When do I do cardio? When do I lift weights?

When to do cardio?  Morning?  Afternoon?  Does it even matter?

According to studies, cardio is best in the morning because we want to induce a catabolic (tearing-down) state in the body to metabolize and burn more fat.  Morning cardio on an empty stomach can result in 300% more efficient fat burning, a concept that's hard to argue with.  Yes, you'll be somewhat catabolic to muscle tissue, but the effects on muscle will be minimal since the catabolism comes to an abrupt halt once food is introduced to the body 30-60 minutes post workout.

Cardio can be done in the PM on a relatively empty stomach (2-3 hours after eating a small meal) and achieve better-than-average results with the same post-exercise eating rule.  Personally, I prefer to do cardio in the morning. It gets me out of bed, gets the day started with a burst of energy, and has the great fat-burning effects I need to shed 30-odd pounds of fat.

Weight training is a totally different matter. I never do weight training in the morning on an empty stomach. The purpose of weight training is to gain muscle. Weight training is intended to be anabolic (building up), and the body needs fuel and materials with which to build muscle. I prefer to have a small meal about an hour before going to the gym on weight days to provide fuel for the heavy weights I will be working with. I follow my weight training immediately with a source of easily absorbed protein (i.e. meal replacement powders).  Eating immediately after weight training prevents muscle catabolism and provides immediate building blocks for muscle repair (and growth!)

When doing weight training on an empty stomach, we will certainly burn fat, but the primary consideration in weight training is to build muscle. The body has a system for fueling itself. It targets muscle glycogen first, then fat, then muscle protein. When one source is depleted (or too slow, or not available), the body goes to the next source in line. Depleting muscle glycogen will take place pretty quickly with weight training, so fat will be the next likely target for energy. Fat, however, isn't a very quick source of energy under such exertions as weight training, so the body moves to burning muscle tissue for its fuel. If you've ever smelled ammonia emanating from your body while working out, you've pushed to the point where the body is consuming muscle for fuel.  Time to stop training and start eating when that happens.

What I accomplish by eating prior to weight training is providing my body with an available source of energy (sugar in the carbs and protein) so it (my body) never begins scouting out muscle mass for energy. I sip a sports drink during the workout to keep my insulin levels a little higher, thus moving the fuel to the muscles and the material to rebuild the muscle tissue. My body never has the opportunity to become catabolic, and muscle mass is preserved. Since a more muscular body burns more calories at rest than its less-muscular counterpart, I figure I'm doing my body a long term service by passing on the short-term fat loss achieved through empty-stomach weight training.

If you're going to do weight training in the morning, I suggest having a small meal before going (3 egg whites, one whole egg and oatmeal, or something similar to that) and having a meal replacement powder afterwards (within 1 hour of weight training).

Since I am naturally a night owl, I generally do weight training in the afternoon to allow myself that extra hour of sleep a couple days a week. Goodness knows, I need the beauty rest!