Should I worry?

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For most of my life, I was a pretty chunky kid. Last summer however, I started an intense exercise program, and lost about 30 pounds. My family was actually a little worried about me, as it seemed I may have lost to much weight. Right now I'm about 6 foot, 165 pounds. In other words I'm pretty dang thin.

Lately I've really been slacking off on the exercise, I don't run much anymore, and I hardly lift. I'm afraid that I am going to get fat again. My brother tells me that my metabolism has probably kicked in, and that I could eat a ton and not gain anything (he is of similar height and weight, and has a very fast metabolism).

Should I be so worried about gaining my weight back? Or can I afford to slack off?

As you get older your metabolism will slow, so even if you stay thin now while eating a lot (and since you were chunky before you may not be the same as your brother), it's likely later on you will need to be careful. It would be a good idea to stay in regular exercise habits. They need not be intense - something you can maintain is what you want.

I would recommend working out again, or, at least doing some sports to keep the weight you lost from coming back.

A good way to excersise without risking more rapid weight loss is to do anaerobic exercise. IE: Weight lifting.