|Eating or Fasting for Fat Loss: A Controversy Resolved
Len Kravitz, Ph.D.
Article Reviewed: Paoli, A., Marcolin, G., Zonin, F. Neri, M. Sivieri, A., and Pacelli, Q.F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International Journal of Sport Nutrition and Exercise Metabolism, 21, 48-54.
The integration of exercise and diet is essential for optimal exercise performance. As well, the proper manipulation of diet and exercise will provide a most favorable environment for achieving weight loss. A long-standing controversy for fat loss is whether a person will burn more fat by fasting in the morning prior to training. This proposed fat loss notion is based on the fact the glycogen (stored form of glucose) storage is much lower from an evening of sleep and that exercise before eating may enhance fat utilization during the workout. Paoli and colleagues (2011) decided to resolve this particular debate with their study.
Subjects: The subject volunteers in this study were 8 trained men (average age, weight, height was 27 yrs, 207 lbs, 70 inches, respectfully).
Testing: The testing for this study was completed over a period of 2 weeks. Each of the male subjects did a fasting (FST) and a food-fed breakfast (FED) test, with one week separating tests. Subjects did a 36-minute cardiovascular workout at 65% of their heart rate reserve on a treadmill. A graded maximal cycling ergometer test was initially completed to determine their actual maximum heart rate. This ensured that the subjects did the exercise sessions at the same workout intensity. Calorie expenditure data (via oxygen and carbon dioxide gas analysis) was collected 12 and 24 hours after each test condition. In the FST condition, the subjects performed the morning workout without food consumption for the previous 12 hours.
Subjects had a normal breakfast after the workout. In the FED condition, the subjects did their training after first having a normal breakfast. Pre-exercise gas analysis was collected in the morning in both the FST and FED to establish base-line data. With gas analysis the authors additionally assessed the respiratory exchange ratio (RER) data. The RER is a meaningful scientific method to measure the contribution of fat and carbohydrate of calories being utilized during any particular period of time (i.e., pre-exercise, during exercise, and post-exercise). RER is determined by collecting the volume of carbohydrate expired per minute (VCO2) in relation to the volume of oxygen consumed per minute (VO2). The subjects were instructed to eat the same diet the day prior to the study and during the entire day of each test session. Thus, food intake in both conditions (FST and FED) was the same, with the timing of breakfast being the only difference (FST condition ate breakfast after the morning workout). The researchers evaluated the dietary intake of subjects using well-established dietary software to ensure subjects complied with the diet instructions.
FST Condition: After completing the pre-exercise (or baseline) data collection, the subjects completed the 36-minute treadmill exercise and then had their breakfast after the workout. During the rest of the day the subjects consumed a standard lunch, afternoon snack, dinner and breakfast the next day. The researchers collected calorie expenditure data at 12 hours and 24 hours post the workout.
FED Condition: In this condition, the subjects did precisely the same schedule as the FST condition, however they had their breakfast prior to the workout. The breakfast was a standard Mediterranean breakfast (25% protein, 22% carbohydrate, 53% fat). This study was conducted in Italy where this diet is considered normal for a large segment of the population.
Oxygen Consumption and Caloric Expenditure
At both data collection times the oxygen consumption was significantly (from the statistical analysis) higher in the FED state. Oxygen consumption is a measure of energy expenditure. The more oxygen being consumed the greater the macronutrients that are being used, and thus the higher the caloric expenditure. In the FED state the subjects were burning slightly more calories at 12 hours and 24 hours after the same 36-minute treadmill workout. For more on post-workout oxygen consumption see Side Bar 1.
Respiratory Exchange Ratio Results
Figure 2 depicts the post-workout 12Hr and 24Hr respiratory exchange ratio (RER) results of the FST vs. FED conditions. At both data collection times the RER was significantly (from the statistical analysis) lower in the FED state. RER is a measure of macronutrients being utilized by the body. During exercise, carbohydrate and fat are the primary fuels. Protein is used very sparingly during exercise, typically during the final stages of exhaustive bouts of training. As RER gets higher, the body is utilizing more carbohydrate for its energy needs. As RER gets lower, the body is utilizing more fat for its energy needs. So, interpretation of Figure 2 shows that in the FED state the RER is lower, and thus the subjects were burning more fat as fuel at 12 hours and 24 hours after the same 36-minute treadmill workout.
Practical Applications for Exercise Professionals
For decades, some exercise 'gurus' have been proclaiming that to lose more fat enthusiasts should do the morning workout in a fasted state. This research clearly refutes this claim. Indeed, if a client seeks to burn more calories and more calories from a fat source, it is recommended to eat a light breakfast prior to the morning workout. There is an increase in metabolism (all chemical reactions in the body to liberate energy that is measured by oxygen consumption) and reduction in RER (thus burning more fat as fuel) after the training session. Encourage clients to eat or drink something easily digestible at least 20 to 30 minutes (or up to one hour) before the morning workout. Because glucose is the preferred energy source for most exercise, a pre-exercise morning snack should comprise foods that are high in carbohydrates, which are easy to digest for the client. This includes foods such as fruits, breads, energy bars and energy drinks. Make sure the client also drinks some water prior to the workout so that she/he is properly hydrated. Fed vs. Fast-a controversy 'busted' and resolved.
Side Bar 1. E.P.O.C. The Post-Workout Energy Expenditure 'After Burn'
One of the variables of interest of this study was the comparison of the excess post-exercise oxygen consumption or E.P.O.C. This represents the oxygen consumption, or energy expenditure, above the baseline (or pre-exercise) level that occurs after an exercise bout. It is sometimes referred to as the exercise 'after-burn', implying the burning of calories after the workout. The body is restoring itself to pre-exercise levels and utilizing additional energy to complete this process. Studies have found that the magnitude (amount of elevation in oxygen consumption) and duration (length of time the oxygen consumption is elevated) of EPOC is primarily dependent on the intensity and duration of exercise (Borsheim and Bahr).
Borsheim, E. and Bahr, R. (2003). Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sports Medicine, 33(14) 1037-1060.