Nope. Not necessary.

Diet is key for losing fat. Weight training is necessary for building muscle mass.

Do some cardio if you want to build stamina and heart health but it is not necessary to reaching your goals. It can help facilitate and quicken the process though.

Specifically for your goal of "lean mass gaining and losing BF"

Yes. Not a problem. Many bodybuilders do the bare minimum cardio (5 minute warmups) because they don't like it. If you don't like cardio, don't do it. It isn't necessary to reach your goal.

Now, if your goal is to be able to run marathons or take part in a triathlon, then no. Those goals require high stamina and that would mean cardio.

However, just a bit of information - Unless you are an obese beginner in weight training, it is unlikely that you can GAIN mass while LOSING fat. Why? Because gaining mass requires a caloric SURPLUS, whereas losing mass (in this case, specifically fat) requires a caloric DEFICIT.

Once you are past the newbie stage (roughly estimated at 6 months by most people), it is practically impossible to build any mass while at a caloric deficit - you will maintain your mass which is great but you cannot expect any significant growth. That is why you will find serious bodybuilders all CYCLE their BULK and CUT phases and it takes years (not months) of consistent training to build up 10-20kg of pure muscle mass.

Oh yes I suppose I should say obese and/or a newbie. Not necessarily an 'obese newbie'. Any newbie can and usually does add muscle mass while losing fat if they maintain a calorie deficit.

AFTER the newbie phase though, it is PRACTICALLY impossible. Not COMPLETELY impossible but results from trying to gain mass+lose fat simultaneously for most would be not worth the effort and time. Most of us are not so genetically fortunate to be able to do this successfully it seems. So it would be easier for most of us to just commit to cutting OR bulking.

Edited to add: I agree about typical cardio being boring (I will use the treadmill or stationary bike if I havent been getting cardio elsewhere though). It is considerably more fun to find a cardio centric activity (like cycling outdoors, walking/jogging to and from places, dance classes, team sports) in my opinion.

This post has been edited by shadowz: Feb 7 2013, 02:03 PM