Well, basically I have a problem, and one of my friends has almost the same issue. Long story short, even if I eat a ton, I lose weight, or at the least stay the same weight. I'm underweight now, for no real reason seeing as I exercise regularly, and eat a lot to make up for that. I don't know if I have just a crazy high metabolism or something, but it's kinda bothering me how even after eating normally or more than normally for a week I might weigh a pound less for seemingly no reason. I've been like this pretty much my whole teenager-life, and yeah. In the past 5-6 months I went from around 125 to 110 for no reason maintaining the same diet (probably around 2000 calories a day) and normal exercise.

Then my friend (A guy) used to weigh like 150 pound (He's 5'11 or so) and he has a similar issue where even though he has a pretty heavy workout routine compared to most people and he eats a TON he still ends up losing weight. And the thing with him is that he weighs like 130 pounds now, but has the same amount or more muscle than he did when he was heavier.

So is there any easy explanation for this? I know this might go against the medical advice rule, but I figure that there might be an easy-ish answer before I check with some medical expert.

Go get yourself a blood test. Glucose, lipids, etc. First instinct says YOU ARE A TEENAGER, second is diabetes.

Yeah. I would assume since you're exercising and such, you're okay (and it's just your nice teenage metabolism) BUT:

If you are drinking lots of water, going to the bathroom a lot

Have headaches, fatigue, weakness, etc.

Breath smells funky

Have flu-like symptoms

(Worst comes to worst) Or you start puking everything you eat, can't eat anything

 

Those are signs of DKA (Diabetic Ketoacidtosis), aka really high blood sugar because you need insulin. Go to a doctor and at least get a finger[puncture] blood glucose test, then they can tell you if you have diabetes or have elevated blood sugar.

 

I have type 1 diabetes, had it for like 8 years now...

You're not eating enough. You think you are, but you really aren't. Of course you have a fast metabolism, so you can seemingly eat a lot more than most without gaining much weight (or even losing it...) but if you're eating a calorific excess of about 500 calories a day whilst working out you should put on about a 1lb a week. All teenagers think they eat a lot and can't gain weight but you might be eating the wrong kind of foods or just plain not eating enough. 2000 calories a day is not a lot, in fact it's about 500 less than what most medical experts recommend. But before I end up with egg on my face could you post what you eat in a typical day?

I don't know about your guy friend (I chalk it up to male metabolism, which is incredible) but I think your weight lose "issue" comes from hormones. If you are on a contreceptive of some sort, the man-made hormone "progestin" can do all sorts of odd things to your body. Not all of which are bad, but weight loss is rather common. Stress can also be an attribute to sudden weight loss (although in some it has the adverse effect). If you don't believe its any of these, then consider yourself blessed with either really good metabolism...or a tapeworm.