Part 5: What Happens To Your Body When You Fast? – Exercising and The Rest
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
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.
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?
About Scott
Scott Kustes loves to cook and loves to eat. He started Real Food University to help you get maximum enjoyment out of the meals that you eat. To find out more about how he has rebelled against the fast food culture and counting calories or carbs, join the Real Food Revolution.
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Great work Scott! Thanks for helping to shed some light on Intermittent Fasting for everyone, as it’s such a great tool to use for fat loss/muscle gain as well as just a healthy and free (from 6x a day while carrying around tupperware containers or portioned off food) lifestyle of eating.
Out of the following two scenarios, is either more advantageous, or are they the same thing:
In both cases, let’s assume a feeding period of 12pm-6pm and a fasting period from 6pm-12pm. In one scenario, I wake up at 7am, workout around 11 and eat at 12. In scenario #2, I wake at 10am, workout at 11, and eat at 12.
Does being awake longer in the morning before breaking the fast allow my body to burn more fat as a fuel source, or does only the total fast time matter? Assume the number of hours of sleep are the same, with me going to bed three hours earlier in one instance.
Essentially, I want to know if that period after waking and before breaking the fast is more effective if it’s a little longer or not.
I work out twice per week, intensely, and have added 10-15 pounds lean since the beginning of the year. Strength is up in terms of what I can lift for three sets of 10 by 100-200% in most things.
I _always_ work out fasted. Yesterday, I was 31 hours in when I worked out (which always kills my hunger), had a very intense and high-energy session, and didn’t eat until a few hours later.
I’ve done a couple of experiments, with blood sugar while fasted, before and after workout, and also with a very cold water dip on this last fast that had me lose 4.5 pounds between 7pm and 9am the next morning.
Here’s my post about it, and there’s a link in there to the previous experiment with blood sugar.
http://www.honestylog.com/root/2008/08/the-hunger-returns.html
Thanks Scott. This series has been a fantastic and thought provoking synthesis of the research
Chris
Scott,
I lift better when I’m fasting. I also sprint and cycle better.
I like to try to set personal records on fast days. Last week, I surpassed my long-term goal of a double overhand grip 2X bodyweight dead lift while fasting. My grip, legs, everything just feels a bit stronger.
Scott, I have thoroughly and completely enjoyed this series. The idea of fasting is fascinating to me, and I really do think I’m going to give it a try. I especially appreciated the common sense concept that the constant processing that current trendy thought on eating many, many meals throughout the day, may not be to the advantage of your digestive system. Giving your stomach a break seems so intuitive.
Thanks for such an indepth coverage. I’ll definitely be featuring this series in an upcoming post. Thanks!
I am curious about what is meant by IF? Eating nothing at all? Is drinking limited as well? I am a Muslim and I fast for 15 hours but no liquids or solids at all, and I’m not sure how safe exercise is without water.
Just wanted to add that last week I was VERY sore after a hard workout the day before and one in the AM. I thought I would put IF to the test so I skipped dinner and got in an extra workout. The next morning (18hr IF) I felt GREAT, almost no soreness and pretty manageable hunger. This is probably going to wean me off advil. .
Asif, we are discussing IF in regards to food and caloric beverages here. That is, I drink only water, tea, or coffee during my fast. No soda (I don’t drink that anyway), alcohol, or other caloric beverages and no eating. I’m not sure exercising during a 15-hour water fast would be a good idea either.
DPS, good stuff!
Cheers
Scott
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This is awesome stuff ! Thank you so much for caring, to display all of that information for free. There is so much wrong information about health and fitness, and I know for some people it’s hard to obtain the truth. And to those who may read this. Scott Kustes articles on fasting is the truth to better health alone with proper diet. I use to be a powerlifter, Thank the Lord those days are gone because I was destroying my body.
From 18 to 21 that was my life I was eating about 10 meals a day, and I got up to weighing 300lbs at 6 foot tall. And yes I was not fasting at this point in my life, and one morning I woke up and said I’m tired of this! I started eating 4 meals a day and fasting for about 2 days a month, and In two months I lose about 55lbs! I was 21 when I started to change my life style of health and fitness. I’m 27 now I weigh in between 190 to 200lbs my body fat fluctuates 7 to 10 percent.
I fast now once a week sometimes up to three days, and I eat two to three meals a day. I switch how often I eat my meals I’ll eat two meals for about a month then switch back to three, and with a 6 hour period in between meals. I take no supplements just a multiple vitamin I eat freah fruits and vegetables, lean meats, & Oats. I just eat a ton of healthy foods, and yes I workout I run on average 30 miles a week & pump iron every day! By the other e-mails It’s a blessing to know that their are other peolpe who do the same 4 health & fitness & to those who are trying to better themselves in health in fitness this is the KEY! KNOWLEDGE IS POWER!!!