Is Fasting an Evolutionary Imperative or Dangerous?




In Maslow’s famous hierarchy of needs, food and water occupy the base and most essential level. Nutritive sustenance ranks with breathing, sleep, sex, homeostasis and excretion for meeting the physiological imperatives for human survival. So why has fasting — voluntary starvation, by one way of looking at it — been practiced for millennia in multiple cultures across the globe? Perhaps more important, what does science tell us about the biological mechanisms and net health effects of fasting?

Fasting is practiced by different people for different reasons. Some fast for religious purposes, others to cleanse and detoxify. Fasting is the basis of many modern weight-loss diets, while some people practice intermittent fasting within a strength-building training program.
It’s a complex realm of discussion, and as with all things related to health-driven activities, it is sometimes a place prone to exploitation by unqualified practitioners. The case of three people who died in October 2009 at the Angel Valley Retreat Center near Phoenix illustrates how fasting might be misused. The motivational speaker who ran the facility, James Ray, was found guilty of negligent homicide in those cases. A total of 18 people were hospitalized in the same incident after paying up to $9,000 for the experience, which included a 36-hour fast prior to the two hours spent in a saunalike sweat lodge.

The 21 victims suffered from heat stroke, dehydration and multisystem organ failure. While most of these effects were due to overheating (hyperthermia) in the sweat lodge, fasting in advance could not have helped. The fact of the matter is that fasting taken to an extreme can be dangerous (long-term, a nutrient-deficient diet can also lead to hair loss).
So why do people voluntarily (outside of famine conditions) fast?

Fasting as a religious practice

There are references to fasting in almost all sacred texts of the world’s religions. But just what is it about fasting that feeds the soul? Here are the explanations from some of these faiths for going without food — and sometimes water — for a day or more:

  • Buddhism. Fasting is done on days of the full moon and certain religious holidays to achieve physical purification and to “free the mind.” Tibetan Buddhist monks fast in conjunction with advanced yoga.
  • Hinduism. Hindus fast on the new moon and during festivals for a complete 24 hours as a means to enhance concentration during worship and meditation, to purify the system and to make personal sacrifice.
  • Islam. During the month of Ramadan, Muslims abstain from food, drink, smoking, sexual intercourse and profanity from dawn to dusk every day, and some adherents fast every Monday.
  • Judaism. Yom Kippur (Day of Atonement) requires adherents to fast according to Jewish law in conjunction with making amends for past transgressions. Additionally, there are six other fasting days on the calendar that mark key moments in Jewish history.
  • Mormonism. Claiming that the general purpose of fasting is to receive spiritual strength, Mormons set aside one “fast Sunday” per month, an observance that includes giving money to the poor that is equivalent to or greater than the money not spent on that day’s meals.
  • Protestantism. While mainline Protestants hold no strong fasting traditions, some are adopting fasting to advance political and social justice agendas. Evangelical Protestants are increasingly adopting fasts for spiritual nourishment and as a nod to the economically disadvantaged.
  • Roman Catholicism. The abstention from food is largely from meat (on Ash Wednesday and Good Friday as well as all Fridays in Lent). The purpose is to exercise control over earthly desires, to receive penance from sins and to experience solidarity with the poor.
  • Seventh-Day Adventism. Heralded for the health practices that are documented in longer, healthier lives overall, Adventists cite verses in both the Old and New testaments of the Bible that note fasts lasting one to 40 days, some including water and some not. There are many reasons given for the purpose of fasting, but a church website claims that “fasting seems to be an outward expression of the person’s inner total commitment and reliance on God’s preserving and rescuing power.”

Most religious traditions allow for special needs of individuals with health issues. A Cherokee spirit healer and registered counselor, Neven L. Gibbs, shared that tradition’s practice with “As part of an overall well-being treatment, fasting may be included as part of a mind, body and spirit modality on a voluntary basis. Moderated and supervised, there should be no long-term risk to the physical health of the individual.”

Fasting to cleanse and detoxify

Pagan cultures (by rights, paganism also qualifies as a religion) speak of how fasting is intended to purify a person, including how it might cleanse the body in the spring from winter eating. While the business of cleansing and detoxification might seem appropriate within a New Age construct, recent scientific inquiry seems to suggest there is some physiological validity to the process.

Most discussion of the physiology of fasting focuses on the digestive track, as in how consuming only liquids for a period of time (one or more days) helps usher out residual wastes, particular in the lower gastrointestinal tract. But a study conducted at the University of Texas Southwestern Medical Center, Center for Autophagy Research (“Exercise-Induced BCL2-Regulated Autophagy Is Required for Muscle Glucose Homeostasis,” Congcong He et al., Nature 2012) explored how individual cells need their own cleaning out of waste.

The process is called autophagy, and this particular study looked at how exercise can promote this type of cleaning. But fasting can promote this cleaning process as well. Under conditions of scarce nutrition, these waste products — macromolecules and organelles — serve as nutrition for the cell. This is good for the cell and good for the individual, since it may well promote lifespan extension for the cells to clean themselves out from time to time. Conversely, the cells of the individual who has a constant supply of nutrition and does not engage in physical exercise never experience the autophagy process — to his or her disadvantage.

Fasting as a type of weight loss dieting

Relative to weight loss, fasting has become very popular in recent years. It works insofar as an absolute caloric reduction over time will lead to weight loss.

The names of fasting weight loss diets are bandied about in celebrity magazines and websites: the maple syrup weight loss diet (also known as the master cleanse lemonade fast) and the liquid fast diet, among others. As is the case in all extreme diets, the results last only as long as the diet is maintained. That, however, is unlikely for several reasons: These diets are unpleasant for most people, they are almost impossible to follow in social settings and they lead to nutrient imbalances, which can have many unintended and adverse side effects.
The conventional wisdom among critics is that the fasting person’s metabolism slows when he or she fasts, so much so that the net “spend” of calories is less. The reasons for this happening are twofold: The process of digestion itself requires calories; therefore, with less food to digest, there is less energy (calories) spent on digestion. The second reason is that all other systems of the body “power down,” so to speak, including the dieter’s interest in physical activity.

Dana Smith (, a blogger and Ph.D. candidate in experimental psychology at the University of Cambridge, offers a very technical reason why fasting for weight loss may be counterproductive: “Nutrient deprivation and autophagy can result in an increase in free fatty acid production in the liver, which in turn elevates hypothalamic levels of the peptide AgRP, involved in increased food intake. Thus, when an individual is able to eat again, they are more likely to consume greater quantities of food than normal.”

Greater quantities of food? Does it make sense to go with too little for a while and then ingest too much later?

Fasting in a fitness program

There is a small subset of the fitness community that believes in a version of fasting as part of a fitness program that is called intermittent fasting (IF). This appears to approximate the process bodybuilders go through as they attempt to lose body fat in preparation for competition. Note that competitive bodybuilders may drop to as low as 3-4 percent of body fat in the days preceding a competition, which is far below the average (16-22 percent) of those who are not bodybuilders. It is almost impossible to maintain on a long-term basis.
The objective is not simply weight loss but loss of fat, not lean (muscle) tissue. This fasting is done while the individual continues a strength-training regimen.

There is not a lot of documented research on this topic; however, some of its advocates readily share online their experiences.

On “Get This Ripped” (, the site authors claim that metabolism does not slow down under controlled fasting conditions of three days’ duration (“Resting Energy Expenditure in Short-Term Starvation Is Increased as a Result of an Increase in Serum Norepinephrine,” Zauner, Scheeweiss et al., American Journal of Clinical Nutrition, 2000). This study explains that energy expenditure of the individual at rest increases in early “starvation” (the fasting period), owing to an increase in plasma norepinephrine. The research was conducted on “lean subjects,” individuals with relatively low body fat to begin with.

At “Mark’s Daily Apple” (, author Mark Sisson writes of several benefits to occasional, short-term fasting: increased longevity (rat and worm studies show remarkable lifespan extensions from caloric reductions), blood lipid reduction, cancer mitigation (such as fasting during a chemotherapy regimen, which shows some promise in limited studies), improved neurological health, mental well-being and clearer thinking. He cites several studies that support the concept of fitness training while fasting, including one finding that Muslim soccer players experienced no negative performance measures during Ramadan, the month of sunup-to-sundown fasting.

Sisson also postulates that absolute abstinence from food, say on alternating days, is easier for some people to maintain than it is to weigh, balance and parcel out limited food quantities every day in a traditional diet. This seems to make sense, but one hopes the nonfasting days are not filled with pizza and donuts.

It appears, therefore, that fasting might work for many people under certain circumstances, which makes sense: A steady supply of food for the vast majority of humans was not available until the 20th century. Our bodies evolved for millennia before then to deal with intermittent hunger on a regular basis. It might be beneficial to return to that state from time to time.