Food is fuel, especially for serious runners who need a lot of energy. It may seem counterintuitive to fast before a workout, but some proponents claim it can benefit your performance. Fasting can also be dangerous, especially when combined with strenuous physical activity. Always consult your physician before making any extreme changes to your diet plan.
A fast generally refers to a time during which you eat no solid food. You can do a juice fast or a true fast, which consists of just water. Fasts can last from just 12 hours -- such as overnight -- to as long as two weeks. A practice called “intermittent fasting” involves alternating days of water fasts with days of full meals.
A study published in the July 2010 issue of the “Journal of Science and Medicine in Sport” found that participants -- especially men -- who exercised after an overnight fast experienced greater training-induced improvements in the ability to use oxygen and energy stores in the muscles. The study looked at the effects of endurance cycling over four weeks rather than running, however. A 24-hour fast prior to running can increase your body’s ability to use fat and lowers the rate at which your body takes glycogen, or energy, from the muscles, noted a landmark study published in 1986 in the “Journal of Applied Physiology.” Endurance runners might fast before a run to train the body to use fat for energy, but these fasts last just several hours, not several days. Proponents of longer fasts claim fasting can increase your energy levels because you use less effort for digestion and can then direct that energy to your exercise efforts. A 2003 study of intermittent fasting, published by the proceedings of the National Academy of Sciences, details some potential benefits, including better memory and a potential decrease in your risk of some diseases. No studies have looked at the benefits of intermittent fasting or long fasts on running performance.
Fasting can leave you with little energy in your tank. You may not be able to go as long or as fast when you have not eaten for a long time. A 1997 issue of the “International Journal of Sports Medicine” published a study indicating that a pre-exercise meal consisting primarily of carbohydrates improves runners’ endurance. An August 2011 edition of the same journal published another study showing that eating the right carbohydrates the day prior -- meaning not fasting -- can significantly improve race day performance among elite marathoners.
Even short overnight fasts before a run can result in nausea, dizziness and weakness. Long-term fasts can lead to nutritional deficiencies and could cause your body to burn muscle for fuel. Burning muscle could leave you weaker in the long run and less able to tackle hills and speed. No scientific studies support fasting as a way to promote health. Fasting can be dangerous for runners with particular medical conditions, such as diabetes.