to agree with the poster who mentioned body image issues. I looked at your profile pictures and you look healthy and in proportion. You don't look 'bulky' or 'manly'. I am not just saying this to try and make you feel better or tell you what you want to do is stupid or dangerous, I am just saying it how I see it. You will always be a tall woman, that you cannot change, but you are not overweight, you don't look even anywhere near bulky too me. I think loosing muscle would be a bad bad idea and I don't think overall it will give you the look that you expect anyway.