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Part 5: What Happens To Your Body When You Fast? - Exercising and The Rest

Posted Sep 01 2008 7:05pm

Here we are nearing the end of our journey through the physiology of fasting. It’s been an interesting, enlightening ride and provides further justification for incorporating Intermittent Fasting of some sort into one’s dietary regimen. One final little post on exercising while fasting and then we’re off to a Q&A session. Then we’ll have to find something else to talk about. I will warn that this post is far more speculative than the previous ones, having reached the end of the scientific studies for the most part.

Is It Healthy To Exercise While Fasting?
The quick answer to that question is, “If it wasn’t, I’d be dead.” The long answer is that it’s perfectly healthy. In fact, I’d bet that in an evolutionary perspective, exercising in a fasted state is much more natural than exercising in the presence of pre-workout shakes and BCAAs and other such things. Consider that it’s unlikely Ug was up looking for a wildebeest after just chowing down on flame-broiled emu. It’s more likely that Ug ate his emu, slept for awhile, ate some more emu perhaps, and when the food was gone and hunger struck, Ug got himself up and went back out looking for dinner.

Here are the results of one study comparing fasted exercise (of the endurance type) vs. carbohydrate-fed exercise:

Thus short-term training elicits similar adaptations in peak O2 whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

As you’d expect, the lack of carbohydrates in the bloodstream means less glycogen breakdown and more fat breakdown for energy. But beyond that study, the science is relatively sparse on meal timing/frequency and the effects on exercise. Many around CrossFit and the Performance Menu have reported very good results while fasting and working out.

I find that when I fast regularly, I am better able to recover from workouts. Any muscle soreness is much shorter in duration. While I’m not taking any blood markers and there could be a placebo effect, on the surface it makes logical sense. Since my body isn’t busy digesting all day, it is able to turn its full attention to healing damaged muscle fibers.

Other Effects
Absolutely nothing to do with this post, but very cool My speculation is that digestion is improved during fasting by giving the stomach a break. When you consider the lifestyle of our evolutionary ancestors, it doesn’t seem reasonable that our bodies are equipped to be in a mode of constant digestion, but that is exactly what the 5-meals-per-day eating plan provides, a constant caloric titration to “ensure stable blood sugar levels.” But digestion is a very energy-intensive process. Unfortunately, there are no studies that I can find specifically looking at “digestion”. In fact, I don’t even know what one would attempt to measure to look at digestion.

However, one thing that may be managed better by fasting is jet lag. A few months ago, this study dropped that showed:

When food is plentiful, circadian rhythms of animals are powerfully entrained by the light-dark cycle. However, if animals have access to food only during their normal sleep cycle, they will shift most of their circadian rhythms to match the food availability.

The study was done on mice, so there’s no 100% proof that it works in humans, but it seems logical.

Learning is also improved by fasting. If you’re ready for your eyes to glaze over, read this:

L-IFD [Long-term Intermittent Fasting Diet] mice showed an increase in low-theta-band oscillations, paired-pulse facilitation, and facilitation of long-term synaptic plasticity in the hippocampus with respect to mice fed ad libitum. In addition, we found an increase in the expression of the NMDA receptor subunit NR2B in some brain areas of L-IFD mice.

As Brian at Spartan Training explains, what this all means in layman’s terms is that the hippocampus is firing harder, indicating better short-term memory and spatial awareness. Further, synaptic strength is being enhanced, possibly by changes in NMDA receptor (a glutamate receptor) composition. Okay, so some of that wasn’t so lay, but someone else can run with it from this point to make it understandable.

Wrapping Up
First, I’d like to give a HUGE thanks to Chris at Conditioning Research for all of the studies he’s posted on fasting. These proved invaluable for me as a starting point in answering this question. Also, big thanks to Mike OD at The IF Life for this page of resources.

One more post to go to answer a couple questions that popped up during these posts. What thoughts are you having on the issue of Fasting?

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