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Fasting and Endurance Sports

Posted Mar 06 2012 8:52am
I recently decided to fast for one day a month, as a way to rest my body and brain. Now I’m not talking about a hardcore fast with no food or liquids, but a modified fast in terms of lower calories. I've done it twice now. On the days I fasted, I had my coffee in the morning, tea at work, and had broth for lunch. Pretty minimal in terms of calories, but plenty of liquids, so dehydration definitely wasn’t a concern.
As these were on Mondays, my day off from training, it seemed like the opportune day for this test.

I already train in a fasted state, as I do not eat before most of my morning workouts and don’t always take in liquids beforehand either. Never really felt it to be a problem, but obviously this was a longer time period without solid foods.

Then it struck me. Tuesday mornings are a scheduled run – how would my fasting affect that? Was it dangerous? What could I expect?
Research was in order.
  • The rationale behind intermittent fasting is that the practice results in metabolic adaptations that reduce body fat levels and improve cardiovascular and brain health. There is some solid scientific evidence that intermittent fasting does indeed have these effects in certain animals. In addition to impairing endurance performance by reduce muscle and liver glycogen and blood glucose levels, intermittent fasting is likely to sabotage performance further by interfering with recovery from training. It is clear from the scientific literature that fasting after exercise retards the various recovery processes, which are inseparable from the processes by which athletes gain fitness in response to training.
  • Method Using a crossover design, 10 moderately trained, active Muslim men performed 60 min runs on a treadmill in the fasted (Ramadan, RAM) and non-fasted (Control, CON) state on two separate counterbalanced occasions. After familiarisation, four subjects performed their CON trial 1 week before Ramadan, while the other six subjects performed their CON trial 1 week after the Ramadan month. The subjects' last meals were standardised before their exercise trials. The 60 min continuous endurance running criteria test consisted of 30 min preloading run at 65% maximum oxygen consumption (Vo2max) intensity speed, followed by another 30 min time trial (TT) where subjects manually adjusted their speeds so as to cover the greatest possible distance. Conclusion Ramadan fasting has a small yet significant negative impact on endurance running performance, although the impact varies across individuals.
  • It may seem counter-intuitive, but endurance athletes actually have higher protein needs than their strength training counterparts. While carbohydrates are the best fuel source for intense training, including generous portions of protein in your diet as an endurance athlete helps prevent muscle catabolism during extended duration activity (Bridge, P. et al. (2002). Muscle protein catabolism during prolonged exercise versus 84hr fasting in endurance trained athletes. Journal of Science and Medicine in Sport, 5(4), 131.).
  • Early studies agree that fasting is detrimental to overall physical performance and to endurance performance in humans; however, a study in rats reported an ergogenic effect where time to exhaustion was increased by a glycogen-sparing effect of elevated free fatty acids in blood resulting from a 24-hour fast. Later studies on humans have also found a detrimental effect of fasting on exercise endurance, with the exception of 1 study which found no difference. The discrepancy between humans and rats could not be explained by level of glycogen sparing, mode of exercise, duration of the fast, physiological differences or level of training. The intensity of exercise, and a potential placebo effect of fasting, are possible reasons for the conflicting results. Despite reduced endurance performance, fasted humans are able to exercise and maintain blood glucose homeostasis; the specific cause of an earlier onset of fatigue during a single bout of exercise in the fasted state remains unclear.
Not an overly aggressive sampling, but once again, as with so many things relating to endurance, results seem to be a mixed bag and no clear benefits or long-term detriments were seen.

I felt fine throughout the morning and early afternoon both times. The first day, in order to be prudent, I decided to eat dinner. I didn't really notice a difference the next morning compared to normal rest days. The next month, I was planning on skipping dinner, but chickened out. A mental hurdle I still need to overcome.

I’m definitely going to continue fasting once a month and, eventually, will make it through a whole day, including dinner. If nothing else, it's good to: 1. Know the feeling of hunger, and 2. Know I can control my eating - maybe I can start doing it more consistently.
Ever fasted before exercise?
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