Common Sense First:
This is another myth left over from the 1980s. It's your total calorie balance (calories in vs. calories out) throughout the day that counts. So why would deprive yourself of calories (that you'll eventually consume anyway) before the one activity in the day that requires the most energy?

Would a lean athlete NOT eat before an event? Just keep daily total calorie intake below your output and use some of the calories before the workout so you have more energy to burn more calories – in other words, don’t add calories to your day, spread them out.

Common Sense is lost it in Fitness Publications

"Common sense is not so common," according to 18th century French philosopher Voltaire. But I think maybe he didn’t have it quite right, because we all possess common sense or our species would have died out centuries ago.

Yet, what’s happened is that as we evolved intellectually, we started micro-analyzing everything. We ended up with more thought-provoking questions about a particular subject that eventually led us back to a common sense answer anyway, or worse—no answer at all because we didn’t need one to begin with. In other words, we either lose our common sense in scientific rhetoric (we read too much) or we ignore the answer we would intuitively have come to and now we are completely lost.

I make the above comments because I never want to have to address this topic again since the answer is common sense. And although the true answer (which I am going to give for the last time) to the question has scientific documentation, I would have to say it was a waste of time proving it because all that does is allow a non-scientist to read the clinical document and draw an erroneous conclusion based on the reader’s lack of knowledge regarding the big picture. Basically, the layperson or media chooses one piece of a complex puzzle (because that’s all they may understand) and takes it out of context to build a case for something entertaining.

Consider the following examples:

#1: Insulin stores fat. Okay, do you think you might be eating too much if you’re getting fatter . . . or is it just the insulin?
#2: Low intensity exercise uses more fat than high intensity exercise. So, does walking lead to more fat loss than running in the same allotted time? Or do you think the harder the bout of exercise the more fat you will lose daily? Ever see a fat sprinter?
#3: Eating carbs before you go to bed will end up as fat on the body. If that’s the case, if you ate nothing all day and ate one piece of bread before you went to sleep it would turn to fat. Or do you think it would depend on how much you ate during the day and the position of the sun has really nothing to do with it?

See, I bet by using common sense you got the right answers. Now, to the topic that brought us here. Should I work out on an empty stomach to burn more fat?

This includes the preposterous notion of doing your cardio in a fasting state (overnight) to increase fat loss. Notice I phrased the statement correctly—increase fat loss—not increase the use of fat for fuel during exercise, which is the only thing someone could have read in a study and taken it out of context. Then, whether accidentally or not, they arrive at a wrong conclusion that gets people to ignore their own common sense.

So let’s re-phrase that and see how this can be taken out of context. Who would lose more fat? Someone who did cardio first thing in the morning before breakfast, but ate 2500 calories for the rest of the day, or someone who ate breakfast before cardio but only ate 2000 calories for the rest of the day. Obviously the personal who ate breakfast before cardio and ate only 2000 calories for the rest of the day. So you see, when you take things out of context, you may end with the wrong conclusion. Now back to the answer.

Of course you will burn less calories if you do not eat before a workout. Why do you think all performance athletes eat their biggest meal before training? So they have a better workout, which will always use more calories (say it with me, "Common sense." Working harder because you feel better obviously burns more calories). We know from research that the higher the intensity of the exercise (which you could never match in a fasting state because you have had no food for the last 6-12 hours) the greater the fat usage post-exercise—and I mean two or three times more. I guess they missed that part.

Exercise itself does not burn a significant amount of fat. It is the contribution of exercise to a person’s total daily energy expenditure (TDEE), including the intensity, that affects fat loss. In other words, exercise simply adds to your daily calorie burn and as long as you do not eat more to compensate (keeping your intake below your needs) your body will lose fat. But to our point—if you break the fast before you come into the gym you will perform better, enhance recovery and burn more calories. The higher the intensity of your workout (which you can now perform thanks to having filled your energy stores), the more fat that is used throughout the day in order to fill your energy deficit.

Don’t forget, I did not say anything about adding food calories to your day. All you are doing is spreading them out further which has additional benefits such as using more calories to digest each meal (10 percent of your TDEE comes from digestion of food), and giving your body a more steady stream of nutrition (enhancing recovery and energy). Finally, eating to break a fast fires up the metabolism. That’s why you are generally hungry sooner when you do eat a breakfast than if you skip it—the body is revved-up and ready to go.

All that said, managing total daily energy intake (TDEI) makes the greatest contribution to reducing fat stores. Consuming an appropriate meal before any workout probably has little affect on fat loss in comparison to alterations in TDEI. Your common-sense conclusion (before you ever read anything) was the right one—do what comes naturally.

On the other hand, eating before you work out is mandatory for performance athletes in order to enhance each training bout, recovery and the final outcome. Therefore, it seems obvious that ingesting part of your energy requirements before you train is only logical for the reasons of:

1. Filling energy stores before a workout (not adding daily calories, just redistributing them)
2. Breaking the fast to bump back up the metabolism and continue a constant flow of nutrients;
3. Increasing workout performance which will use more calories and allow for a higher intensity workout that burns two to three times more fat throughout the day following exercise
4. Enhancing recovery to improve maintenance or growth of muscle which also adds to the metabolic rate; and finally,
5. Increasing spontaneous physical activity (SPA) from never being in a fasting state beyond rising in the morning; your body naturally wants to move more.

So, eat before you train. The common sense that is innate to us would have easily told you to eat if you have not eaten for the last 6-12 hours and you are about to perform an activity that requires more energy than anything else you do all day. It takes calories to burn more calories.

When you train first thing in the morning like I do, simply use your meal replacement shake appropriately. The liquid delivery allows rapid absorption so it’s working almost immediately. And remember, you are not adding calories you are re-appropriating them.

THE NEXT QUESTION YOU MIGHT GET COULD BE SOMETHING LIKE, "WHAT TIME OF DAY IS BEST TO TRAIN?" NOW THINK: FIRST REACTION—COMMON SENSE.

Here's an additional discussion on when to exercise.

When to exercise
Suggestions that morning workouts lead to more fat loss are often made because of a partial understanding of the "afterburn" induced by exercise. Intense exercise of a significant duration may cause Excess Post-exercise Oxygen Consumption (EPOC) or afterburn. The contribution of EPOC to 24-hour energy expenditure is still debated, but several studies have shown extra calories are burned after an exercise bout because of the exercise. This indirect expenditure of energy has been shown to last from 30 minutes to many hours post-exercise.

Often claims are made that if one exercises in the morning they will have a faster metabolism for the rest of the day. The time of day for the exercise should not affect the intensity (amount) of EPOC since extra energy is extra energy.

Exercise performed at various times throughout the day burns the same number of calories as long as heart rate and duration are held constant. The number of calories a person burns during exercise is a product of their heart rate and the time spent at that heart rate. A person exercising for 30 minutes at 165 BPM will burn the same calories whether it is performed at 8:00 p.m. or 8:00 a.m. One study on time of day showed only one significant physiological difference; body temperature is higher at night.

Perceived exertion was found to be higher in the morning along with a slower rise in body temperature - which may lead to a decrease in exercise intensity. This suggests that a longer warm-up may be necessary in the morning. The best time of day to exercise therefore remains the time of day one is most likely to do it for those seeking fat loss. For those seeking performance goals other factors play a role such as the time of day when energy levels are highest or performance can be optimized.
The second area of discussion of whether to eat before a morning workout. The typical argument for morning exercise on an empty stomach is that it will result in more fat burned. The explanation is that after a night of sleep, glycogen stores (in the liver) and blood sugar are low. If one performs cardio exercise before eating, the body will use a higher percentage of calories from fat and that this translates into more fat lost for a dieter.

The first part of this argument - that more fat is oxidized as a result of little carbohydrate availability - is easily supported by science. Research shows that in a fasted state more fat is used during cardiovascular exercise, but if food was consumed in this case carbohydrates before exercise, there is a greater reliance on burning carbohydrate rather than fat. Why? Carbohydrates are preferentially oxidized or burned when present.

The insulin response from carbohydrate ingestion reduces lipolysis, fatty acid release and fat oxidation. In fact, what one has consumed in the 24 hours preceding an exercise session can affect fat oxidation. Looking at fat oxidation by itself during an exercise session to determine the efficacy of the session for fat loss misses the lion's share of the story. The respiratory quotient, or RQ, is the "ratio of the oxygen inhaled to the CO2 expelled by the body." RQ is influenced by macronutrient percentages in the diet and tells a scientist is how much fat or carbohydrate oxidation changes when all else is held constant. In addition to diet, a negative energy balance will offset the RQ and decrease it in favor of fat oxidation.

Carbohydrate and protein oxidation are both directly coupled to acute changes in their intake. In other words, a diet high in carbohydrate will increase carbohydrate oxidation (RQ) and decrease fat oxidation. A large increase in protein consumption will create the same compensatory decrease in fat oxidation. Whether these changes in macronutrient intake increase fat stores on the body depends on energy balance.

So what does all this mean if one just wants to know what to eat before morning cardio? Just because a study shows fasting participants had a higher fat oxidation during exercise than fed subjects does not mean that an equivalent amount of body fat won't be lost at the end of the day as long an equivalent amount of calories were expended.

Consider a study by Schneiter, et al., which showed that the percentage of fat burned by those exercising in a fasted state is higher than when the participants ate before exercising.

Both groups consumed the same calories during the eight-hour study period. The only difference between the two trials was that one trial was done in a fasted state with all food consumed after exercise. The other trial was performed with the same number of calories consumed before exercise. The biggest result from this study was that total calories expended during exercise and the eight-hour study period was the same in both trials. The meal-before-exercise group burned slightly more carbohydrate during the eight hours and the exercise-before-meal group burned more fat. Keep in mind that the groups did 45 minutes of high-intensity exercise (~8 METs) yet the fasted group burned only 10g of fat more in the eight-hour study period than the group that ate before exercising. At this rate, it would take 45 days of exercise to burn 454 grams of fat with the differences between the groups! Thankfully, this is not where the bulk of fat loss resides for a typical client.

In reality, while the percentage of fat used during exercise may vary depending on how much carbohydrate is available, the most important thing for a client seeking fat loss is how many calories are burned during their training. The more calories used during exercise, the greater the total energy expenditure will be for that day, and the more stored fat will be lost at the same calorie intake.

An example: Amy is an individual with a fat loss goal and maintenance calorie of 2000 per day. She eats 1500 calories daily and exercises four times per week. If she follows a 20-60-20 diet, she will consume 60% or 900 calories of carbohydrate daily in addition to 600 calories of protein and fat. If she wakes on Monday and exercises on an empty stomach for 30 minutes at 65% Vo2 max, she will burn fat and muscle glycogen as illustrated in the above studies.

Throughout the rest of the day she has 1500 calories to consume. She will use about 500 calories of stored fat to make up for the energy deficit. If she eats 300 calories of carbohydrate on Tuesday before exercising with the same duration and intensity, she burns much of the exogenous carbohydrates and less fat during exercise than on Monday. She now has only 1200 calories to eat for the rest of the day and is still in a 500- calorie deficit. The result is that she will burn more stored fat during the post-exercise period on Tuesday than on Monday in contrast to the extra stored fat lost during exercise on Monday. The loss of stored body fat for Monday will be identical to Tuesday because her energy intake and expenditure is identical on both days. If the eight-hour study by Schneiter were performed on individuals in an energy deficit over a 24-hour period, the 10-gram difference would have been erased as the energy deficit forced the use of stored fat throughout the day.

The bottom line for whether or not an exerciser should eat before a morning workout comes down to preference. Some people need to eat before they exercise or they get lightheaded, dizzy or fatigue quickly. Others just feel better with some food in their stomach, which may lead to a higher-intensity workout and more calories burned not only during the routine but also afterward (excess post-exercise oxygen consumption, or EPOC). For those who consume food before exercising in the morning, they need to be aware that certain types of food might have a negative effect on their performance and should avoid too much or certain foods that upset the stomach.

Athletes have long known that performing cardiovascular exercise for more than an hour at a high intensity may require carbohydrates to sustain the activity. A study by Schabort, et al., concluded that subjects who consumed a breakfast before exercising had an increase time to fatigue compared to the fasted subjects. Since most dieters seeking fat loss never exercise to exhaustion, much less exceed an hour this is a moot point. This discussion does not change the fact that performance/endurance athletes need to be adequately nourished before and during endurance activity.

HIGH VS. LOW INTENSITY CARDIO:
The third area for discussion is which intensity to use for maximum fat loss. Keep in mind the first law of thermodynamics when considering exercise intensity. What is most important for fat loss is the extent of the energy deficit. The more calories burned daily in comparison to calories consumed, the more fat is burned to make up the difference. Again, taken out of context, individuals can miss the entire point of cardio, which is to increase calorie burn for the whole day.

Most studies show as exercise intensity increases the percentage of fat used decreases. I.O.W more carbohydrates are used for energy as exercise intensity increases.
At high intensities, however, it appears that more fat is being oxidized in total, albeit a lower percentage.

For the purposes of our discussion, enhanced fat oxidation from higher-intensity exercise doesn't matter if the subjects were in energy surplus. For exercisers in an energy deficit, what matters most are the total calories expended by the exercise.
Consider a recent study by Melanson that measured energy expenditure (EE) for 24 hours following bouts of both low intensity and high intensity exercise. Both bouts were performed until energy expenditure reached about 500 kcal. There was no difference in fat oxidation between groups. The biggest difference between these trials was the time to achieve the 500 kcal expenditure.

The males in the study exercised for 86 minutes in the low intensity group and 49 minutes in the high intensity group. The females in the study exercised for 112 minutes in the low intensity group and 66 minutes in the high intensity group. In other words, even though the low intensity group of exercisers took about twice as long to burn their 500 kcals, they burned no more fat than the high intensity group. The higher the intensity, the more calories are burned per unit of time. Such exercise expenditure contributes to 24-hour energy expenditure and allows for a greater deficit (fat loss).

The graph below show at high intensity (on right) you may burn 39% fat (82Kcal) and at low intensity (on left) you may burn 50% fat (73 Kcal). So even at high intensity you burn more fat, but a lower percentage AND you also burn more calories.

Summary
The bottom line to the intensity question is the higher the intensity; the more calories will be expended. The more energy expended per minute, the more efficient your exercise time will be for fat loss.