Negative energy balance -- that's the goal if you want to achieve weight loss. Some say you can facilitate this process by exercising on an empty stomach. The reasoning is that your glycogen stores are low from fasting; therefore, your body will have to tap into your fat stores to provide energy to fuel the exercise.  It seems plausible, but evidence from a small study conducted in Italy suggests the contrary.

Researchers evaluated the differences in fat metabolism between training in the fed and fasting states. Each subject underwent two testing conditions. They were randomly assigned to exercise either under the fasting or fed condition first. They then returned to be tested in the opposite circumstance, with an interval of one week between testing situations.

In the fasting scenario, subjects exercised in the morning after a 12 hour fast; they then ate a standard breakfast. In the fed condition, subjects ate a standard breakfast before exercising. In both testing conditions, all subjects exercised for 36 minutes on the treadmill at an energy expenditure that was equivalent to 65 percent of their heart rate reserve. Oxygen consumption was measured before, during and at 12 and 24 hours post exercise. Substrate utilization (e.g., fat versus carbohydrate metabolism) was estimated from the respiratory exchange ratio (i.e., the amount of carbon dioxide produced to oxygen consumed).

The results indicated that although an increase in carbohydrate metabolism occurred initially under the feeding condition compared to the fasting state, metabolism later shifted toward utilizing fat stores at 12 and 24 hours post exercise for the fed scenario. Furthermore, oxygen consumption values remained higher at 12 and 24 hours post exercise in the fed state, indicating that metabolism was faster.

The investigators concluded that eating prior to exercise will result in a greater improvement of fat utilization over long periods when compared to exercising on an empty stomach. It should be noted, however, that the subject sample of this study was quite small--only 8 men were tested. Further research with a larger sample size is warranted. 

An important point is that individuals react differently. You may need to experiment with the timing of meals and the type of foods consumed in order to find the weight management protocol that works best for you.


Note: Prior to beginning and exercise program or increasing the intensity level of a current routine, a physician's approval should be obtained, especially for older adults or those at risk for or who currently have chronic health conditions.

Reference
International Journal of Sport Nutrition and Exercise Metabolism, 21, 2011, 48-52, Antonio Paoli, et al.