In our never-ending search for a magic cure for all of our ailments, health and fitness gurus are constantly trying out new methods to burn fat and build muscle. One of the more unique methods praised by both internet sensations and health professionals alike is intermittent fasting. As with any popularized “diet” plan, people are singing the praises of intermittent fasting and all of it’s wonderful, life-changing power.

So what is it? Does it work?

Is it the magic solution we’ve all been dreaming of? (spoiler alert: no)

What is intermittent fasting?

Although fasting practices have been around forever, intermittent fasting has recently been popularized as a weight loss tool. The basic idea behind intermittent fasting is that you eat at designated times and then fast the remainder of the time. Advocates of fasting as a weight loss technique will tell you that you can lose fat, build muscle, and increase your energy all while eating more food than before.

Sounds amazing, right?

Of course, as a dietitian, my initial reaction to this trend was “Yea, ok.”

However, it’s not fair to judge right off the bat just because it’s a trend. So let’s break down the science and see what’s really going on.

 Types of Intermittent Fasting

There are several different ways a fast can be set up. Some are more extreme, while others seem more reasonable. It’s important to note that NONE of these methods are an excuse to eat twelve hamburgers and an entire sleeve of oreos during feeding times. In order for this to work, you still have to eat a reasonably healthy diet or at the very least not be eating more than you were before to see a change (Boo.). So without further ado, here are some of the most popular methods of fasting:

16/8 fasting

Probably the most reasonable and sustainable of all the fasting regimes. In this method, you eat during an 8 hour “feeding window”. The other 16 hours of the day, you are only allowed zero calorie beverages. So if you ate your first meal at 10 a.m., you would stop eating at 6 p.m.

Pros: reasonable feeding times, adjustable to individual schedules, fairly normal meal pattern

Cons: hunger during fasting periods, difficult to adapt to last minute changes

5:2 Fasting

In this method, you eat normally five days a week. The other two days, intake is restricted to 500 calories. In some plans, you may not eat at all on these days.

Pros: Most of your week is fairly normal

Cons: Two days a week you’re going to be VERY hangry



This may be the most challenging of all the fasting methods. In this version, you take a 24 hour break in between meals. Meaning that you eat dinner at 7 p.m. the first day and then don’t eat again until 7 p.m. the next day. This is customizable to your meal of choice, so if you prefer lunch to lunch or breakfast to breakfast, it works just the same.

Pros: Eating one giant mega meal

Cons: Eating one giant mega meal, getting seriously hungry


In all of these scenarios, there seems to be one glaring issue and that’s hunger. Is it really worth it to spend your whole day focused on your next meal because you’re starving? Let’s see what science has to say.

 Let’s Talk Science

The biggest problem with getting a scientific opinion on intermittent fasting is that research is all over the place. To be fair, there is a TON of ground to cover. What population do we look at? What kind of fasting should we study? Do we include exercise?

Research on  this topic is fairly new, but here’s what we know so far:

1. It wasn’t as much of a difference as we thought/hoped for.

For example, one study found that while the fasting group lost a significant amount of fat, they were also in a calorie deficit compared to the non-fasting group. Another study compared intermittent fasting to good ol’ energy restriction and found that fasters had a 3 – 8% reduction in body weight while those on the energy-restricted diet lost 4 – 14% body weight. Meaning that the reason intermittent fasting works may just be because it puts you in a calorie deficit (more on that later).

2. Biomarkers improved

One significant change noted between those who fasted and those who didn’t was a change in biomarkers. Most studies found changes in biomarkers that mimic a calorie-restricted state (note: some studies found no significant difference in biomarker changes), despite the fact that the fasting group was only in a slight calorie restriction (meaning you wouldn’t expect to see those changes).

What does this mean? The results would suggest that intermittent fasting tricks the body to thinking it’s in a calorie deficit even if the deficit is only minor (“Yay, we’ve found a solution!” Hold that thought).

3. Better synchronization with circadian rhythm

This last point is based on the assumption that your feeding window is during the day time. In animal studies, food intake has shown to cue circadian rhythms. To put it simply, food intake appears to cue the body as to when it is supposed to be awake and alert. It’s important to know that this hasn’t been replicated yet in humans, but the possibility still exists. Despite that lack of human studies on circadian rhythm influence, we do know that late night eating can disrupt sleep patterns and that people tend to experience decision fatigue towards the end of the day which can lead to poor food choices.

Some  important things to note…

ALL of these studies were small (we’re talking less than 100 people). Many studies also reported participants complaining of hunger during fasting times and that this hunger did not decrease over time (mainly noted in alternate day fasting). There were also a wide variety of populations study (fit, healthy, obese, exercisers, non-exercises etc.) which means that the results can’t be easily generalized to the population as a whole.

So…what does this mean for the average person?

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It means it might work for you, depending on your situation. If you’re an otherwise healthy person, fasting every now and then is certainly not going to hurt you. If you’re someone that struggles with frequent snacking or find yourself craving a giant bowl of ice cream right before bed, setting a feeding window might help you control these impulses. It also doesn’t hurt to practice self discipline, especially when you know you’re only restricted for a portion of the day. However, if you think you might die of hunger if you don’t eat for 16 hours, there’s no evidence to suggest that this method is any better than your standard calorie deficit. Healthy eating and fat loss is all about what is going to work for you as an individual, not what works best for an instagram star (#fitfam).

So how do you know if it will work for you? While not all inclusive, the following is a quick list of reasons to try or not try intermittent fasting:

Try intermittent fasting if…

  • you’ve been eating healthy and exercising but your weight/fat loss has plateaued
  • you struggle with snacking outside of meal times or temptation at certain times of day
  • it works with your schedule

Avoid intermittent fasting if…

  • you have a medical condition that would be affected by fasting times
  • you currently have or have had a history of eating disorders
  • you’re constantly miserable and hungry when you don’t eat


To sum it all up, healthy eating is about finding a balance that works for you. If you find that you make better choices when you set time restrictions for yourself, then give intermittent fasting a whirl. On the flip side, if you find that fasting makes you grumpy and tired, there’s no reason to force yourself to do it. While a useful tool, intermittent fasting has yet to be proved to be superior in comparison to other weight loss methods. Find a plan that will work well with your lifestyle and you’re more likely to maintain your progress long term.



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Written by: Rachel Pulley, MS, RD, LD

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