Hi Clint-
This was a question that I stuggled with too and asked around different people and what I have found is that to be pretty acurate is pasted below from someone else:
First off, this is a very general rule to go by, and takes 1st precedence.
Resistance training should always take priority over cardio. Here's why....
The long term gains of increased LBM (lean body mass) and increased secretion of hormones like GH and test far outweigh the calories burnt during a cardio session..
By doing cardio before weights you are simply reducing the amount of glycogen available that could have been used more effectively to fuel your muscles throughout the weight training session...This point cannot be stressed enough.
Here are the top 3 scenarios for doing weights and cardio in order of importance:
#1 - On non-weight training days
#2 - weights in the am and cardio in the pm
#3 - light-cardio after weights
Now, that being said, here are a couple scenarios that are most proposed/asked about by folks, so different protocols need/can be effective. For the person who is lean and wanting to save muscle mass while burning fat, then the above scenarios cannot be altered in any way! You are asking for loss of lean tissue if you do cardio on an empty stomach or any such nonsense. You are already into a catabolic state from the overnight fast, and you are compounding this by doing cardio without fuel. Cortisol will be running rampant, and eating at your lean tissue for fuel. The body is very adept at breaking down amino acids for glucose, and will greedily do so. Cortisol will liberate amino acids to produce glucose (glucose cannot be synthesized from fats) and can lead to muscle loss. I would propose the best scenario here would be to treat your cardio session as you do your resistance training session. Albeit a different PWO formula (protein/fiber/fats will fit the bill here, no impact on insulin, thus keep any lypololysis induced during your cardio session intact)), but still with the same intensity and purpose. You would not dream of working out on an empty stomach, would you? Never work out on an empty stomach. True, you will burn a buttload of calories, but they will be as much lean tissue as fat. Never sacrifice lean tissue (long term results) for a short term fix.
It is also perfectly fine to do a “warm up” session of cardio before your weight training session if you are mid range, not real lean but not lots of fat stores either. But, here you are on that borderline of getting a lean hard body, and building lean tissue should be your number one priority. But make sure this is no more than a warm up, just to get blood flowing. 5 minutes at most should suffice. I personally prefer to warm up with the exercise I am going to do…i.e…weights.
Now, on the other hand, if you have plenty of fat to spare, it can really become a non issue. I believe this is/should be obvious. You can do your cardio even before if you like, or even on an empty stomach, as in this case total calorie burn is your issue and not saving lean mass, for you probably don’t have enough to save or you have as stated plenty of fat stores to give up. Your body will freely give up fat stores in this case, but still, make your weight session your priority as per the long term benefits.
But, no matter what your physique may be at the time, do not center your workout around cardio. Make certain you incorporate resistance training, as this will in the long run be your best friend in the battle of the bulge. Lean tissue is a fat burner 24 hrs. per day, a cardio session ends when you stop doing it. (A little different with HIIT, but this is another whole subject.)
