Optical zoom lowers quality? DSLR?

I have a Nikon D3100 and I was wondering, if I take a shot of something, but I zoom in with my 18-55mm lens, do I lose quality, since what I'm doing is sort of a crop?

Much appreciated.

Absolutely NO. Optical zoom does nothing to reduce the quality. You're confusing optical with digital zoom. Digital zoom is nothing except cropping, reducing the size and quality of any image.

Look at any sports photo taken by a pro. Most likely, they used a 200, 300, or 400mm lens. With a professional grade, fast lens, there's no loss of image quality.

Here's some shots I took using a 70-200mm and a 135-400mm lens.
http://www.sportspagemagazine.com/content/fb/c-fb/gal-c-fb/ncaa-football-kraft-fight-hunger-bowl-arizona-stat.shtml?55371#14