Most people agree that exercise added to proper diet is the best way to induce and maintain weight loss. Several studies have shown the contribution to weight loss by exercise is quite small compared to the contribution by a low calorie diet. This article will not cover the diet portion of weight loss, but will instead discuss various aspects of exercise in relation to it. Specifically, there are three areas of debate concerning the ideal use of exercise for losing fat: the best time of day, the proper intensity, and whether to consume food beforehand.

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 of EPOC since extra energy is extra energy whether it is in addition to that required by a sleeping person or one working an active job. Many EPOC studies that measured RMR the morning after an exercise bout still showed an increase. The only area remaining where the time of day can influence exercise-energy expenditure is during exercise.

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.

Eating in the morning

The second area of discussion is 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. According to some, the way to test this is to measure RQ in relation to the food quotient (FQ). FQ is the ratio of "carbon dioxide produced to oxygen consumed during the oxidation of foods representative of the habitual diet." When the RQ to FQ ratio (RQ/FQ) is less than one, it means that calorie intake is less than 24-hour energy expenditure. When RQ/FQ is greater than one, it means the opposite.

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. This adds up to about a third of an ounce of fat. At this rate, it would take 45 days of exercise to burn one pound 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 probably 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 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.

Intensity

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 imbalance. The more calories burned daily in comparison to calories consumed, the more fat is burned to make up the difference.

The intensity issue arises from studies that show as exercise intensity increases the percentage of fat that fuels the exercise decreases. Most studies agree that between low (30% VO2 max) and moderate (up to 65%VO2 max) cardiovascular exercise, percent fat oxidation is higher up to 30% VO2 max, but total fat oxidation is higher at 65% VO2 max for the same time. The explanation is that more calories are burned per minute for moderate intensity exercise, which offsets the smaller percentage from fat.

At intensities greater than 65% - 70% VO2 max the pendulum swings the other way. For example, at about 85% VO2 max less FFA are available in plasma (Romjin) and less total fat is oxidized during exercise.9, However, it appears that the high intensity results in more fat being oxidized in the hours following the exercise bout.

Some studies on high-intensity endurance activity illustrate this Two such studies by Schrauwen et al and Tuominenet et al found exhaustive exercise in a trained person may also lead to increased fat oxidation up to 44 hours later. , Several resistance-training studies show increased fat oxidation for hours afterward including one showing RQ was still lower 15 hours after training. This is significant because during resistance training almost all energy is supplied by CHO. Some do not show enhanced fat oxidation or EPOC from high-intensity endurance exercise. The differing results may stem from the intensity and duration of the exercise or the number of hours after the bout that energy expenditure and fat oxidation were measured. For the purposes of our discussion, enhanced fat oxidation from higher-intensity exercise doesn't matter if the subjects were in energy balance. For exercisers in an energy deficit, what matters most is the 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 (LI) at 40% VO2 max and high intensity (HI) exercise at 70% VO2 max. 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 LI group and 49 minutes in the HI group. The females in the study exercised for 112 minutes in the LI group and 66 minutes in the HI group. In other words, even though the LI group of exercisers took about twice as long to burn their 500 kcals, they burned no more fat than the HI 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) at constant calories or a greater calorie intake at a given rate of fat loss.

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.

Summary

One benefit of exercise is improved conditioning. If one trains at low to moderate intensity, improved conditioning means an increased ability to use fat as a fuel Increased carbohydrate oxidation occurs from increased VO2 max at high intensities. An improved endurance capacity at both levels will increase time to exhaustion (Gollnik) Increased training state can also lead to a longer workout of higher intensities which may bring with it EPOC even though it probably accounts for less than 50 kcal/exercise session it is better than nothing.