Cardiovascular exercise has several myths surrounding it. The largest myth being that one should perform low intensity cardio in a fasted state. The logic being that if one is in a fasted state, their glycogen levels will be low and will force their body to burn fat. Unfortunately, this idea is misguided.
While performing cardio in a fasted state may indeed increase the amount of calories that are burned from fat stores, it will also increase amino acid oxidation. Cardiovascular exercise while in a fasted state is a great way to increase cortisol release. Cortisol will liberate amino acids to produce glucose (glucose cannot be synthesized from fats) and can lead to muscle loss.
Additionally, I find it ironic that many people take such great care to time their meals so that they do not go for more than 2-3 hours without eating in order to prevent muscle loss. However, they purposefully induce this state and then perform work on top of this!
Research has shown that the type of substrate used during cardiovascular work makes little overall difference on fat loss. This is most likely due to the fact if one relies mostly upon fat stores during cardio (i.e. low intensity cardio), the body will burn predominantly glucose at other times of the day. Likewise, if one mainly utilizes glucose for energy during cardio (i.e. high intensity cardio) the body will customarily rely on fat at other times of the day in order to spare muscle glycogen.
Training in and of itself causes the body to preferentially spare muscle glycogen and burn fat. It makes sense that one should strive to do their cardio on their 'off days' from lifting (as to not further hinder their recovery), and plan their carbohydrate intake similar to their lifting regime.
Cardiovascular work will increase nutrient partitioning towards muscle tissue and away from fat tissue. One should take advantage of this by consuming the bulk of their carbohydrate intake around this time. The benefits are that these nutrients induce fat storage, but will rather be stored in muscle tissue.
Why would you want to deny your muscles nutrients at the most crucial time of the day, but then provide them during rest? It does not make sense. Treat your cardiovascular work like your lifting.
