The Physiology of Fasting

A look at 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.


  1. 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.

  2. 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.

  3. 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

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  5. 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!


    • I’d say keep calories low, especially calories from dense carbs, eat fruit veggies and good protein, you’ll return to normal. Try not to overeat in compensation for the fast day.

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