Intermittent fasting and endurance training (part 2)

First off, there are lots of data about Intermittent Fasting (IF), training while fasted, nutrient timing, “train low,” glycogen depletion, etc… So, it’s easy to get confused when you begin to discuss the interaction between nutrition and training.

So, I’m going to make it simple and introduce “IF plus training” one step at a time. In this post, I present ONE aspect (the simplest aspect) of how IF can be incorporated with endurance training.

NOTE: This article compliments the video I posted on the ECo Insider group page and my Part 1 post on the health benefits of IF. If you are an ECo Insider member, refer to the video for more details on how I suggest incorporating IF and training. If you aren’t an ECo Insider member, join today to watch the videos, ask question, provide insight, and join in on the conversation!

Here are the conditions in which I frame my conversation. This scenario pertains to athletes who are:

  1. Endurance training
    • primarily aerobic training (<70% VO2 max)
    • minimal High Intensity Interval Training  (HIIT) or Super-High Intensity Interval Training (SHIIT)
    • minimal strength work
  2. Base-building period
    • off-season through the prep phase
  3. One training session per day

Over the next couple of posts, I’ll make the situation more complex (two to three sessions per day and adding strength work and/or HIT/SHIT work). But for now, let’s understand the basics.


The goal of endurance training is to increase efficiency (use of energy) and be more economical (less calories to do certain amount of work). Simply put, we want to be able to go longer faster (or faster longer) and use less energy doing so.

How do we measure this? In simplest terms, we look at the Fick equation to determine our “fitness.” The Fick equation states that the amount of oxygen we use at any given time is equal to the product of our cardiac output (CO) and the amount of oxygen our muscles can extract. So, to make this REALLY SIMPLE, there are two “things” we can modulate when increasing our fitness: 1) we can increase the amount of blood our heart pumps and 2) we can become more efficient at extracting and using oxygen at the level of the working muscles.

Incorporating IF into endurance training specifically targets point #2 (becoming more efficient at extracting and using oxygen). So, when I look at how I can manipulate nutrition to increase these variables, “training low” (glycogen depleted) is an attractive method. I’ll explain why…

NOTE: there are lots of other interesting things I could discuss related to this, but to keep things simple, I’m focusing simply on the effects of “training low.”

Data points to “training low” being an effective strategy for increasing the “extract and use O2 at the level of the muscle” side of the equation. Why?? Well, there are a couple of ways to increase this variable 1) increase capillarization (deliver more blood (more O2) to the muscles) and 2) increase mitochondria density (number) and/or size.

Exercising in the glycogen-depleted state (“training low”) increases AMPK, which is an “cellular energy signaler” that notifies the cells that energy availability is low. AMPK signals for the cell to increases fatty acid uptake and oxidation. AMPK also signals the cell to make more mitochondria (called mitochondrial biogenesis). More and larger mitochondria leads to increased oxidative capacity. In other words, exercising with low levels of glycogen can increase fatty acid uptake and also increase mitochondria density and size.

How do we make this happen?

Training LONG is one way – numerous studies show that extended endurance sessions lead to an increase in capillarization AND an increase in mitochondria number and size.

But exercising while fasted may have a similar effect!

And this was my primary draw to continuing to pursue “training low.” It doesn’t feel good. I can’t perform HIIT sessions as well. I sometimes don’t have much motivation to exercise. But… the molecular gains and potential long-term gains seem worth the short-term issues.

My strategy is to do my a.m. session fasted. When coupled with the 8-hour IF protocol, I’m essentially training “low.” In other words, my IF strategy has me in a glycogen shortage.

Some studies show that endurance training in a glycogen-depleted state can lead to improved performance compared to training with high glycogen, but care needs to be taken when applying this to your program.

Risks include: higher risk for overtraining, decreased performance in the short-term, and decreased high-intensity performance.

These risks lead to the importance of periodizing IF and “training low” as part of a complete picture. The “train low” approach appears to have a lot of promise in terms of substrate utilization and mitochondrial gains. However, it is clear that the “train low” strategy won’t cut it when it comes to ramping up the intensity (2 to 3 sessions per day) or actually fueling during race-intensity sessions and the competitive season. Therefore, care must be taken to periodize the “train low” strategy as part of a well-constructed long-term training program.


This is part 2 of a 4-part series on Intermittent Fasting and Endurance Training. Click here for Part 1.