Intermittent Fasting: Feast or Famine for Endurance Athletes?

One of the trends among the high fat community is intermittent fasting. If you haven't heard of it, check out this great overview from Precision Nutrition.

Quite a few people have asked me if I use intermittent fasting as a way of improving my fat metabolism. Now, I don't want to debate its effectiveness or whether it should be used—there are countless arguments on both sides, and there are too many individual nuances to make generalizations. However, I'd like to discuss my personal experience and why I'm cautious with fasting in my own training.

The length and the regularity in which people use intermittent fasting varies. Some people do shorter lengths with more regularity (e.g., five days a week for 12-16 hour fasts), while others do longer in length with less regularity (e.g., once every two weeks for 24 hours). Whichever way you look at it, the goal is to give the body the opportunity to utilize fat as a fuel for prolonged periods of time in order to become more efficient at it. Personally, I would agree that, in a healthy person who is not underweight, is fat adapted, and does not partake in high volume training, this could be a good tool. But I don't think it's for me. Here's why:
Let's say that I decided to subscribe to fasting 12-16 hours, five days a week. Of course, sleeping at night (8 hours) would take up the bulk of the fast. Let's see how this would pan out:
  • Stop eating at 5pm
  • Go to bed at 9pm
  • Wake up at 5am
  • Run 15 miles fasted at 6am
  • Break fast at 8am
Normally, this 15-hour block of time would be a great window to train the body to utilize fat as fuel. The biggest problem I have found with this is the amount of calories I would burn during a 15-mile run (approx. 1500-1800) would translate to at least 20 hours worth of "sedentary activity." When this is put into the equation, I would be looking at the equivalent of 35+ hours of "fasted calorie expenditure." The margin of diminishing returns on fasted calorie expenditure of 35+ hours compounded five times a week seems to be an issue here. I would imagine there would eventually be some adrenal or other health issues caused by this.

To avoid undue stress, I typically break my overnight fast before my morning run on the days I do one. I don't eat a full meal, but I do aim to send a signal to my body saying that calories are not scarce. This usually takes place in the form of drinking my morning coffee or tea along with some combination of coconut milk, heavy whipping cream, Xendurance Xecute and/or 3Fuel, and taking a Vespa Ultra Concentrate. I do use raw honey from time to time, but this is becoming more rare and is heavily dependent on where I am at in my training cycle. All in all, the coffee/tea usually amounts to between 100 and 300 calories (this again depends on the workout time and intensity). During really heavy training blocks, I will also shorten the fast by eating a small snack before bed, or pushing dinner later. These meals are nearly always high fat, moderate protein, and very low carb (unless I happen to have just done a workout beforehand—then a few more carbs may appear in this meal/snack depending on what my next day's workout will look like).