Q: In question 146, you mentioned that we should not do cardio on an empty stomach. My personal trainer actually advised me to. I think his reasoning is that my body will burn more fat storage ( as oppose to first burning the food i just ate)....I just wanted to know a little bit more about this issue. Do you recommend eating carbs and protien before cardio/weight training?

A: Let me try to expand on why I recommend not doing cardio on empty stomach and let's put some numbers on it... Let's say your cardio burns 300 calories, your daily BMR (not counting cardio) is another 2500 calories, your breakfast (either before or after cardio) is 300 calories, and you eat another 2000 calories during the rest of the day. Now let's look at the two scenarios...

Under your trainer suggestion you burn 300 calories by cardio (which will come from a mix of muscle and fat because your body is famished and has no particular reason to preserve all the muscle while using the precious fat), then eat 300 calorie breakfast, then go on with the rest of your day burning 2500 calories and eating 2000. The net total is calorie deficit of 500 calories per day and some of those 300 calories during the cardio came from burning muscle.

Now let's look at my recommendation. You eat 300 calories, go run burning 300 calories (few if any of these calories will come from burning muscle since you just ate and your body has fresh energy reserves), then go on with the rest of your day burning 2500 calories and eating 2000. The net total is, once again, calorie deficit of 500. But this time you burned less muscle during the cardio run!

The notion that cardio on the empty stomach somehow burns more fat is based on fallacy that assumes that somehow the rest of your day is the same calorie-wise as when you eat and then run. But clearly, if you are comparing apples to apples, then you should assume that your total daily calorie intake is the same in both cases (like I did above). Once you do that, you realize that you'll still have to eat that breakfast and those calories will still be in your system. In the end, total calories in vs. total calories out are the same in both scenarios but under my recommendation you do not jeopardize burning muscle. Hope that makes my position clearer.

If you do cardio in the morning, I recommend to at least have protein before it (giving your body less reason to burn muscle for energy since you give it readily-absorbable protein instead), but ideally you would have a regular breakfast. For weight-training in the morning having a nice big breakfast before is even more important -- your body needs fuel to push those weights around!