Training Board is…

Training Board is basically my gym notebook.

In theory, I can also blog in a more public spirited way about weightlifting, nutrition, weight loss, etc. But mostly, I just use this blog to track my workouts. And surfing.

If anything I have posted seems useful to you, let me know with a comment.

About Me

I'm a level I "sports performance certified" USAW coach, and I train, and work as a trainer at Asheville Strength and Conditioning, a great little gym here in Asheville, N.C.

I work with clients who want to get strong and fit, and am especially happy with helping older or detrained individuals.

My own training now is more focused on developing strength and using basic lifts and classic strength and conditioning techniques. A lot of the older portions of this blog deal with CrossFit and reflect my journey into fitness self-awareness. I first discovered CrossFit, back in late 2008. I learned a lot from CrossFit's "fitness as sport" model of training -- and benefited tremendously from training with my friends at CrossFit Asheville -- and furthermore I must credit my interest in CrossFit for opening my eyes to the larger world of training for strength athletics, Olympic-style weightlifting, powerlifting, and other arcane approaches to moving the Iron around.

Support Wikipedia

Support Wikipedia
mcbalz's items Go to mcbalz's photostream
Health Reports • March 16, 2010, at 10:39 pm

The Physiology of Fasting

Today I completed a 32 hour fast. I enjoy fasting, and plan to continue using it in my life as a tool for focusing, cleansing, and recuperation. Yet fasting remains something of a mystery to me, and I want to understand the practice better. After my last fast, I promised to begin doing some research in the medical literature to try to understand the physiology and health benefits of fasting. Today, in an effort to keep my promise, Training Board presents the first in a series of short notices — making no pretense to a comprehensive scientific treatment of the subject — in which I examine and report on some aspect of the medical literature related to fasting.

Review: Kernt et al., Fasting: The History, Pathophysiology, and Complications

This time round I’ve selected for review an article from The Western Journal of Medicine from 1982:

Peter R. Kerndt, MD, James L. Naughton, MD, Charles E. Driscoll, MD, and David A. Loxterkamp, MD “Fasting: The History, Pathophysiology and Complications,” Western Journal of Medicine 137:5 (1982) 379–399. PMCID: PMC1274154.

The article is available online here.

As it happens, this article is a good starting point for this series. The subject matter and focus of this piece is perfect for those who want to know more about the basic physiological processes involved in fasting, including any risks that may be attached to the practice.

One of the things I appreciate about the article is its sensitivities to the psychological, cultural, religious and political dimensions of fasting. The authors demonstrate that there is a long tradition of appealing to fasting as a mechanism of spiritual and physical cleansing, and as a generalized therapy for various disorders, both physical and spiritual, that afflict human beings. Citing a variety of studies and sources, the authors discus the spiritual use of fasting in religious antiquity (pg. 380) as well as its political use in protest and revolutionary movements (pg. 381). They also report on the history of interest in and study of fasting among western health-advocates and medical doctors (pg 380), who have studied fasting to shed light on starvation, and have experimented with therapeutic uses of fasting and ketogenic diets for treatment of obesity and convulsive disorders (pp. 380-381).

The study itself was also quite interesting. Much of the data was obtained by running weekly and daily blood tests on a 41 year old monk who wished to embark on a medically supervised 40 day fast. During the fast the monk took only communion, reckoned at 60 calories a day (pg. 381).

The article offers interesting answers to the kinds of questions I’ve been asking about fasting:

How long can a person fast?

We have enough glycogen stored in our livers for 18-24 hours of energy (385), so fasts of that length are not likely threaten life. People of typical body weight have enough fat stores to last several months without food (385). Under medical supervision, on hyper-low-calorie diets, people have been known to exist in a state of functional starvation for more than a year (380).

Where does the body get energy for activity during a fast?

Your body needs the equivalent of 150-200 grams of glucose a day to live, most of which is used up in the brain. In the absence of eating food, there are three primary stores of energy in the body, and three pathways for accessing them: stored glycogen in the liver (accessed via glycogenolysis), the protein in body tissues (accessed via gluconeogenesis), and stored fat (accessed via lipolysis/ketogenesis). In the initial period of the fast, your body rapidly depletes its glycogen stores (about 70 grams worth) from the liver (384-385). This is mostly completed after 18 to 24 hours and glycogenolysis ends entirely by day three (385). You also can convert stored muscle glycogen into blood glucose, but only after it is released into the blood as lactate, and taken to the liver where it is reconverted to glucose (386). In lipolysis, the body begins production of ketones from metabolizing body fat; a ketone is a molecule that can be used as a replacement for glucose in the blood and nervous system. On the other hand “gluconeogenesis” is the synthesis of glucose for the blood from non-carbohydrate sources in the body, especially amino acids. For the first 10 days, the body gets fuel from a steadily increasing activity of both gluconeogenesis and lipolysis, with a slight bias towards burning fat. After 10 days, gluconeogenesis drops off substantially, and a greater portion of the energy you expend comes from stored fat. In other words, while fasting, you do burn some muscle and protein tissues. But, in later stages of fasting the body relies more and more on ketosis. But the longer you fast, the more your body relies on fat rather than muscle for fuel (379; 384-385; 386). This spares your body from excess “protein catabolism” (and death from loss of bodily tissue). See figure 3 from the article, reproduced at right, above.

In truth, some aspects of the energy production in fasting are FAR more complex than I’ve described here; it starts to get over my head. In particular, gluconeogenesis is quite complex. Apparently the body likes to use the amino acid alanine to synthesize glucose in the liver; and muscle tissues synthesize alanine from other ingredients they have readily to hand, in order to release it for the liver to convert it. As the body enters ketosis, gluconeogenesis is reduced significantly.

How much weight do we lose while fasting, and what tissues do we lose?

On the first several days of a fast, people will lose 2-4 lbs of weight per day while fasting (1-2 kg); this is due mainly to a “negative sodium balance,” or “salt and water diuresis” — in other words we lose mostly water weight (384). Later, the body shifts to ketosis, and we lose more and more of the weight from body fat; but the authors hypothesize that it is the switch to ketosis which leads to the initial rapid weight loss through diuresis (384). After three weeks of fasting, people begin to lose about 2/3 lb per day of the fast (0.3 kg) (pg 379, 382, 384).

What are the effects on our hormonal system during fasting?

As we might predict, insulin levels drop as we cease to ingest carbohydrates. Concomitantly, glucagon levels rise (384-385). As insulin falls and glucagon rises, lipolysis is enhanced (388-389). Things are a bit different for the obese person, whose insulin levels remain higher (due, I would think, to the lingering effects of insulin-insensitivity). Levels of growth hormone also increase markedly during fasting and continue to rise as the fast is prolonged (389). There are some changes in thyroid hormones but they do not appear to be a serious concern (390).

What happens when we start eating again after fasting?

If we eat a lot of carbohydrates, even if we remain in a low calorie consumption mode, we immediately halt diuresis and sodium and water retention are induced, leading to rapid weight gain and even edema (swelling and inflammation) (384).

Does fasting feel good?

In my experience it does. Really my question has been, is there any medical basis for thinking that fasting produces a feeling of well being or not? The answer is yes. The authors note that lucid mental states, feelings of euphoria, and other psychological effects are frequently reported, and are hypothesized to be associated with ketosis and the release of β-endorphin (394). Some studies have suggested that there may be a build up of alcohols synthesized as metabolic byproducts during fasting, and that these accumulations could produce acute intoxication (394).

Are there any risks associated with fasting?

Obviously, there are few risks to short duration fasting. But as fasts are prolonged, your body goes through changes, some of which might be beneficial, and others of which are potentially deadly. Deaths — from heart failure, renal failure, and other weird causes — are sometimes reported during extended fasts (395). There is a whole list of “complications” that people who fast have reported (table 7, on page 395). People have every reason to be cautious about not eating for extended periods of time.

As you fast, your metabolic system slows down, as measured by, among other things, “bradycarida”, or in layman’s terms, a slower heart rate. Accompanying this slowness of heart rate is a steady decline in blood pressure the longer the fast is extended. These symptoms correlate with reduced levels of the hormone norepinephrine in the blood. There is some reason to be concerned that extended periods of fasting can also produce atrophy of the heart muscles, producing irregular electrocardiogram (382-383). Blood levels of some essential minerals such as potassium, magnesium, calcium, etc., are affected during fasting (392-393); but the authors do not note a risk associated with these changes. There is an increase in the production of uric acid during fasting, increasing risk of gout (394).


In short I recommend this article, especially for scientists and doctors or intrepid readers. It leaves many questions unanswered, such as how differences in dietary habits prior to fasting might affect metabolism during the fast; for instance, does an already lean person who already eats a relatively ketogenic high protein, high fat diet, more readily enter ketosis than does a person used to a high-carbohydrate diet? How does vigorous physical activity influence the fasting state? Is there a hierarchy of tissue breakdown for gluconeogenesis? or does the whole system suffer losses to protein based tissues equally? I’ll be returning to this article in the weeks ahead as I pursue the study of fasting in Christian antiquity… I want to take a look at some of those sources in the footnotes… but that’s another matter.

8 comments to The Physiology of Fasting

  • Do you do a crossfit workout after a fast, and then eat a steak with a sweet potato?

    I did my first real fast on Feb 17th (I’ll do a 48 hour one from 9pm April 1st-730pm April 3rd) and it wasn’t bad. It helped that I’d already learned how to restrict myself.

  • Why not? Sounds like an excellent plan.

    Father G one of the things I learned from this article is that our body tends to rely on a combination of protein and fat catabolism in order to meet energy needs during fasting, and that we flush water, salts, and metabolic by products from the system rapidly during fasting, especially the first couple of days.

    If you eat carbs at the end of the fast, there is a dual effect; part of it is good, part bad. First, you release insulin, which halts gluconeogenesis, which is good; muscle catabolism is not desirable for most athletes. So breaking the fast with a little carbs is a good thing. A secondary effect is to halt diuresis, which could cause rapid weight gain through water and salt retention, leading even to edema (swelling). So I would caution against overdoing the carbs at the end of a fast.

    It seems to me that the best approach would be to eat a Zone balanced small meal at the end of any fast, workout or not. Concentrating on low glycemic vegetable sources of carbs and clean protein and good fats.

    At the end of my fast, I started by drinking some lemon juice in water, then had a dinner of broccoli, cauliflower, a spinach salad with avocado, shredded yellow beet, and an olive oil and lemon juice dressing, some mashed sweet potato with coconut oil, and salmon. Later, for a late night snack I ate more meat and some fruit (apple and orange), and had some nuts.

  • ron landry

    please send me a copy of any reply to my email above..

    i just completed a 40 water/tea/honey fast. i ended the fast with miso soup that i added tomatoes, cucumbers, and tofu. had this throughout the 41st day. Second day of my breaking of the fast i had same mixture of miso soup and added apple slices and mango. Day 3 of my breaking I did the same miso soup combo with the apples. I did add Tamari Pumpkins Seeds.
    Today is day 47 of my post fast and i am experiencing Slight Swelling in my ankles and my calves. no apparent pain associated with it. i did notice that there is slight pitting in my calves.

    will this swelling go down as my body completely switches over from ketosis to glucose?

    should i be concerned about the swelling?

    Thank you so much for your responce.

    • Ron, thanks for your comment. It looks like you got the classic post-fast edema talked about in the article I referred to in this blog entry. It will go away, but, if you ever do this again, you should take note of the causes of the edema. Apparently, the edema is caused by sudden retention of water and salt that happens when we reintroduce carbohydrates to the fasting system. The solution is to eat small, balanced meals that include a high protein, high fat content. I wouldn’t have recommended your slow, water and salt heavy (miso soup?) reintroduction of food. I would say: first meal: a raw egg yolk, a little olive oil, a few salad greens, a bit of orange. Subsequent meals: egg, salad greens, tomatoes. Easy on the water and NO ADDED SALT. Matt

  • I really love the theme on your blog, I have a website too, and I would love to use this theme. Is it a free style, or is it custom?

  • Justin

    Hey Matt,

    Thanks so much for this post, it has relieved some concern. Two weeks ago, Monday the 23rd, was my 40th day of a water only fast. I broke the water fast with the Edgar Cayce apple diet (3 days with only apples, then a big dose of olive oil at the end of the last day). After that I ate fruits, veggies and back to more substantial (but mostly natural and organic) foods quite qucky. The past 4 days I have had severely swollen feet, ankles, and legs. I assumed my body was in some sort of hoarding mode coming off of the fast.

    Any idea how long I can expect this to persist? Do you know of any other literature on this topic, any websites?

    Again, thank you so much for this post and any additional info you may have!


Leave a Reply




You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

hi mom!