Perhaps the most dreaded word in a bodybuilder's vocabulary is "cardio." Unfortunately, cardio is a necessary evil of pre contest prep for most people. Few are able to achieve contest bodyfat levels through diet alone (i.e. ectomorphs with extremely fast metabolisms).
In men, cardio increases lipolysis in visceral fat (surrounding organs), especially in the stubborn abdominal area. In women, cardio increases lipolysis in the stubborn subcutaneous buttocks and thigh area in women. This is due to innervation and blood flow, which aerobic activity influences much more than diet alone 17.
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.
Another question that often arises regarding cardio is the argument "Low-Intensity vs High-Intensity" cardio. Many people automatically assume that low-intensity cardio is better; citing that high-intensity cardio primarily utilizes glucose (anaerobic metabolism), while low-intensity cardio primarily burns fat (aerobic metabolism).
Once again, the substrate used during cardiovascular work is not as important as the caloric deficit created by the cardiovascular work. In actuality, high-intensity cardiovascular work is superior to low-intensity cardio for several reasons
High intensity cardio has a much stronger effect on GLUT-4 translocation in muscle cells due to the increased force of muscle contraction. This means that high-intensity cardio creates a much stronger nutrient partitioning effect towards muscle tissue than low-intensity cardio.
Low periods of low-intensity exercise tend to "overtrain" the fast-twitch muscle fibers and convert the intermediate muscle fibers to slow-twitch fibers. This is not a desirable effect as the fast twitch muscle fibers are those that have the greatest chance to hypertrophy. If your body has less fast twitch fibers, then you will experience less hypertrophy from training.
The body's hormonal response to high intensity cardio is similar to the body's hormonal response to resistance training (i.e. increased insulin sensitivity, gh release, Igf-1 release, etc) without placing the same strain on the nervous system as resistance training.
High-intensity cardio causes the body to preferentially store more carbohydrates and burn more fat.
High-intensity cardiovascular exercise increases oxygen expenditure and forces the body to adapt by becoming more efficient at oxygen transport (increase in VO2 max). More efficient oxygen transport to the muscles will increase fat oxidation as fat oxidation is dependant upon the presence of oxygen.
High-intensity cardio seems to be more muscle sparing. Several studies have shown that high-intensity interval training (aka HIT) burns less calories when compared to continuous lower intensity cardio. However, the skinfold losses were greater with the HIT group than in the continuous intensity group. This means not only did the HIT group lose more fat, they also spared more muscle tissue by burning less overall calories .
At this point I am going to refer you to several articles that I think are some of the best I've seen regarding cardiovascular work. I urge you to read them as they will re-emphasize what I have already stated, as well as help you gain a further understanding of how cardiovascular work effects your metabolism.
(if you only read one article READ THIS ONE!)
I also implore you to read this thread from the bodybuilding.com message boards as it may help you better understand the subject and or answer any questions you might have.
In conclusion, I suggest performing HIT cardiovascular work on their off days only. One should treat this cardio session like a weight session and eat accordingly (as outlined in the diet section). If you must perform cardio on your lifting days then do it on the day you train your weakest body part and divide up your carbohydrate intake in view that you leave enough carbohydrates for both pre/post lifting and cardio.