A Perspective on the Physiology of Fasting & Weight Loss – Part Three

So now we move on to what happens during a fast. I have already discussed the various reasons for fasting and what may be accomplished in various time frames in my previous post ‘To Fast or Not? For How Long?’ Here I will focus on the physiological changes that take place during a fast.

In preparation for the fast one should change to a reduced calorie diet composed primarily of cleansing fruit, vegetables and water for a couple of days. This prevents adverse reactions, which might occur due to a sudden change in diet when you actually start the fast. Once you start the fast, the body goes into the initial ‘not fed’ state that normally occurs between meals. This means that during the day the body is using ‘aerobic respiration’ to break down glycogen stores into glucose. At night, the ‘at rest’ state of aerobic catabolism (breaking down) of fats (lipolysis) will take place as normal. As long as one is not exercising, these normal states of energy production will usually last 24 to 48 hours for females and 48 to 72 hours for males. [3]

If one is on a ‘water only’ fast, The body will have used up most of the stored glycogen and readily available fatty acids by this time. Its first reaction is to begin scavenging non-essential protein as it is more readily available than stored fat (triglycerides). The protein is turned into glucose, which goes primarily to the brain. However the body does not like losing protein mass and if no other energy source is available it will soon switch to stored triglycerides using a process called ‘Ketosis’. In this highly efficient state, the liver begins converting stored fat and other nonessential tissues into ketones, which can be used by the brain, muscles, and heart as energy. It is at this point in the fast that sensations of hunger generally go away, and many people experience normal or even increased energy levels. Hormone levels and certain functions become more stable in this state as well. The goal of most fasts is to allow the body to reach the ‘ketosis state’ in order to burn excess fat and unneeded or damaged tissue. Thus, fasts longer than three days are generally recommended as therapy.

Weight loss occurs most rapidly during the first few days of a fast, up to 2 pounds per day. In following days, the figure drops to around 0.5 pound per day. An average weight loss of a pound a day for an entire fast can be expected. [3] One reason for this is that the body recognizes that there is little or no nutritional input and it turns down your basal metabolic rate, so that you require less calorie input to maintain your existing weight.

One side effect of burning fat, is that any drugs, prescription or otherwise, nicotine, and environmental toxins previously stored in the triglycerides will be released into your blood stream. Your liver, kidneys and lymphatic systems will be taxed to eliminate them. Support them by drinking lots of water and herbal teas. You may feel some lightheadedness or even headaches. These are not unusual ‘healing crisis’ symptoms. If the symptoms become severe, you may be experiencing a ‘Herxheimer’s reaction’, which means toxins are being released faster than the body can eliminate them. So, slow down the process.

If instead of a ‘water only’ fast, you choose to use juice or broth, you will be slowing down the burning of stored fat and the release of toxins as the body will have a small input of carbohydrates, which will interrupt the state of ketosis and reduce weight loss. In fact you may never enter the ‘ketosis state’ if you use juice, shakes or broth fasts, but weight loss is still posible.  Experienced fasters recommend slowly diluting the juice or broth with water until you are drinking mostly pure water by the end of the fast. My experience is that I have lost up to 20 lbs. on 28 and 30 day juice and broth fasts. This may not be the maximum possible, but it is certainly significant.

Fasting is not starvation, but rather the body’s burning of stored energy. Starvation occurs when the body no longer has any stored energy and begins using essential tissues such as organs for an energy source. Therapeutic fasts are stopped long before this happens. It is estimated that even very thin people can survive for 40 days or more without food. [3]

I hope this material has been helpful. Please register and leave comments.

Go back to Part Two

Don Gillmore, Licensed Massage Therapist(15 yrs), B.S.- Life Sciences

       1.  Fundamentals of Anatomy & Physiology, Third Ed., Frederic H. Martini, Prentice Hall, Englewood Cliffs, NJ, 1995.
       2. Prescription for Nutritional Healing, James F. Balch, MD & Phyllis A. Balch, CNC, Avery Publishing Group, Garden City Park, NY, 1993.
       3. Gale Encyclopedia of Medicine – Fasting, 2nd ed.. Douglas Dupler, MA, Gale Group, 2002.

Tags: , ,

6 Responses to “A Perspective on the Physiology of Fasting & Weight Loss – Part Three”

  1. […] Self-Help « A Perspective on the Physiology of Fasting & Weight Loss – Part One A Perspective on the Physiology of Fasting & Weight Loss – Part Three […]

  2. dgillmore says:

    I want to note that the the processes of breaking down protiens and triglycerides during fasting are both Ketosis. During the process of switching from the normal ‘not fed’ state of Glycolysis to Ketosis, some muscle protein may be utilized. Once the body has switched to using triglycerides (fat) for the ‘ketosis state’ it will continue to scavenge damaged protein from body cells (not muscle tissue) to create ketones for energy production. This is an additional side benefit as it cleans up unproductive tissue, which might otherwise inhibit proper cell functioning and accelerate ageing. Dr. Michael Eades, MD explains this protein scavenging process better than I can on his blog. Certain body cells can use ketones as energy directly, while the liver creates Glycogen from Ketones in a process called Glyconeogenesis.

    Many have hailed the ‘Low Carbohydrate’ and/or Atkin’s diet as a revolutionaray weight loss plan because the body remains almost constantly in the ‘ketosis state’. Whether that is a healthier lifestyle or not (there is some evidence that it is) may be debated for a long time. However, even if you are in the ‘ketosis state’ permanently, if you consume more calories of fat and protein than your body uses on a daily basis, you will not burn stored fat and therefore you will not lose weight! It comes back to the simple equation of energy in vs energy used. A suplus input results in fat storage.

  3. dgillmore says:

    Mike, a close friend of mine who is also an MD emailed some comments on this post, which I would like to include. The first is a clarification on the Herxheimer reaction.

    The Herxheimer reaction is an immune response to proteins inside of spirochetes and some bacteria from rapid lysis with the commencement of treatment (seen frequently with syphyllis). It is a rash and fever.

    He goes on to add his own perspective on fasting, weight loss and exercise.

    Initial weight loss is largely water… [W]ater and electrolyte supplementation to correct this helps reduce some of the renal stress of fasting. After use of glycogen stores, there will be protein as well as fat catabolism. Nutrition and exercise during weight loss are necessary to build new protein (and, unfortunately, some fat) to offset this.

    I would like to underscore Mike’s comment about additional water. It is essential to replace lost fluids and electrolytes during a fast. Water is also essential for increased triglyceride (fat) break down and therfore weight loss whether one is fasting or not.

  4. water help to loss weight…

    I don’t normally comment on blogs but your post was a real call to action. Thank you for a great read….

  5. fasting to lose weight…

    I stumbled across your post Friday titled A Perspective on the Physiology of Fasting & Weight Loss … when searching for info. related to fasting to lose weight. You should continue writing more as I will certainly return to your site. Good …

  6. admin says:

    “Big Fat Lies” a great article, which discusses several commonly held misconceptions about fat loss. http://www.smh.com.au/lifestyle/wellbeing/big-fat-lies-20091118-imkh.html