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Part 6: What Happens To Your Body When You Fast? - Q&A

Posted Sep 07 2008 2:01am

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…
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.

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!

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