Part 6: What Happens To Your Body When You Fast? – Q&A
Recapping The Goodness
First, I’d like to give a quick bullet-point run-down of some of the effects of fasting that I reported on in the last five posts.
- Liver glycogen levels are depleted within 8-10 hours. Muscle glycogen falls by 50% over 24-hours, even without exercise.
- After depleting glycogen, amino acids are recycled to be broken down for glycogen through gluconeogenesis.
- We see increases in three of the four hormones driving lipolysis, indicating a propensity towards fat burning. Somewhere around 12-18 hours, lipolysis becomes a major energy pathway, producing energy from body fat.
- T3 levels fall slightly, indicating a slightly lower metabolic rate. Urinary nitrogen excretion falls, indicating less catabolism of muscle proteins.
- Beta-hydroxy butyrate, hGH, and IGF all increase. Proteins that protect cells from stress also increase.
- Inflammatory markers decrease. Insulin sensitivity improves. AGEs likely decrease.
- Cancer protection increases, healthy cells are better protected from chemotherapy, and markers of heart disease decrease. General immunity seems to improve.
- Brain neurons are protected from stressors, BDNF increases (helps grow brain neurons), and the brain is better protected from Alzheimer’s and Parkinson’s Diseases. Fasting after a brain injury lessens the damage of the injury.
- Exercise during a fast shows a higher rate of fat burning for fuel.
- Learning is enhanced and jet lag may be reduced.
If you’re just joining us, you’ll be well served to jump back into the previous articles to learn more about the brief synopsis above.
On To The Questions…
Cortisol
Scott H asked:
This may be covered in a future part, but I was under the impression that increased cortisol (during fasting, or otherwise) is a bad thing (this is primarily from reading MOD’s blog and posts). Just wondering.
Cortisol is most commonly known as a stress hormone, one of the family of hormones known as glucocorticoids. And if you’ve listened to commercials for supplements like Corti-Slim, it’s also an all-around bad thing. But that’s not entirely true. I can’t think of a single hormone in the body that is a bad thing. Evolution would’ve selected against such a thing. In fact, without cortisol, you’d find it exceptionally hard just to get out of bed in the morning. The key is to keep stressors acute, rather than chronic.
Let’s look at this a different way. There’s another hormone that has received quite a bad rap in the past few years: insulin. So is insulin good or bad? It depends. Insulin is necessary for life; without it, nutrients don’t get into your cells. However, when it’s constantly way above baseline levels, as in Type II Diabetes, it wreaks havoc, damaging arteries and keeping the body in fat storage mode. We want hormones to come and go, do their jobs and then retire themselves, not stick around.
In excess, cortisol can impair insulin sensitivity and cause additional breakdown of muscle protein for gluconeogenesis. However, cortisol is quite anti-inflammatory. The glucocorticoid family is also quite immunosuppressive as well, which bodes very poorly when dealing with chronically high levels of cortisol. Oddly though, slightly elevated levels during fasting are not doing major damage to the immune system since we’ve already seen that immunity is enhanced by fasting.
So I think an episodic shot of cortisol is likely not a bad thing. Given all of the hormonal changes arising while fasting, it’s possible that cortisol is involved in kicking some of them off. The key is to make sure it’s acute. Intermittent Fasting is an acute stressor. Fasting for two weeks is not acute.
Metabolism
Tony K wanted to know:
One thing I’m wondering about is what actually signals the body to notch down metabolic rate. I have heard people say that it’s the calories ingested, but I suspect it’s not that simple. I wonder if consumption of muscle protein does this for example. Do you know what triggers it?
As discussed in the second post, one of the effects of short-term fasting is a decrease in the thyroid hormone T3. We already know that T3 (and T4 to a lesser degree) is directly responsible for metabolism, so it’s not too hard to deduce what causes the metabolic rate to fall. T3 conversion falls, metabolism falls – simple as that. But why does T3 conversion fall?
Well, for that, we’ll look back up to the previous question and just say one simple word: cortisol. Cortisol blocks the conversion of T4 to active T3, along with promoting the production of a thyroid hormone called “reverse T3″. This mirror image of T3 has an empty iodine receptor; it binds to thyroid receptors in cells, but does not activate them. By binding to these receptor zones, it blocks the action of T3. It’s easy to see why chronic stress, and therefore chronically elevated cortisol levels, helps push one towards obesity. It’s not just the late nights at work with fast food for dinner (though that doesn’t help); the stress is actually causing a chronic depression of the metabolism, along with the immune system as seen above.
There’s another interesting effect that I found. A hormone called “Thyrotropin-releasing hormone” (TRH), produced by the hypothalamus, stimulates production of “thyroid-stimulating hormone” (TSH) by the pituitary gland. TSH tells the thyroid to create T4 and T3. During fasting, TSH-pulse amplitude decreases, though the frequency of pulses remains the same. That means less TSH is being released and therefore the thyroid is being stimulated at a lower rate.
There may be other effects going on that cause a change in the metabolic rate, but these seem to be the most prominent. Again, while it seems to be a negative, we’re dealing with acute changes in metabolic rate, not chronic depression. As this study points out, decreased T3 during fasting leads to energy conservation and decreased protein breakdown, beneficial changes for the organism in the short-term. As most IFers can vouch, once the lid comes off and the food starts going in, the body lights up like a heat lamp. If I add a good bit of medium-chain fats from coconut (oil, milk, etc), I can feel my body throwing off crazy amounts of heat, when just hours before I was chilly.
Intense Exercise While Fasted
steveSCENE of Becoming Adonis says:
Quick question though. Did you ever experiment with fasting during your endurance/weight training?
I don’t know what it is, but I can’t picture working out strenuously (either with weights, running, or otherwise) without having eaten for 12-18 hours. Is it something the body acclimatizes to over time?
Yes, I typically workout fasted. The only time I changed this was during the day-in, day-out glycogen-intensive work of sprint training. I would have a small snack of fruit, nuts, and hard-boiled eggs a couple hours before the workout. This seemed to help me keep the intensity higher. However, I’ve set many personal records on CrossFit benchmark workouts while fasted and I feel crisper, lighter, faster when fasted.
It’s going to largely depend on the type of activity you’re doing. Extremely glycogen intensive workouts may require a small boost beforehand, similar to my snack above. Lower intensity exercises shouldn’t require anything. That would be traditional “cardio” (at least in the way most people do it) and weight-lifting. This assumes you’ve adequately adapted your body to burning fat for fuel, which is key.
I don’t advise jumping straight into an 18-hour fast. It’s better to slowly increase the period of fasting to allow the body time to adapt. As the body adapts to fasting and switching between energy pathways, intense exercise becomes easier. Check out some of the comments on Part 5 to see some other people’s results with working out while fasting.
This Or That…
In the comments of the last post, Kyle asked:
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.
Now this is an interesting question. Basically, is it the fasting period that determines the body’s change in energy production or does when the fasting hours occur matter? In both scenarios presented here, we’re going from 6pm to 11am before a workout, a period of 17 hours. The only difference is a three-hour earlier wake-up in one scenario, 7am rather than 10am. The fast is broken at noon in both scenarios, immediately following the 11am workout.
My speculation, which is possibly completely off-base, is that there is little, if any, difference between the two scenarios Kyle has presented here. The total awake time and total sleep time are the same in the two scenarios, a three-hour shift being the only difference. As we saw in Part 1, just sleeping is enough to nearly deplete liver glycogen and a 24-hour fast with no activity depletes muscle glycogen by about 50%. So a 17-hour fast with sleep is going to leave the body with little in the way of glycogen and force burning of fat for fuel. Given that you can store 200-400g of glycogen in the muscles, at that point, muscle glycogen is probably still sufficient for any workout of under an hour. You’re still sitting on 100-200g, or 400-800 calories, of glycogen.
The stark reality is that there’s precious little evidence regarding exactly what is the best setup for Intermittent Fasting. Is it Alternate Day Fasting or is it a 15-hour fasting period or 18 hours or is 12 hours enough? The truth is that until real studies start rolling out comparing various fasting protocols, we’re all working with the “study of one.” Basically, test different scenarios on yourself and see what works. For instance, I’ve tried Alternate Day Fasting and found that if I’m going to be sitting at a desk at work, it makes me too lethargic to eat in the morning. Given that, I typically eat nothing while at work, but will sometimes grab lunch or a small snack in the afternoon if I’m really hungry. But if I’m going to be out and about (or “oot and aboot” for you Canadians), I can grab breakfast and have no feelings of lethargy.
So the end result is that you have to play with it for yourself and see what works best for you. What setup gives you the best energy for working out? What setup best helps you hit your target body composition?
Further Reading
Okay, so we got pretty science geeky here for the last couple weeks. I’m sure there have been more than a few eyes glazed over like so many donuts that you weren’t dunking in your coffee. Oddly, I’m weird enough to find this stuff exciting!
Anyway, if you have a hankering for even more science, check out Brad Pilon’s best-selling e-book Eat Stop Eat. This is a culmination of Brad’s graduate level studies in Human Biology and Nutritional Sciences. And depending on how big your hankering for more is, you can check out Brad’s Advanced package with several hours of audio files to really dig into the science behind IF.
If you have other questions or better answers to the above questions, have at it!
Table of contents for The Physiology of Fasting
- Part 1: What Happens To Your Body When You Fast? – Energy Production
- Part 2: What Happens To Your Body When You Fast? – Playing With Hormones
- Part 3: What Happens To Your Body When You Fast? – Inflammation and Blood
- Part 4: What Happens To Your Body When You Fast? – Healing and Longevity
- Part 5: What Happens To Your Body When You Fast? – Exercising and The Rest
- Part 6: What Happens To Your Body When You Fast? – Q&A
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Scott:
You mentioned getting chilly during a fast. Also my experience.
However, I’ve been experimenting the last few of fasts with excellent results. I’m still in the phase of trying to shed BF, so my fasting and workouts are rather intense (two 30-36 hour fasts per week, with a weight session somewhere near the end).
What I’ve been doing is that when I get really hungry, about the 10-12 hour point into the fast, and begin to feel chilly as well, I’ll get myself into some cold water. My gym has a 60 deg cold dip, so I’ll go over there and do 5 minutes or so, basically until I get a pretty intense shiver going. Or, a swimming pool for longer (they’re usually kept in the neighborhood of 80 deg).
Once I get out, dry off and wait a bit, I find that I don’t feel as chilly, and, my hunger is gone. Now, I’m starting my fasts with breakfast as the last meal when it used to be lunch to dinner next day. The reason is that if I eat breakfast at 9 or 10, I’m generally not hungry until around dinner anyway. So, now I trot over to the gym (I’d use the bathtub and a thermometer if I didn’t have that resource) in place of dinner, get a really good sweat going in the sauna, steam room, and Jacuzzi, then go in the 60 deg water. Kills my hunger until bedtime, and then that combined with the sleep keeps the hunger off until well into the afternoon the next day. Then I’ve only got a couple of hours with the hunger back prior to my workout, and then 5 minutes into that, hunger gone again until a couple hours after, at which point I feast.
I’d be interested to see if anyone can duplicate this. Regarding the cold water, it’s just like fasting in that it take getting used to. However, for me, enduring 5 minutes of that cold in exchange for hours of no hunger is highly worth it.
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@Richard: Wow, now that is a great suggestion, the cold water notion. Very cool.
I’ve been wondering about the “food panic” feeling I get, not when fasting, but when I’ve done extreme low calorie diets in the past. At some point late in the day I get that “gorge” reaction where I will eat the feet off the couch if the varnish has a trace of protein in it.
Do you get the same sensation during fasting? Only one way to find out……..
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I like doing exercise, and what i have readen in this page fasting is good. But i have a fear, while your doing exercise and fasting you will problably look like a skull or like models that have problems with food (bulimia, anorexia).
How fasting works, that really make us look and feel better ?
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So far I think this series is an excellent source of information about IF. Here in Europe and especially in Germany fasting is used as cleansing (of the testiness) and also curing of all kind of illnesses. Those aspects you didn’t cover yet…could you do that?
Another aspect is that the impact of IF may be enhanced bij doing IF at specials periods of times: Full Moon and New Moon. You may read something about that on a website based on very ancient knowledge about the phases of the moon and things you can do better in a certain moon phase, like have a hair cut, exercise, operations, curring wood and sowing etc etc: surf to http://www.paungger-poppe.com/ (also in English!)……
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Richard, I have never tried that. Interesting stuff. I don’t have a cold bath anywhere handy during the day though unfortunately so I can’t give this a try.
Carlos, read the comments on Part 5 of this series. You’ll see lots of reports of people getting better looking bodies when they incorporate exercise into fasting. Also, check out My Intermittent Fasting Success Story. I’m pretty far from looking bulimic or anorexic.
George, what aspects did I not cover that you’d like me to?
Cheers
Scott
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cleansing and curing
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Just wanted to say that I really enjoyed this series and it got me to take the plunge. I’m fasting now from 15-18 hours most days and it’s not a problem for me at all.
I have rheumatoid arthritis (an inflammatory condition) and my life is pretty high stress right now, so it’s been difficult for me to keep my condition under control. With dietary interventions I’ve been off all of my medications for a while now (under a doctor’s supervision no less) but I’ve had a lot of minor flare-ups recently. I wasn’t able to pull out of my latest flare until I started doing IF, and even though I’m not totally out of the woods, it has definitely helped. Thanks.
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” I’m fasting now from 15-18 hours most days”
“…I started doing IF…”
Fasting “most days” is not IF. That’s chronic. IF is about crating acute stressors, intermittently.
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The study posted about people fasting during Ramadan was people fasting for about 14 hours every day for a month.
Those people had significantly lowered markers in their systems for inflammation.
Since I have a serious inflammatory condition, I have to lower inflammation as much as possible.
I believe that we are not supposed to eat regularly throughout the day. Digestion is a huge strain on the body and is all tied up with our immune systems. I think lots of small meals contributes to insulin resistance, especially when those meals are higher in carbs.
It’s intermittent because I’m not fasting for days on end. That’s the way I look at it, anyway. I change it up by fasting for a different period of time each day — sometimes 14 hours, sometimes 20 hours. Whatever feels right to me.
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When your body burns fat, what happens to the waste fat? When you eat food, we know what happens to the waste. We flush it. But, I wonder what about fat, does the body completely use it or is there any waste to it?
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Susan, I do the same. I basically just eat when I’m hungry. Some days I eat 3 meals, some days I eat 1 or 2. It’s rare that I eat more than 3 times though.
Jeff, not sure what you’re asking. There may be a few metabolic breakdown products of fat metabolism, such as ketones, but your body will either use those or pass them out in your urine. The body is too smart to waste much in the way of fuel.
Cheers
Scott
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This is a question for the author…I am not sure if you have covered this or not in your writings….
I am wondering what the effects of fasting and going without water while exercising or doing manual labour for 8 hours or more?
Thank you,
Peter
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Mike OD Reply:
@ Peter – Scott wrote the article….but I’ll throw in my $0.02. Fasting for extended periods of time while doing manual labor may result in more increased muscle wasting (than just fasting and not doing any intense exercise). Going without water is also not recommended. There are many variables but if you are increasing the energy requirements of your body, not providing any additional nutrients, it will take the energy from within which can be stored fatty acids for lower intensity movements but can also involve the breakdown of muscle/amino acids for fuel as well. Of course this is furthered if this is a daily routine vs just once in a while and you are eating around it normally.
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Hi I am new to IF and hungry to learn more about it. I first did an 18 hour fast then did a few 14 hour fasts (based on Leangains philosophy). Someone told me that 14 hours was ok for CR, but not for the hormonal benefits. So today I did an 18 hour fast and the last 2 hours were really tough. Once the fast ended I really wanted chicken and pasta, but had to settle to yogurt and toast. I was ravenous after 18 hours unlike the tepid feeling of a 14 hour fast.
Richard remarked that fasting everyday is not IF. I would like to know what he thinks about 14 hour fasts most of the week and two 18 hour fasts during the week. I want to lose weight and curb my bad snacking habits (which IF helps with).
Thank you!
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Mike OD Reply:
@Mary – There are many hormonal benefits with using intermittent fasting. The biggest ones being lower blood glucose/insulin levels. IF usually hormonally speaking kicks in around the 15-16 hr mark and then progresses from there. You can also make great strides with your weight loss with eating right the other times as well. A “real” intermittent fast means…..”intermittent”, so not every day. But that doesn’t mean that using condensed eating windows IF style will not be effective for weight loss and health. I suggest going here to get the full report series on Intermittent Fasting which may help explain it more.
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Thanks Mike,
I have a new question about IF– If I am doing two 18-24 hours fasts a week what would you think the non fasting days would be like? I find I’m still not eating for a certain period of time like 12 or 14 or 16 hours. Is that bad? Is that chronic fasting? I don’t want to negate to good things IF can do for me.
I did download the free book thanks!
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Mike OD Reply:
@Mary – Think of it this way….you fast everytime you go to sleep and wake up. Don’t sweat over it too much. It’s more important to make sure you are eating healthy and enough calories (without going into a starvation mode). Not about starving yourself afterall, just getting some deficit and burning fat.
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Just a few short comments on some claims here. Sorry if this has been covered somewhere among the comments, I haven’t read through them all.
“Liver glycogen levels are depleted within 8-10 hours. Muscle glycogen falls by 50% over 24-hours, even without exercise. ”
False on both accounts. Liver glycogen is completely depleted in approx 28 hrs (Cahill et al). And the second statement is also incorrect. In humans, muscle glycogen is barely affected at all after 24 hrs, assuming no strenous activity.
“After depleting glycogen, amino acids are recycled to be broken down for glycogen through gluconeogenesis. ”
This is an ongoing process, but liver glycogen does not need to be depleted in order for amino acids to contribute to the maintenance of blood glucose. The longer the fast, the greater the contribution – for example, after 16 hrs, aminos will start contributing more than 50% to the amount of glucose in your blood stream. After 24-28 hrs, 100%.
“We see increases in three of the four hormones driving lipolysis, indicating a propensity towards fat burning. Somewhere around 12-18 hours, lipolysis becomes a major energy pathway, producing energy from body fat. ”
Yes. In fact, after an overnight fast, 2/3 of the energy burnt are free fatty acids. Eat breakfast and you’ll be putting the brakes on this process, of course.
“T3 levels fall slightly, indicating a slightly lower metabolic rate. Urinary nitrogen excretion falls, indicating less catabolism of muscle proteins. ”
No. Are you looking at rats now again? Because the downregulation of T3 takes more than 72 hrs to occur ih humans. A short fast in the 16-24 hr range certainly doesn’t impact negatively.
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Thanks for the info Martin.
Cheers
Scott
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Hey Martin,
Had a chance to actually read through what put up there. I remember when I wrote this a little over a year ago that you wrote me asking for the source and I pointed out that it was a rat study. Looks like I forgot to go back and change the post.
Thanks for clarifying a couple things. I’m going to update the series where applicable and give you credit.
Cheers
Scott
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No problem, Scott.
Rodent studies on metabolism/nutrition are often used as an example to what occurs to humans under similar circumstances, but we’re actually quite different in some instances.
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