I get asked this question all the time..the answer is AFTER!! It really is ideal to separate weight days and cardio days, however if you are trying to get it all in in one session, do the cardio after your weight training session. Reason being is because if you do cardio before that long and intense of an interval will almost, if not totally, exhaust your glycogen storage. Why is that a problem? Lifting weights does not utilize as much of your glycogen as a cardio session would, but it still needs that glycogen because it is your muscles source of energy for contraction. With no glycogen stores, your weightlifting workout will pretty much be a waste of time. The energy won’t be there to push you through your last reps, meaning a less efficient workout. Another reason this is an issue is because during a cardio session, protein synthesis decreases, and protein breakdown increases. Meaning protein synthesis (your body's mechanism to build muscle) drops after your cardio.
Doing cardio after lifting weights is beneficial for both the goals of gaining muscle and losing fat. You also want to make sure to not go too long of a cardio session after weights because you don't want to burn muscle. Stick to a 45 minute weight lifting session and 25 minutes of cardio after. This is the ideal way to target fat supply for fuel, burning fat as a result, YES!!!