Is Fasting Healthy?

            Fasting is the act of limiting how much you eat or drink for a period of time.  The length of a fast can vary depending on the objective.  Some medical test (colonoscopies, blood test, sedation, etc.) only require an overnight fast.  For other purposes fast may be restrictive in food type or duration. A fast may also be intermittent in nature. For example, not eating after 6:00 PM until the following morning would be a partial day fast that discourages ‘grazing’ after dinner.  Often, a fast will allow nonnutritive liquids like water or black coffee but nothing else.

            As an alternative health aid or medical treatment, fasting has been used as a way to:

  • Complement some cancer treatments
  • Treat depression and osteoporosis
  • Eliminate white blood cells and encourage replacement by new cells
  • Lose weight
  • Reduce fat mass and maintain lean body mass
  • Increase appreciation of food
  • Enhance spiritual awareness
  • Detoxify
  • Reduce basal insulin, blood glucose, and triglycerides
  • Live longer

            For weight loss, fasting has the appeal of simplicity since it requires no food preparation, no calorie counting and might even provide one or more of the above benefits.

            While fasting has been practiced for thousands of years it’s not recommended for everyone.  There are many situations in which fasting can be harmful. For example, fasting is not recommended if you have liver problems, kidney problems, a compromised immune system, are taking certain medications, have been on a nutritionally deficient diet, have a wasting disease, or pregnant to name a few.  Any fast planned to last for an extended period should be preceded with a nutritious diet, be followed by a nutritious diet and be under close medical supervision.

            Usually, fasting does not result in sustained weight loss unless complemented by other strategies.  Most gain their weight back when they resume eating according to their normal routine.  Because fasting lowers your metabolic rate, what you eat immediately after a fast is likely to put pounds back on quickly unless portions are significantly decreased.  For most, a weight loss program based on a healthy diet and exercise is likely to have better long-term results than extended duration fast.

            Fasting can be an aid in detoxifying the body of accumulated toxins.  This happens when the body runs out of carbohydrates and enters a fat burning state called ketosis.  As the body burns fat, it also rids itself of toxins which are predominantly stored in fat.  Whether a long fast is more helpful for detoxification is debatable since we naturally fast at night while we sleep.  The use of colon cleansing as a boost to fasting detoxification carries risk.  Usually, our body does an excellent job of detoxifying itself so long as we eat nutritious foods in moderation. A health concern with colon cleansing is that it washes away good bacteria in the gut and disrupts the electrolyte balance. 

            Advocates of fasting are many and their claims are many.  Unfortunately, few credible human studies exist to support many of the claims.  As you might expect, finding volunteers for long-term trials of fasting is challenging, hence, most research has been with animals or short-term human trials.  In one study, mice had better insulin control if forced to fast every other day, while eating twice the normal amount of food on non-fasting days.  In hundreds of animal studies, fasting has led to a longer length of life.  These studies lend credibility to the theory that excess calories shorten our lives.  Perhaps this is because we evolved to survive in a world of scarcity and today live in a world of never-ending abundance.  While there is much we don’t know about human fasting, it’s likely that in the ‘hunter-gatherer’ phase of our evolution we frequently missed meals (fasting).

            While fasting for extended periods may have an almost mystical appeal, the dilemma is that for most, fasting is difficult and not a prescription many will follow.  This reality has caused researchers to look for hope in short duration calorie restriction diets that are easier to stick with.  A recent article, “Can a Diet that Mimics Fasting Turn Back the Clock?,” offers hope through a study that demonstrated positive results in a human trial.

            Researchers are only beginning to understand the underlying biology of fasting. A recent Harvard study demonstrated how a less demanding short duration calorie restriction diet based on intermittent fasting increased lifespan by manipulating mitochondrial networks.

            Based on work from animal research like the Harvard study, a wide variety of intermittent fasting protocols have been promoted that split the day or week into ‘eating periods ‘ and ‘fasting periods.’

            Several variations of intermittent fasting have been promoted in popular books, magazine articles and advertisements. The list below highlights the distinguishing feature of a few of the most popular intermittent fasting techniques.

  • The 16/8 Method (also called the Leangains protocol) involves skipping breakfast and restricting your daily eating period to 8 hours, for example from 1 pm to 9 pm. Then you “fast” for 16 hours in between.
  • The Eat-Stop-Eat method requires fasting for 24 hours, once or twice a week. This could be done by not eating from dinner one day until dinner the next day.
  • The 5:2 Diet involves eating only 500-600 calories on two non-consecutive days of the week. On the other 5 days, you eat normally.
  • The Alternate Day Fast (ADF) involves a 24-hour fast followed by a 24-hour non-fasting period.
  • The Warrior Diet – This involves eating only over a four hour period and then fast for 20 hours.
  • The One Meal Fast requires eating one meal per day followed by 23 hours of fasting.
  • The UpDayDownDay fast involves eating very little one day and eat normally the next. On the low-calorie days, that means one-fifth of your normal calorie intake.

            Pros and cons of these fast are explained in the article, “Which One is Right for You.” An important point about intermittent fasting is that it’s a relatively new idea with little solid research in human studies to back it up as a successful long term method.  For more perspective on the pros and cons of intermittent fasting read, “Experiments With Intermittent Fasting .”

If you have an urge to tiptoe into fasting then intermittent fasting is the easiest way to fast. If you have ongoing health issues then first consult with your physician.  Be aware that the topic of intermittent fasting has been hijacked by a wide range of business interest as a way of promoting supplements, meal plans, and other products that are unlikely to be helpful.   Should you choose to venture into one of these intermittent fast, here are a few reminders.

  • Eat nutritious foods between fasting periods.
  • Drink plenty of water. If not getting enough water then enjoy teas, coffee, or water with a twist of lemon.
  • At first, try a short overnight fast where you avoid eating for 12 hours (8 hours of sleep + 4 more hours).
  • Begin your fast on a busy day.   You are unlikely to be distracted by thoughts of food when busy. 
  • Pair intermittent fasting with consistent exercise.  Exercise is a distraction from food.  Depending on intensity it can also stimulate burning calories and release of endorphins.
  • Adjust your attitude. If you are like most, you have spent a lifetime being bombarded by advertisements warning of possible calamity if you don’t get enough nutrition.  Read about the benefits of fasting until you understand that missing a meal will not send you into a nutritional death spiral.  Missing a meal is not starvation.  The feeling of missing a day of food is not even hunger.  More likely it’s an addiction driven desire for refined carbohydrates and sugar. Knowledge can prepare you.  Breaking up with the one you love (junk food) is never easy.

            With an abundance of fast food, snack food, and prepared food about us, it’s easy to overeat.  As an aid in reducing the amount we eat, fasting can be helpful but not the first line of defense.  For most, the first step is moving toward a lifestyle with daily exercise supported by a diet with adequate, micronutrients, fiber and essential macro nutrients. 

            Perhaps you are planning for a healthier lifestyle this year.  Come share your plans with me on a Saturday morning walk.  I’ll be rooting for you. Do remember, winter mornings are wonderful when bundled in double layer clothing, hat and mittens. Hope to see you.

            Nancy Neighbors, MD

By Nancy Neighbors, MD

... Dr. Neighbors provides a blend of traditional family medicine and evidence-based lifestyle medicine in Huntsville, Alabama. When indicated, lifestyle change is recommended as the first line of therapy.

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