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Dear Doctor: Intermittent Fasting- A Winner for Weight-loss or a Diet Dud?

Answer Provided by Jessica Bartfield, MD

Fall 2017

At any given time, nearly half of the adults living in the United States are trying to lose weight. In doing so, most people try to eat less and exercise more. The majority of weight-loss experts agree that changing eating patterns usually leads to greater weight-loss than increasing exercise.  Bottom line, calories need to be cut to lose weight. Decreasing calories by about 500 per day can lead to safe and effective weight-loss. But how best to do this?

For at least the past 20 years, the focus has been on what to eat for caloric reduction and weight-loss:

  • Low fat
  • Low carb
  • High protein
  • Gluten-free
  • Mediterranean
  • Vegetarian
  • Vegan
  • Paleo

All of these have been recommended at one time or another as the best way to drop pounds. This can be very confusing for a person who is trying to lose weight. Interestingly, when these different eating patterns for weight-loss are compared to each other, they tend to all result in the same amount of weight-loss, as long as people keep eating the same way. There lies the greatest difficulty- being able to follow a certain eating pattern over many weeks, months and even years. If and when people return to previous eating behaviors, weight regain occurs.

More recently, the focus has shifted from what to eat for weight-loss, to when to eat for weight-loss. Imagine that instead of having to think constantly about what to eat:

  • Choosing which types of foods
  • Measuring portions
  • Counting calories

All you simply have to change is the timing of meals and snacks to effectively lose weight. The concept is called intermittent fasting, and it is quickly gaining attention among top scientists and weight-loss experts as an effective way to lose weight. People may find intermittent fasting easier to stick to over the long term. There are different types of intermittent fasting options (listed on the following page).

Intermittent Fasting Options

For example, one plan recommends not eating for 24 hours one or two days per week. You would eat dinner and then:

  • Skip any nighttime snacks
  • Breakfast
  • Lunch
  • And all other snacks until dinner the following day

All other days of the week you eat as you normally would.
Another plan recommends alternate day fasting:

  • Eating only about 400-600 calories total for one day
  • Then eating as you normally would the following day
  • And repeating this pattern every day

Finally, a third option called timed restricted feeding recommends eating as you normally would during a specific period of time (i.e., 8 hours), then fasting during the remaining time (i.e., 16 hours).

How Long has Intermittent Fasting Been Around?

Although considered a “new” idea for weight-loss, intermittent fasting has been around for hundreds of years, often used for religious or spiritual purposes. Christians often fast during Lent, Jews fast during Yom Kippur and Muslims fast during Ramadan. Many people will find it a familiar practice, possibly easier to understand and follow for weight-loss purposes. But two key questions need to be answered:

  • Does it really work for weight-loss?
  • What are the risks?

Researchers are conducting many studies using intermittent fasting to answer these questions and learn some useful information. Both animal studies and human studies have offered interesting results about not just weight-loss, but these condition as well:

  • Changes in blood pressure
  • Changes in blood sugar
  • Aging process can change

Looking specifically at weight-loss, some animal studies have shown hopeful results. For example, mice that fasted every other day did not have much greater weight-loss when compared to mice who followed a reduced calorie diet, but did have significantly less visceral fat and smaller fat cell size2,3. Visceral fat is the dangerous fat that surrounds internal organs- (i.e., stomach, liver, gallbladder) and can lead to the development of diabetes, high blood pressure, high cholesterol, etc.

Human studies on intermittent fasting have also shown some positive findings specifically for weight-loss. People who did intermittent fasting for 3 months lost about 5-7 percent of their starting weight (average 12-14 lbs.) 2,4. Although it may not seem like a very high amount of weight-loss, this amount may lower blood pressure, improve cholesterol and diabetes and even lessen joint pain from severe osteoarthritis.

In addition, people who used intermittent fasting for at least 3 months decreased their waist size by 2-3 inches. This is very important as high waist measurements (greater than 35 inches for women and greater than 40 inches for men) indicate high amounts of visceral fat (dangerous fat) and thus higher risk for many different diseases. Therefore, lowering your waist circumference improves health and decreases disease risks. Other studies using intermittent fasting for weight-loss have shown similar results.

Is Intermittent Fasting Effective?

So to answer the first key question – yes, intermittent fasting does result in significant weight-loss. We still do not know, however, if it is better than other specific eating patterns for weight-loss, or if it is more effective than just overall cutting calories. It may not be. One study found that male veterans who used intermittent fasting for 6 months lost about the same amount of weight as those who just decreased their calories by 500 every day 5. We also do not know whether it may work better for specific populations, such as people with diabetes, men versus women or certain age groups.

Problems with Intermittent Fasting

Now, how about potential problems with intermittent fasting? Unfortunately, there are very few studies in animals or humans that have specific safety information. Potential problems with intermittent fasting would likely occur if the fasting is not done correctly. People who fast for too long of a time or too frequently may risk:

  • Starvation
  • Dehydration
  • Vitamin or mineral deficiencies
  • Anemia
  • Hair loss

Some people may feel fatigue, irritability, difficulty concentrating, increased hunger or all of the above. Not helpful for losing weight and keeping it off over the long-term!
People with a history of:

  • Emotional eating
  • Binge eating
  • Or other disordered eating behaviors

May have a higher risk of overeating on the non-fasting days, and not get any benefits. So the answer to the second question is also yes, intermittent fasting does carry some risks. We do not, however, know yet if these risks are very low, more concerning or who might be at greatest risk.


Overall, the last word on intermittent fasting has yet to be written. For now, it offers a different way to change eating patterns for weight-loss. Rather than concentrating on what to eat, concentrate on when to eat. Many people may find this type of change more attractive and less overwhelming than trying to sort through all the different diets reported for weight-loss. If the diet is easier to follow, then it will be easier for people to keep up with it over the long-term and reduce their risk for weight regain. And that is the ultimate goal for successful weight-loss- not only losing weight, but also keeping it off permanently! As with any weight-loss plan, you should always talk with your doctor first. Your doctor can help determine if intermittent fasting is a good option for you, how best to do it and monitor you to ensure that you are experiencing both safe and successful weight-loss.

About the Author:
Jessica Bartfield, MD, completed a postdoctoral fellowship in clinical nutrition and obesity at the University of Alabama Birmingham. She is board certified in internal medicine and obesity medicine. She has practiced obesity medicine for the past seven years and recently joined The Weight Management Center at Wake Forest Baptist Medical Center. She is a member of The Obesity Society and the American Society for Metabolic and Bariatric Surgery. 

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