Should You Do Your Cardio on an Empty Stomach?
Many bodybuilders prefer to work out on an empty stomach. They base this on the belief that growth hormone release is inhibited by the presence of sugar released by recent carbohydrate ingestion. A similar belief exists among those doing cardio for fat loss. The most obvious extreme example of this is how it feels when trying to run after a big meal.
In the case of cardiovascular exercise for fat loss, this phenomenon is best understood in terms of the rate of fat oxidation. Fat oxidation (fat “burning”) produces much of the energy used in such exercise. However, after consuming carbohydrates, the primary source of energy shifts the sugar now in the bloodstream. Thus, it appears that fat is burned at a faster rate when the cardio is performed on an empty stomach.
Eating Before Exercising?
However, it’s not so simple. While “running on empty” may burn more fat, it may also reduce muscle mass. Studies have shown that cardio on an empty stomach uses more protein for energy than does running after a meal. Since protein is the building block of muscle, and more muscle generally translates into a faster metabolic rate, using protein as an energy sources can slow the metabolism. This could offset the increase in fat burning achieved.
It also appears that any gain in calories burned through fat metabolism may be minimal when considering the time and sacrifice necessary to achieve it.
Thus, it appears from the current state of evidence that those concerned most with total weight and fat loss are best advised to do their cardio on an empty stomach. However, those who don’t want to lose muscle mass should do their cardio with some fresh nutrients in their bloodstream.