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.
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