Eating properly is only half the battle in losing body fat, the other half is physical activity. Losing weight is simple math.  If you consume more calories that you burn in a day, you are in a "positive energy balance".  When you are in a positive energy balance you will store the extra energy as body fat, whether it be from carbs, protein.  On the flip side, if you burn more calories than you consume in a day, you will lose weight.  For every 3500 calorie deficit, one pound of fat is lost.  Lastly, if the calories you are consuming and expending are equal, you will maintain your current weight.

Cardio training is an effective way to manipulate your calorie expenditure so you lose or maintain weight, specifically body fat.  Cardio exerts several benefits; it expends energy by burning calories (hopefully in the form of stored body fat), it affects hormone levels in the body that can increase your metabolism, and it enhances cardiovascular and mental health and enhances endurance.  There is no question that cardiovascular exercise is very important for overall health, but too much cardio can actually impede muscle growth.

It is more effective to lower your calories than to increase your aerobic exercise in achieving your ultimate physique.  Increasing cardio sessions to burn more fat will physically tax your muscles energy reserves.  This can negatively impact muscle growth.  Remember, to lose fat you need to expend more calories than you consume.  The simplest way to do this is to intelligently reduce you caloric intake to a 3500-7000 calorie deficit each week.  This method will elicit a 1-2 pound decrease in fat each week and preserves maximum muscle mass while dieting.

Any larger of a calorie deficit is not practical and probably unhealthy, not to mention the fact that you risk muscle loss.  From a health standpoint, some cardiovascular training should always be done.  The key is to strike a balance between your cardio and weight training so that you maximize the fat-burning benefits and reduce the negative impact the cardio can have on building muscle.

It is possible to build muscle and lose fat if you take the right approach.  You have to train hard to build muscle and let your diet and cardio burn the fat.  Some people have the misconception that it is effective to burn fat through circuit training.  This involves doing all of your resistance exercises consecutively with no or very short rest periods.  People train like this under the assumption that you are burning more calories and keeping your heart rate elevated.  But training this way causes you to lighten up on the weights and this does not have a significant fat burning effect.

You must realize that weight lifting is not an aerobic exercise.  Increasing your reps and using lighter weight does not increase fat burning, but it will decrease muscle size and strength.  There is a direct correlation between your muscle size and the amount of weight you can lift with good form.

Many bodybuilders lighten their loads while preparing for a competition and start to train with very short rest periods.  This is a big mistake because what built the muscle is going to keep the muscle.  If you train in this style, you may drop body fat slightly, but at the expense of losing a significant amount of your hard earned muscle.  Train hard and heavy and save the fat burning for your cardio sessions.

Besides all of the physiological reasons to do cardio, there are many psychological reasons of doing it.  Research has shown that cardiovascular exercise of any kind improves mood and reduces depression like symptoms.  I'm sure almost all of us have experienced this after a sprint session or 20 minutes on the Stair Climber.  It may feel like torture while doing it, but afterwards you feel like a million bucks.

A factor that serves to improve mood during cardio is the endorphin rush that is created. This occurs when hypothalamic neurotransmitters are released from the anterior pituitary gland.  These endorphins can block pain sensation, promote a feeling of euphoria and reduce feelings of stress in the body.

TIME OF DAY

Cardio helps to raise hormone levels in the body, specifically norepinephrine.  Yet when you do cardio makes a big difference in how your body handles the hormonal changes. Performing cardio on an empty stomach allows norepinephrine to readily target fat cells, which triggers maximal fat-burning.  On the contrary, if you eat before doing cardio (particularly carbs), the fat-blocking hormone insulin rises, making your body less effective at burning fat.

At the onset of exercise, muscle glycogen is used as the primary energy source of fuel to the muscles.  The sympathetic nervous system triggers hormones, norepinephrine and epinephrine, to be released into the bloodstream.  These hormones stimulate glycogenolysis (the breakdown of glycogen) to fuel the muscles during physical activity.  Epinephrine is turned on later than norepinephrine, unless the exercise is at a very high VO2max and intensity.

Insulin is an anabolic hormone that increases after eating and stimulates glucose uptake.  It is critical for glucose and free fatty acid storage.  So if you eat immediately before you do cardio, your body will not be in a state to burn fat optimally.  To ensure your body is in the optimal fat-burning mode, you should perform you cardio at a certain time of day.

The best time to do cardio is first thing in the morning, before breakfast.  Other times during the day will certainly burn calories, but you will not be burning fat effectively.  You can drink your morning coffee (without cream or sugar) to burn more fat.  No carbohydrates should be consumed before your cardio session.  Otherwise the carbs will fuel your workout versus your stored body fat.  You can take 6-10g of mixed amino acids or a small amount of whey protein powder mixed in water beforehand.  This will help prevent muscle breakdown during cardio.

Let’s say you do cardio later in the day, after lunch.  All you will be doing during that cardio session is burning the carbs and calories of the food you just ate.  When you perform cardio first thing in the morning, there are no carbs to burn because you haven’t eating anything for the past 8 or so hours while you were sleeping.  When the body does not have carbs to burn, it turns to stored body fat for energy to fuel the aerobic activity.

Another time during the day when your body is depleted of carbohydrates is immediately after your weight training.  Your body uses muscle glycogen to fuel your anaerobic workouts.  After an intense hour of power workout, your body will burn fat more effectively during cardio because your muscle glycogen levels are low.  Post-workout is the second best time to perform your cardio training.

The only drawback at doing cardio post workout is that it can impede muscle growth.  Your body is depleted at this time and needs to recover and recuperate to rebuild muscle tissue.  You run the risk of overtaxing your body by performing your cardio in a catabolic state. This is counterproductive in building muscle mass.  I recommend taking in a small amount of protein or aminos before your cardio session if you do it at this time, it will prevent muscle breakdown.

Never do cardio immediately before your workout.  A lot of people like to do this to feel really warmed up before tackling the weights. However, this will just fatigue you prematurely and you won’t be able to overload the muscle properly or train intensely to stimulate muscle growth.  Not to mention you will not be burning fat effectively.

The third best time to cardio is later in the evening.  You have to make sure that you have not taken in complex forms of carbs for hours beforehand though.  Later in the day, if you are taping your carbs like you should be, the body will have less carbs to burn off during cardio.  Surplus calories can also be burned off at this time, preventing them to be stored as body fat before going to bed.

MODE AND FREQUENCY

There are numerous fashions cardio training can be carried out.  You can go for a fast walk or run, swim, jump rope, take an aerobic class, ride a bike or use equipment at the gym.  The choices are endless, all these modes of exercise will burn calories and kick start your metabolism into high gear.  One is as good as the other when you are consistent.

I recommend performing cardio on a Recumbent Bike, Stair Climber or Treadmill.  These modes allow you to easily control the intensity while keeping your heart rate elevated and gauge your workout.  The bike is great for the quads, especially the outer sweep.   The step mill is great for developing the quads, hamstrings, glutes and calves.  While the treadmill tends to burn the most calories.

You should progressively work up the frequency of the sessions.  I believe that 3-5 days per week is effective at burning fat while preserving muscle.  If you are not achieving results by doing this, you should adjust your diet accordingly to restrict more calories.  If your goal is muscle hypertrophy, I suggest limiting your cardio to three times per week.  When your primary goals is fat loss, go ahead and increase the frequency up to five times a week.

It is not a good idea to perform cardio on leg day.  You should save your strength and energy to exert during your anaerobic exercise.  And your leg muscles are in a state of deprivation after the workout.  It is best to schedule your cardio on non-workout days or days that are less taxing on the body, such as arm day.  You can, however, do cardio if your legs are sore the following days, this will actually help in removing the waste product lactic acid.

INTENSITY AND DURATION

A huge benefit you get from cardio, from a fat burning standpoint, is its effect on your resting metabolic rate (RMR).  Your RMR is the amount of energy your body expends when you are not exerting yourself, at rest.  Cardio training has a dramatic elevating effect on increasing your body's resting metabolic rate.  This increases your body's overall fat burning capabilities, making your body much more efficient at burning body fat.

It is important to take a smart approach to your cardio training.  Intensity and cardio volume are the two energy expenditure barometers that can be manipulated through aerobic exercise.  There are lot of different things you can do to burn calories, but you want to burn them most efficiently without the cost of muscle loss.  Low intensity and long duration cardio sessions burns muscle, that is exactly what you don’t want. The fat you burn from doing cardio this way is indiscriminate.  You will burning calories but end up being a smaller

A good real-world example of the difference in effect of low intensity and long duration cardio has on athlete’s physiques is to look at sprinters and long distance runners.  Sprinters are very lean and extremely muscular.  They train with short bursts of maximum intensity.  Long distance runners are skinny with little muscle, yet possess higher body fat levels than the sprinters.  They train more inline with the conventional cardio approach, which is low intensity levels for longer periods of time.  Which type of physique would you rather have?

The percentage of fat you burn during aerobic exercise can also differ for each individual.  It depends on a number of things including your VO2 max, lactate threshold, mitochondrial density and efficiency, and hormonal mileau, to name a few.  Despite your genetics, research shows that in general, the lower intensity that you work at, the higher the percent of total calories expended will come from fat.  However, when you exercise at a higher intensity you burn more total calories, but a lower percentage of those calories come from fat.

For example, let’s say a 200 lb. man performs 30 minutes of cardio at an intensity level of 50% VO2 max.  He will burn approximately 250 calories and around 65% of the energy will come from fat.  In other works, 162.5 calories comes from fat and 87.5 comes from glycogen or carbohydrate.  If that same man does the 30 minutes of cardio at a 75% VO2 max intensity level, he will burn approximately 350 calories and about 35% comes from fat.  So 122.5 calories comes from fat and 227.5 calories come from carbohydrate.

So then how do you go about reducing body fat percent?  Create a caloric deficit.  You need to lose weight to decrease your fat stores.  If you look at the total amount of calories that you burn through aerobic exercise it isn't too much.  Aerobic exercise is not your key.  It helps with cardiovascular and metabolic health, but does not contribute substantially to weight and fat loss.

It really boils down to calories.  If your goal is fat loss, then it is best to concentrate on your diet and resistance training.  Continue to engage in aerobic exercise, but do not make that a focus of your training.  The cardio you do perform should be of higher intensity and shorter durations in order to burn more calories and have less of a negative impact on muscle breakdown.  15-20 minutes is an optimal time span that will allow you to put forth maximum intensity without pacing yourself.

Interval work is the best way to burn calories without sacrificing muscle.  This involves alternating high heart rate work with brief recovery periods.  It is better to perform two short duration cardio sessions of 20 minutes each than to do one 40 minute session. Shorter durations like this will maximize your metabolic rate without tapping into lean muscle tissue for energy. The intermittent intensity levels during the 15-20 minute sessions results in greater overall energy output than on a steady resistance setting.

Two ways to increase the intensity on a piece of cardio equipment are to increase the resistance and/or increase the speed.  The speed can easily be increased on the Recumbent Bike, Stair Climber and Treadmill. You can increase the incline grade in order to increase the resistance on the treadmill.  The bike’s resistance can also be increased by setting at different intensity levels, they usually range from 1-20, 20 being the most difficult.

Reading your favorite novel or focusing on the closed-captioning of the news should be out of the question when performing cardio, your sessions should be fairly intense.  That is one reason I prefer doing interval training, it is more mentally stimulating than training at a steady intensity. Give yourself a five-minute warm-up on the manual setting before beginning the interval training. Then crank up the intensity and do one minute of high intensity cardio by increasing the speed or resistance. Follow that up with two minutes of moderate intensity cardio. Alternate back and forth for the duration of your workout.

An example of this are using the intensity levels on the Recumbent Bike.  After your warm-up, say on level 5, turn up the intensity to level 15 for one minute.  Then bring it down to level 10-11 for two minutes before repeating.  If you prefer to do your cardio outdoors then do sprints after a short warm-up.  Sprint all out for 15 seconds.  Then turn around and walk back where you started, it should take about 45 seconds.  At the start point, sprint for 15 seconds again.  Perform 15-20 sprints to complete one cardio session.

One implication of doing cardio through interval training is that it can be harder on the joints that sustained level exercise.  If you find intervals to be tough on your joints, I recommend doing your cardio with some set parameters for the duration of your workout.  You should determine your target heart range, which should be 65-75% of your maximum heart rate. This is easy to calculate.  You take your age and subtract it by 220.  Theoretically this is your max heart rate.  Then multiply your max heart rate by 0.65 and 0.80.  This will give you a target heart range to stay in while performing cardio in order to burn fat more effectively.

Here is an example for someone who is 28 years old:

Max Heart Rate = 220 - 28 (age) = 192 BPM

Target Heart Range = 192 (0.65) = 125 BPM and 192 (0.75) = 144 BPM

So the target heart range for this individual would be 125-144 BPM.

You can measure your heart rate while doing your cardio by taking your pulse.  This can be taken at the side of your neck next to your windpipe or at your wrist.  Measure with your finger because your thumb has it’s own pulse.  You can hold your finger at the neck or wrist for a full minute, but a simpler method is to count your heart rate for 10 seconds and then multiply by six.  For example, 21 beats in 10 seconds gives a pulse rate of 21 x 6 = 126 beats per minute.

Another way you can gauge your intensity level without performing intervals is by your perceived level of exertion.  Your breathing is a good indicator of your training zone and the talk test can be used to determine your intensity level.  You should be breathing enough where there are exchange of gases, but not so much where you are out of breath and can’t talk to someone next to you.  After the cardio session, always end with a 1-2 minute cool down period.  This allows your body to slowly return to it’s resting state.

SUMMARY FOR CARDIO TRAINING:

1. Train heavy when resistance training and save your fat burning for cardio. Do not lighten the weight in attempt to burn fat, this does not help fat-burning.  It will decrease muscle size and strength.

2. Perform your cardio first thing in the morning on an empty stomach or immediately following your weight training in order to burn fat effectively.  Do not do your cardio before your weight training as you will be dramatically weaker.

3. Schedule your cardio for 3-5 times per week. Any more than this frequency can impede muscle growth. It is better to manipulate your diet for increased fat loss.

4. Be consistent with your cardio workouts, there are several modes of exercise that can be used to vary your program.

5. Cardio exercise should be of short duration (15-20 minutes) and high intensity (interval training). This burns calories more effectively and reduces muscle tissue breakdown.