In college, my exercise physiology professor was one of the coaches for the Team USA track team. He filled me in with a bit of insider wisdom that I have still not seen a lot of research on today. He was involved in a study regarding elite athletic performance and muscle hypertrophy. Muscle hypertrophy for the layman is just muscle growth; the vital gains we all seek to make when we go to the gym.
Well, when you do cardio after weight training for hypertrophy, your body wants to take care of the muscles taxed the most in your workout. Seeing as the energy stores you use to fuel muscle contractions and lift weights pulls energy from a different system than the slow-twitch fibers involved in endurance training, it makes sense to want to train both in one day. If your routine is cardio + weight training, and your goal is performance and aesthetics, then you've got to do your endurance work in before you lift weights because you don't want to inhibit your SIRT enzyme.
We have many SIRT enzymes that facilitate a ton of processes in the body. SIRTuins are a class of proteins that possess either mono-ADP-ribosyltransferase, or deacylase activity, including deacetylase, desuccinylase, demalonylase, demyristoylase and depalmitoylase activity (that's a mouthful of de's). SIRTuins regulate important biological pathways in bacteria, archaea and eukaryotes. There is a particular SIRT enzyme that gets inhibited when you do endurance training and that inhibition shuts down muscle growth. It turns the enzyme into an inactive site and muscle is not produced. That's not gonna be what you want.
What your gonna want to do is avoid this shutdown at all costs. You want a lean, attractive, strong, and powerful body so you want to maximize your potential to do that by following these guidelines:
1) Training cardio before weight training
2) Incorporating HIIT after your weight training sessions instead of endurance work (the benefits are just as plentiful, some even moreso)
3) Do cardio and weight training on separate days
4) Do cardio in the morning, weight train at night
That's gonna be your recipe to hold onto all your gains while slimming down and getting into your best overall shape. It's a philosophy I have lived by ever since my professor told me. It's worked for me, that's all I can say. And science seems to support it. Go get 'em.
