May 11, 2021 As a dietitian, I’m always steering people away from fad diets. These “get thin quick” approaches tend to run rife with unhealthy and unsustainable methods to lose weight. So, I’ll be honest, I’ve been very hesitant when it comes to advocating intermittent fasting (IF). And, after much research, I still wouldn’t advocate it as the best way to lose weight, but I have come to better understand the plans and some of the benefits. Since I often get questions about it, I wanted to share some insight with my followers here. First off, when it comes to the term “intermittent fasting”, there are multiple definitions. Among the most popular types of IF are:

Eating on Feed Day

Eating on Fast Day

# of Fast Days

Alternate Day Fasting (ADF)




Modified ADF (MADF)








Time Restricted Eating (TRE)




T The first three versions of IF listed above share the fact that on days of fasting, there is dramatically reduced intake or no intake of food at all.  For instance, with the ADF plan, a person eats what they want one day and then fasts completely the next. During the fasting day, the only thing allowed is non-caloric beverages (ex: water, coffee, tea). A modified version of this highly restrictive plan is known as MADF and allows for intake of approximately 500 calories on fasting days, or about 25% of one’s normal daily caloric intake. Again, this is a one day on, one day off approach, though because it is less restrictive than ADF, this plan typically has better adherence results. The 5:2 approach listed above involves eating normally 5 days a week and then on the other 2 days, restricting intake to 25% (or approximately 500 calories). The days are selected by the individual, but it is recommended that the 2 fasting days be spread apart and not done on consecutive days.  While there are other approaches to IF, these are the most common ones that involve fasting days. The last type listed in the table is time restricted eating (TRE), which involves eating every day, but restricting the hours during which one can consume food.  Again, there is much variability when it comes to this plan, as some call for 4, 6, or 8-hour daily windows of eating and specify eating only early in the day or late in the day. On these TRE plans, there are no days of complete fasting, however, so they differ from the others in that respect. Now that we’ve established what IF entails, the big question is, is it effective?  The answer, as you might expect, is not cut and dried. Many clinical trials have shown that people with insulin resistance may indeed benefit from IF more than they would from a calorie restriction (CR) diet. So for certain populations, IF may be a great idea. That said, when it comes to weight loss, fat loss, total cholesterol, triglycerides, blood pressure, glucose and A1C numbers, there is really no difference shown between subjects who simply follow a CR diet versus those who engage in IF. So, if you’re among the many who have thought of trying IF simply for weight loss, my recommendation is that you steer away from it and try an approach that doesn’t involve fasting. Rather, work on cutting calories by reducing consumption and increasing exercise. Working with a registered dietitian to come up with a plan tailored to your body, taking into account your likes and dislikes, is the most effective and sustainable way to approach weight loss.

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