Akicita & Jayalan: Congrats to both of you on your accomplishments!.. Here’s my take on cardio and why you cant gain muscle and lose fat at the same time. First, I believe that if your high in BF%, some moderate cardio is definitely called for. My guess is it’s sort of a bell curve - if your very high in BF%, it should be very light, as your heart is not used to the extra stress. As you start getting down below 20%, you can increase it - you litterally have fat to burn. As you get below 10%, cardio should be decreased greatly - at this point, your probably burning a lb. of muscle for about 1 lb of fat - not a good scenario. I think what you’ve both experienced is great, but the process will need to be changed as you get more experienced and your body composition changes. The other thing is goals - the primary goal of the poster is to gain muscle, but he want to still look ripped the whole time. For many people that haven’t been training long, the primary goal is to lose the fat. I personally wish I was a little more educated when I started training seriously about 3 1/2 years ago, as I probably wouldn’t have done as much cardio and burned precious muscle. But once again, it’s a matter of goals and education (we’re too educated to look at scale weight rather then body fat, so I kept going hoping to get my scale weight lower, killing off muscle at the same time). And here’s a place that I have to disagree with Jayalan - a successful (long term) diet and exersice plan sacrifices ZERO muscle, and should gain some (regardless of your age). If you’ve lost any muscle, the plan isn’t working for you (you’re probably over training).
Now, as to why you can't gain muscle and lose fat at the same time... well, this isn't entirely true - for beginners, this is entirely possible. My guess is because training and dietary changes are such a shock to the metabolism, that it burns whatever it can get to keep up... but there's plenty of fat, and not much muscle, so it goes for fat, even though that's a "harder" gram to burn (it is harder to burn a gram of fat then a gram of muscle). Plus, your muscles are in attrophy, so the shock of lifting causes them to overcompensate - they are completely unused to the new load, but are terrified that your going to do it again to them, so they better compensate (i.e. grow). But now as your body fat gets lower, there's little fat to burn, and your metabolism is jacked up. So, you go into sort of a self preservation mode, and too much energy expenditure forces you into a catabolic state. There are other reasons that poloquin and many other have gone into regarding cortisol levels and T levels on why cardio doesn't work at low body fats.
I'm not sure about the whole biochemistry aspect of this, but I suspect that it is impossible to convert a fat molecule into a muscle molecule, so you can't get enough energy from fat to convert it into muscle - what the fat does is provide energy to do the work that the muscle is requiring, and possibly provide it with fuel in order to keep it alive. But it doesn't convert into muscle - that's done by muscle cells splitting or getting larger (depending on who you listen too). Anyway, hope this helps (I've blabbered long enough). Congrats again!