Fasting – is that what you’re really doing?

Run a search on the word “fasting” and you will undoubtedly get back hundreds of results, most of which will lead you to websites loaded with a mix of information ranging from half truths to excursions into the absurd.  So, what’s the deal with fasting?  Is it good, bad, or somewhere in between?

The fasting state

Abstinence from food for 18 to 48 hours is called the “fasting state” (1). During the fasting state, glycogen (stored carbohydrate) is depleted and amino acids from muscle protein break down to provide fuel for gluconeogenesis (production of glucose by the liver in the absence of carbohydrate intake) and the body enters a state of ketosis.  (If you thought only low carb diets promoted ketosis, you may want to look at fasting and starvation as something a bit more extreme than low-carb, in that they are both “no-carb”.) Large amounts of nitrogen are lost through urine to keep up with the body’s high rate of muscle protein breakdown and glucose synthesis by the liver (1).

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The starvation state

Abstinence from food for more than 48 hours is called the “starvation state” (1).  While a 24 hour fast is generally a good idea every now and then, starving oneself is not.  So, to be clear:  if you’re abstaining from food for more than 48 hours you are not fasting – you are starving your body of important nutrients.

When fasting turns into starvation, fat stores become the main suppliers of energy as the body tries to hang on to important proteins (enzymes, antibodies, hemoglobin).  Fatty acid levels in the blood increase sharply and the brain and skeletal muscles adapt to using ketones for energy.  The more fat deposits a person has, the longer s/he can survive – in theory.  In practice, the detrimental effects of starvation on the immune system leave people susceptible to infections which, in the absence of protein in the diet, can lead to death.

Purposeful starvation

Most people undertaking a fast, do so for more than 48 hours, which means they are actually engaging in starvation.  On purpose.  I don’t know how long such people keep up this feat, but I suspect most do not run out of fat stores by the time they start eating again.  Thus, they are in no danger of starving to death.  However, there are side effects to starving oneself.

Fasting and starvation rob the body of essential amino acids (EAA), essential fatty acids (EFA), and essential vitamins and minerals (1).  They’re called “essential” for a reason.  We can not produce them ourselves, so, we must get them from food.  If you think you can overcome this problem by juicing while you’re abstaining from food, I hope to relieve you of such fantasies in the next sections.


Juicing entails the removal of pulp and nutrient rich skins from fruits and vegetables. As a result, some essential vitamins and minerals, as well as most of the fibre are lost.  This is no big deal if you enjoy the products of juicing as part of an adequate diet, but if you are relying on juice to get you all the nutrients your body needs as you forego solid foods, you are far from reaching your goal.

Protein intake is significantly limited, meaning you’re very likely to experience the side effects of continuously robbing your body of essential amino acids.  I use the word “likely” because, in theory, if you consume enough juice, you will eventually meet your EAA needs.  In practice, this is virtually impossible, since the amount of juice needed would be of astronomical proportions.  Limiting amino acids, particularly lysine, methionine and tryptophan would keep your body from producing adequate amounts of proteins (antibodies, enzymes, hormones, etc.).  The longer you keep it up, the longer your body has to make do with robbing muscles of their proteins to compensate for self-imposed limitations.


Our stomachs and adipose cells under the skin secrete a hormone called leptin.  Its job is to keep track of how much fat we store and to suppress appetite after eating by stimulating the release of melanocyte stimulating hormone (MHS) (1).  In addition, leptin alters immune system function.  Fasting decreases basal leptin levels in the body.  Since inducing starvation in human subjects is considered unethical, we can only extrapolate what happens to starving people based on animal models and the study of malnourished persons or those suffering from eating disorders such as anorexia nervosa (2,3).  The effects of leptin imbalances during fasting and starvation are visible in as little as two days in animal models, with immune system suppression occurring at just 48 hours (4).  It should come as no surprise if, during a bout of starvation (water “fasts”) or adhering to a liquid diet yielding inadequate essential nutrients (particularly protein), people succumb to viral, bacterial or fungal infections, among other things.  All it takes is to be in the right place at the right time (in other words, exposure).

Other stuff

In a surprising turn of events, researchers in Israel found that some of their initially obese test subjects, who had no prior diabetic symptoms, developed diabetes after a course of very low calorie dieting (5).  This may have been the result of insulin resistance which usually occurs during early stages of starvation.  The subjects were not starving, but the low caloric intake may have mimicked starvation enough to lead to insulin resistance.

One day fasts and alternate day fasting

This I can support.  There is a fair amount of evidence that one day fasts and alternate day fasting have a number of benefits ranging from weight loss and maintenance to decreased risk of some cancers and cardiovascular disease (see my post on caloric restriction).  Although the most likely to be beneficial, alternate day fasting is the toughest of the two in terms of adherence.  Most people get tired of it within a week or two, many much sooner.  Alternate day fasting results in lowered caloric intake which, as I mentioned in a previous post, has been linked to higher quality of life and possibly longevity.


If you’re abstaining from food for more than two days, you are not fasting. You are starving.  I suggest you stop.  If you are engaging in one day fasts or alternate day fasts, more power to you.  If you think juicing while fasting or starving is enough to keep all bodily functions operating properly, you are kidding yourself.  Juicing is the equivalent of a very low calorie diet which is to say, you’re not getting essential nutrients and you’re setting yourself up for infections, weight re-gain after the regimen is over and possibly other issues not readily visible.

I did not touch on the claim that fasts of any kind or starvation of any length act as a way to detoxify the body or liver because I already covered it here.


  1. Gropper SS, Smith JL, Groff JL.  Advanced Nutrition and Human Metabolism, 5th Ed. CA: Wadsworth; 2009.
  2. Marcos A, Varela P, Toro O, Lopez I, Nova E, Madruga D, Casas J, Morande G.  Interactions between nutrition and immunity in anorexia nervosa: a 1 year follow-up study.  Am J Clin Nutr, 1997;66(2):485S-490S.
  3. Lord GM, Matarese G, Howard JK, Baker RJ, Bloom SR, Lechler RI. Leptin modulates the T-cell immune response and reverses starvation-induced immunosuppression. Nature 1998;394:897–901.
  4. Faggioni R, Moser A, Feingold KR, Grunfeld C. Reduced leptin levels in starvation increase susceptibility to endotoxic shock. Am J Pathol, 2000;156(5): 1781-1787.
  5. Koffler M, Kisch ES.  Starvation diet and very low calorie diets may induce insulin resistance and overt diabetes mellitus. J Diabetes Complications, 1996;10(2):109-12.