Yes! Intermittent Fasting Works

Posted: Mar 19 in Lifestyle by

Why Intermittent Fasting Works

Intermittent fasting in Arizona

Periods of very-low-calorie intake force our bodies to turn the metabolic switch to burn fat to meet energy needs.  The burning of fat leads to the release of ketones, the breakdown products of fat.  Ketones cause us to make less ghrelin, a hunger hormone released from the stomach.  Some studies indicate that intermittent fasting leads to an increase in metabolism and sparing of muscle.  In summary, intermittent fasting leads to decreased hunger, even on non-fasting days, and increased fat burning.  It should be noted that limiting carbohydrate intake is necessary even on non-fasting days to prevent shifting our metabolism from fat burning to sugar burning.

What to Expect During Intermittent Fasting

 A fast consists of a very-low-calorie day lasting 36 hours.  For example, if Monday is a fasting day, then from dinner Sunday until breakfast Tuesday, only 600 calories are consumed (in the form of a prescribed medical meal replacement).  A 200 calorie shake is consumed for breakfast, lunch, and dinner.  Note that each protein shake provides 1/3 of daily nutrition, something that is not possible with an over-the-counter shake.  The result is that you will receive 100% of your daily needs on only 600 calories.

The first day on intermittent fasting is the most difficult part of the process, and it quickly gets easier with time.  It is during this time our bodies use up sugar reserves (glycogen) in our liver and muscles.  Hunger usually lasts until our bodies use up the last glycogen stores, at which time you may “hit the wall.”  As our bodies convert from sugar burning to fat burning, you may experience a phenomenon similar to marathon runners who hit the wall late in a marathon, also known as “bonking”.  You can expect to feel a bit sluggish or have a headache.  This usually lasts up to ½ a day.  We encourage increased salt intake (pickles or soup bullion) to help these feelings pass

It is important to work with a medical provider to help adjust medications for high blood pressure or diabetes during this process.  Fasting often leads to rapid decreases in blood pressure and blood sugars, and our specially trained providers will guide you if and when to make changes in these medications.  Once this feeling passes, and ketones are released, the headache usually passes and energy improves.  Many patients tell us they have no hunger and marked improvement in feelings of well-being after the first fast, and that these feelings persist during weight loss.

The following day is non-fasting, usually consisting of a low-calorie, low-carbohydrate meal plan (whole food or partial medical meal replacements) of 1000-1200 calories.  It is important to avoid increasing carbohydrates during these days, as that will shut off fat-burning, lower ketone levels, and increase hunger.  If your carbohydrate intake is too high, you may hit the wall with each fast. 

Who Benefits from Intermittent Fasting

There is no single diet that works best for everyone.  Many of our patients do best on a continuous low-calorie diet, such as an 800 calorie full meal replacement plan, or a partial plan with some meal replacements and some whole food.  Finally, some do best on a low-carbohydrate meal plan.  For the greater population, studies have shown that IF is no better yet no worse than continuous low-calorie diets.  As experts in obesity medicine, we can often predict the best and safest approach for you, given your medical history and past results with dieting.  Studies support the combination of the best meal plan, medical support, education, long-term accountability, and anti-obesity medications to result in the most dramatic and sustained weight loss.

The History of Intermittent Fasting

Fasting has been a part of religious tradition for centuries.  In 1965, at 456 pounds, Angus Barbieri set the world record for the longest fast.  He fasted for 382 days, consisting of tea, coffee, carbonated water, electrolytes, and vitamins.  He lost 276 pounds, reaching his goal weight of 180 pounds. It should be noted that he was monitored closely by physicians during the fast and that undertaking a fast of this sort would be quite dangerous without close medical supervision.  Severe electrolyte disturbance, such as low sodium, potassium, magnesium or phosphorus can lead to deadly heart, brain, and kidney consequences.

Over the last decade, several studies have demonstrated the benefits of IF, including reduced cholesterol and improved diabetes parameters.  Dr. Michael Mosely personally used alternate day fasting that lead to a weight loss of 20 lbs. and reversal of his diabetes.  He ate 500 calories twice per week and ate with no limitations on the other five days, a regimen now called 5:2 intermittent fasting.  Further studies by Varady and Hellerstein in animals demonstrated alternate day fasting (ADF) was as effective as continuous low-calorie diets in improving cholesterol and insulin.  Halberg et al showed similar health improvements after 2 weeks of ADF. 

Time-restricted feeding (TRF) consists of limiting eating to a window during the day has led to modest weight loss and improvements in metabolic parameters, but further studies are needed.  In our experience, the 5:2 36-hour intermittent fast offers better promise than TRF.


 NEJM 381;26 Dec 26, 2019.   Effects of Intermittent Fasting on Health, Aging and Disease by Rafael de Cabo Ph.D. and Mark P. Mattson Ph.D.

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Varady KA, Hellerstein MK. Alternate-day fasting and chronic disease prevention: a review of human and animal trials. Am J Clin Nutr. 2007;86:7–13.

Halberg N, Henriksen M, Soderhamn N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol. 2005;99:2128–2136.

Lihn AS, Pederson SB, Richelsen B. Adiponection: action, regulation and association to insulin sensitivity. Obes Rev. 2005;Feb 6(1):13-21.

Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005;81:69–73.

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