If you are concerned about your health, chances are you have heard of intermittent fasting (IF). It’s become so popularized that in 2018, “IF” was one of the top 10 diet related searches on Google’s search engine. 

Followers of the IF diet are lining up for what is many times mistaken as a diet in and of itself, being used to restrict or scale down caloric restriction. It’s been highly touted as the “quick fix” to drop pounds. 

Taking on many different names and forms such as “intermittent energy restriction , or IER,” or “alternate day fasting,” intermittent fasting has no single definition, but  generally is used in one, or a combination, of several ways:

  1. Reducing calories by 50-70% every other day, or perhaps two days in a row, followed by a day or two of regular eating
  2. Fasting one day followed by a “feast” day, where participants eat as much of any food that they want. Please note, reputable programs recommend healthful eating on the non-fasting days.
  3. Time-restricted feeding: Fasting 12 to 21 hours per day and having a specified eating window to consume food without restricting calories
  4. Modified fasting: Periods of severely limited food intake (5:2= 5 days normal eating, 2 days restricted intake) 


Guaranteed caloric restriction is the obvious benefit of intermittent fasting, so how can this be any different than simply reducing your overall caloric intake with smaller portions, less snacks, desserts, and beverages?  

A 2015 study  of 12 clinical trials compared IF with a simple, continuous reduction of calories and found there was “no significant difference in weight loss amounts or body composition changes (Seimon et al., 2015).  A similar study was conducted in 2018 finding the same results, “intermittent energy restriction was comparable to continuous energy restriction for short term weight loss…” punctuating that while IF has become a popularized diet, it’s not necessarily superior to becoming more aware of and cutting calories more routinely.

Case in point, a 2017 study analyzed people who used an alternate-day fasting program had similar weight loss profiles as those individuals who ate an overall lower-calorie diet. 


There are clear benefits to our cutting calories and increasing food portion awareness   in the super-sized Standard American Diet (SAD). Most Americans realize they eat too much 24-7; doing something about it is the hard part, and IF effectively delineates a routine for taking action to cut calories. 

Dietary changes almost always yield weight loss benefits, and so researchers have  been equally interested in how IF modulates management of body fat, diabetes, heart disease, glucose, insulin/insulin resistance, and cholesterol levels. A few research studies discovered overall good results including reduction in visceral body fat (the type of fat strongly associated with increased risk of heart disease and type 2 diabetes), fasting insulin levels, and insulin resistance.

Health gains of any type is great news for IF followers and when implemented effectively, IF includes a healthy diet to include increased fiber with more fruits, veggies, and whole grains, which always yields better health metrics and increased weight loss. 

Unfortunately most research results on IF are mixed and lack longitudinal studies.


Generally speaking, no serious side effects have been noted when IF is implemented in research studies, but one small study using alternate day fasting across 8 weeks showed that a few people reported constipation, weakness, dizziness, and bad breath. Happily these individual did note improved mood!

That said, there are a few groups of people who should NOT fast. These include:

  • Pregnant or breastfeeding women
  • Anyone with an unhealthy relationship with food, or a history of eating disorders or disordered eating habits
  • Anyone with diabetes, or who experiences low blood sugar
  • Women with a history of irregular periods or who are trying to conceive are urged to proceed with caution, as IF may adversely impact hormone levels.


Female fertility and loss of muscle mass are top concerns for those following an intermittent fasting protocol, since some studies have shown ovulation and fertility as being altered both in frequency and length of a woman’s menstrual cycle. For example,  one study reported a longer than average menstrual cycle in women who followed a program of two consecutive days of reducing calorie intake by 70% each week for 6 months.

Please note menstrual cycle alterations can create a cascade of additional health concerns! 


Those with diabetes could experience hypoglycemia on fasting days, and so they need to pay careful attention to their blood sugar levels as well as consult with their physician before attempting an alternate day fasting program.


Finally, since energy is reduced and muscle tone can be altered in a strict IF practice, long-distance endurance and intense exercise enthusiasts may want to consider a modified IF schedule.


Intermittent fasting may be a helpful program for individuals interested in losing weight and improving general health. However, more high-quality, large-scale controlled and longitudinal research on intermittent fasting with large groups of people is necessary. 

Harvie and Howell (2017) sum up what we can safely conclude about IR: “We do not know conclusively whether long-term IER [intermittent energy restriction] is a safe and effective method of weight control for subjects who are overweight or obese or whether IER may confer health benefits to people of any weight independent of weight loss. High-quality research comparing long-term outcomes of IER and CER [continuous energy restriction] are required to ascertain any true benefits or detrimental effects which IER may have for controlling weight and improving metabolic health in the population.”

It’s too early to know the long term benefits and potential health concerns of fasting. 

If you decide to try intermittent fasting, check with your physician first and keep these tips in mind:

  • Reduce usual calorie intake by 50-70% every other day, or perhaps for 2 consecutive days each week.
  • Choose a healthful, balanced diet that includes lean sources of protein, vegetables, fruit, whole grains, and healthful sources of fat every day, even on so-called “feast” days.
  • “Feast” days are not a license to overeat or consume large amounts of sweetened, processed foods.