A Registered Dietitian Nutritionist shares her thoughts.

If the start of the New Year has you pondering a popular weight loss approach called intermittent fasting, or IF for short, read on for the pros and cons from a nutrition expert before signing yourself up.

What is it?

You’ve probably read about IF because dozens of celebrities from Beyonce and Hugh Jackman to Justin Theroux and Nicole Kidman are all rumored to follow a varied version of IF. But what is it?

IF is an overarching term used to describe diet regimens that involve cycling brief periods of fasting or very low -calorie intake with unrestricted calorie intake on non-fasting days, with the ultimate goal of weight loss. This approach attempts to combat the body’s metabolic response to a lower calorie intake, which is to slow the metabolism, by varying the amounts of calories eaten between very low and more typical amounts.

The three common IF methods include the following:

Whole Day Fasting or 5:2   

  • Whole day fasting, or the 5:2 technique, caps your calorie intake at around 25% of daily calorie needs (500-600 calories) per day on two non-consecutive days of the week. On the five non-fasting days of the week, a ‘normal,’ though unrestricted, diet is eaten.

Time-Restricted Feeding (TRF)

  • The TRF method involves a designated window of time in which to eat each day. For example, you eat all meals within an eight-hour timeframe, say 10:00AM to 6:00PM, and then fast for the remaining 16 hours.  TRF may not be that much of a stretch for individuals that refrain from eating after dinner and prefer more of a brunch than breakfast.

Alternate Day Fasting

  • Just like it sounds, on alternate day fasting you feast one day and then fast the next.

Does it work?

After one overcomes the initial symptoms of hunger, fatigue, irritability and headaches, there is some evidence to suggest that improvements in fat mass, total weight, cholesterol, fasting insulin and inflammation may occur. It’s important to note that at this point, most studies have been conducted in animals and robust research in humans is lacking.

My thoughts

Adopting an IF method into your lifestyle may train you into a new eating routine, however, the lack of guidelines to follow on the non-fasting days may not help you learn how to make nutritious food choices or address any deeper food relationship issues. Additionally, if non-fasting days are treated as ‘cheat days’, one may end up counteracting any potential benefit of the fasting behavior. Eating a restrictive and poorly balanced diet may also lead to eventual nutritional deficiencies.  

Further, a fasting regimen is certainly not recommended during pregnancy or breastfeeding or for individuals with certain health conditions, that are of adolescent age or have had a history of disordered eating.


If you still plan to kick off 2019 with the IF style of eating, I would strongly encourage a discussion with your physician to review any potential issues or concerns related to your health history and/or medication usage. Of course, I would also recommend a consultation with a registered dietitian nutritionist (RDN) that can better address the ‘why’ behind your choice of this approach over others and assist with meal planning and supplementation to prevent nutritional deficiencies. When it comes down to it, many of the known potential benefits of IF can be achieved by making more traditional and sustainable behavior changes such as eating more whole grains, fruits, vegetables, fish, nuts, seeds and beans, moving daily, getting enough rest, eating mindfully, etc. Perhaps it’s worth taking a closer look at these less trendy habits instead of the fad?  

— Beth Stark, RDN, LDN

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