For most of the last year, I've been working out 6 days a week, three days of cardio (30 min run/walk) and three of reasonably vigorous weight training for about 45 minutes. I generally keep my carbs in the 35-50 range and my calories in the 1700-2000 range if I'm trying to lose weight, a bit higher if I'm maintaining. For me, the calories actually make a bigger difference than the carbs -- I can do just fine at 20-25 carbs for a couple days, but I'm noticeably weaker and more tired if I cut calories below normal for a couple days. I eat a little more protein than most LCers -- 125-150g per day -- and I think that's the right amount for me, because I start craving it if I eat much less.

I should say, though, that I think my current eating is the *minimum* I need to seriously work out. It's low enough that if I don't also get enough sleep, I can really wear myself out in a hurry -- I can handle any two of carb, calorie, and sleep deficits, but not all three at once for more than a couple days. I'm also pretty small to start with, which makes a difference. If you've got more to lose, you will definitely want to eat at least a bit more than that.

My trainer gave me the same spiel about losing muscle and all that, but I've actually gained about 2 lbs of muscle and lost 11 lbs of fat, which dropped my body fat from 25% to 18%. I consider those pretty acceptable results .