Is fasting a good idea for triathletes?

Fasting is the willing abstinence or reduction from food for a period of time. Many religions fast, health enthusiasts preach it, and now it has seeped its way into endurance sports. But is it a good idea?

In a new study published in the Journal of International Society of Sports Nutrition, researchers took 12 healthy male athletes and had them carry out a caloric restriction nutrition plan. Over a period of six weeks, the participants practiced every-other-day fasting, decreasing their caloric intake by 33 per cent relative to their usual diets. The nutritional plan had a near identical distribution of carbs, fats and protein as their regular diet. Participants did both maximal exercise and blood tests before and after the restriction period to determine the effects of fasting.

Following the six weeks, the caloric restriction showed no changes to blood parameters related to iron metabolism, tissue damage, glucose levels, lipid profiles or fatty acid composition. The intervention did significantly reduce body weight, as well as fat and lean body mass. Post-intervention maximal exercise tests showed a decrease in energy expenditure rate and the energy cost to run one meter at various exercise intensities. The intervention also appeared to delay the onset of the anaerobic phase when exercising – the phase when you start producing lactic acid.

Though there was a significant increase in performance and energy efficiency, there’s a caveat. Blood tests did reveal a decrease in magnesium, potassium, zinc, folic acid and calciferol. Therefore, if you do practice caloric restriction, make sure you’re aware of any micronutrient deficiencies.

For people new to endurance sports, it’s recommended to begin fueled for your workouts before introducing fasted or depleted exercise.