Food While Fasting?

Nutrition and Diet

Can the right diet give you all the benefits of fasting without going hungry?

If you’re not familiar with the very popular diet trend of Intermittent Fasting, you can read a bit of the backstory here.

But First, A Story

A few weeks ago I came across a company selling a drink mix that you take while you’re fasting to help maintain normal electrolyte balance and obtain essential vitamins, minerals, and amino acids. I laughed out loud because obviously everyone knows that fasting means you’re consuming nothing…right? I even went so far as to call it “peak nutrition BS” on my social media because certainly there was nothing sillier than a product that claimed to provide nutrition support during a time when the very point is that you’re not taking in any nutrients.

It turns out, I was wrong.

I was wrong about the fact that THAT was peak nutrition BS, because things have only gotten more out of control from there. A few days ago, something even more ludicrous made it’s way onto my Twitter TL… an energy bar that you eat while you’re fasting!

Let That Sink In…

Let’s just let that sink in for a minute. A bar that you eat while you’re…fasting? Now, a little electrolyte drink mix with a measly 36 calories per serving is one thing. Although, I’d need some serious evidence to convince me that it’s different in ANY WAY from the branched-chain amino acid (BCAA) powders you can buy from supplement stores for a fraction of the cost. But a full-on nut and honey energy bar?

Come on now! Take a look at the nutrition facts and ingredient list below…


I had to share this discovery on my instagram story (@stephthedietitian) and I tell ya, I was not disappointed by the responses…. my followers are the best BS spotters around!

I’ve seen similar nutrition profiles and ingredient lists on products at my grocery store. There is absolutely nothing in this bar that makes it worth $3.50 each. It’s not even keto!

All Jokes Aside

So, after we all finished laughing, I wanted to look into this company a little further to see if they were legitimately trying to market this with a straight face, or if I was going to find out the whole thing was from The Onion.

Just so we’re all on the same page, the very definition of fasting is ‘to abstain from food and drink’. Many fasts observed for religious reasons do not even allow water, whereas most fasting diets do allow water- some even encourage coffee or tea (the caffeine suppresses appetite and keeps your energy levels up).

So, what did I find?

The whole idea of the ‘Fast Bar’ originates from something called the “Fasting Mimicking Diet”, or FMD for short. So now it’s a little easier to see where the concept of eating while fasting might come from, if the whole thing is based on the idea that there’s a certain food combination out there that makes your body think it’s fasting even though it isn’t.

This FMD diet is patented by a company called ProLon, which is an offshoot of the company L-Nutra which was founded by Valter Longo, a researcher in the US who does work on aging and longevity. He’s written a book (of course) called the Longevity Diet which breaks down exactly how the FMD works.

If there’s one thing I’ve learned in our current state of nutrition science, it’s that just because there’s a conference talk, TEDx talk, or even a few studies on a website, doesn’t necessarily mean something is proven or legit. Just look at the flat-earthers and anti-vaxxers, they have conferences all the time!

So, what is the Fasting Mimicking Dieting (FMD)?

It’s essentially a very low-calorie diet (725-1100 calories per day) that you complete 5 days out of every month. This very low calorie allotment is meant to stop you from being hungry during your fast (because you’d certainly be good and full on 725 calories per day?), while still allowing your body to maintain it’s ‘fasted’ state. Of course ProLon will sell you all of the food you need for your 5-day FMD (in perfectly pre-portioned packages) for only $250.00. What a steal.

The other days of the month it’s recommended that you follow the general guidelines in the Longevity diet which is almost exclusively plant-based with limited added sugars and saturated fats, and no meat other than fish (which at the most you can eat 2-3 times per week). It recommends a very low protein intake, high intake of omega-3 fats, and limiting your number of meals per day while only eating within a 12 hour window.

Much of the research and examples used in the book are based on “Blue Zones” throughout the world, where there are the highest populations of people living past the age of 100. Their lifestyle habits have been of great interest to people wanting to live longer, so studies have been conducted on their diets, daily routines, and physical activity regimens.


The 5-day FMD part of the longevity diet is low in total calories, protein, and carbohydrates which promises all sorts of fancy-sounding things like ‘stem cell rejuvenation’, ‘improved metabolic health’, and of course the big thing that tends to draw people in… weight loss.

ProLon FMD

All this for the low low price of $250.00!

Fasting Mimicking Dieting Claims

So, let’s take a closer look into these FMD health claims, shall we?

Claim #1: The fastest way to lose weight

Well, since we know that the ONLY way to lose weight is to be in a caloric deficit, it’s pretty reasonable that a very large caloric deficit (such as the 725 calories you’re expected to consume during days 2-5 of the fasting mimicking diet) would get you more weight loss than a small caloric deficit over that same time period. Unfortunately though, that initial rapid weight loss (maybe a little fat, but certainly a lot of water and glycogen weight) will result in a slowing of your metabolism, which in turn makes it harder and harder to lose weight over time.

Extreme calorie restriction and deprivation can also be a major trigger for binge eating for a lot of people so, rapid weight loss may actually wind up causing weight gain in the long run. Sure, they recommend following the Longevity Diet guidelines outside of those FMD days, but if these types of restrictive diets “worked” for people, we wouldn’t have the billion dollar diet industry we do today. Their claims that lean muscle mass is somehow protected and maintained during these fasts is also total BS due to the extremely low protein intake. During times of fasting muscle mass is the first thing to go since it’s so metabolically expensive. When your body thinks you’re starving it does not care about keeping up the gun show.

It’s like a mini yo-yo diet sent right to your door, each and every month!

What about the non-weight focused claims?

Can the FMD really pump the brakes on aging and help us live longer?

Claim #2: Stem cell rejuvenation

How are your stem cells feeling today? Sluggish, or… Rejuvenated?  One of the major claims made by promoters of intermittent fasting or the FMD is that during times of fasting, our cells undergo a process called “autophagy”, which is when they become more efficient at getting rid of waste products or damaged cell components. It’s one of the hallmark features of the idea of longevity- if our cells are in autophagy more often or are better at it, we are more efficient at avoiding DNA damage, we slow down the aging process, and we reduce our risk of many diseases like cancer.


It actually may surprise you to find out that autophagy is a real thing. Yes, it sounds a bit made-up and too good to be true, but it is a real physiological process. What I always feel compelled to point out during discussions about autophagy though is that our bodies DO routinely enter this state on a daily basis- during sleep or after physical activity. Yes, when we are sleeping we are fasting, which is a period of time when many restorative functions in the body are taking place. After a bout of exercise our bodies have to recover and repair damaged cells, so it’s another opportunity for autophagy to do it’s thing.

I am highly unconvinced that there is any benefit of fasting during daytime hours if someone is getting adequate rest at night, and/or if they’re maintaining an active lifestyle. I also am unconvinced that intermitted fasting or FMD is an adequate replacement for proper sleep or exercise, and that we can simply bio-hack our way into good health.

Claim # 3: Better blood sugars, blood pressure, and blood lipid profile

Certainly all of these things will likely improve with a few days of very low calorie eating. The issue really is that many of the benefits are likely short-term only. If you go back to your usual diet in between fasts all of those benefits will quickly be erased if your eating patterns the other 25 days per month aren’t particularly nutritious. Yes, the longevity diet book contains specific recommendations for what to eat outside of the FMD days, but just like every other diet out there, it’s hard to follow in the context of every day real life.

Since the actual evidence for the value of the FMD protocol is so limited, I can think of some much better ways to spend $250.00.

Claim #4: The promise of prevention

The idea of disease prevention is a surefire way to make a sale in our “wellness” culture. It’s a very simple and often used marketing tactic to create an anxiety around an issue (ie- tell the consumer there’s something wrong or missing in their lives), then sell them the fix. The diet industry has us wrapped around their little finger, because what’s more anxiety provoking for most people than illness or death? The marketing teams behind these companies don’t even need to create an anxiety, it’s already prominent in our culture. No one can truly promise disease prevention, and guess what? None of us are going to be on this planet forever. It’s a hard truth to accept, but realizing it might keep some of your hard earned money in your pocket. Shelling out for gimmicky diet programs or stressing about which days this month you’re going to do your fast is just a waste of your time, money, and energy.

Fasting Mimicking Dieting: Backed by evidence?

If you’re looking for a more in-depth review of the book itself, Red Pen Reviews recently did a great piece on the Longevity Diet book. Essentially most of the healthy eating advice is reasonable- eat more plants, avoid highly processed foods, etc. They also note however that the evidence behind the actual FMD protocol (especially the minimal amount of protein encouraged and the demonization of meat) is non-existent, and felt that the overall dietary strategies in the book would be really hard to follow for most people. You can check out the full review here.

My advice?

When it comes to your own personal lifestyle habits, just keep it simple! Eat your fruits and vegetables, get enough sleep, and maintain an active lifestyle. When I read articles on these Blue Zone populations (or look to my Grandma who’s turning 92 years old this year), one thing is clear to me- older adults haven’t reached these ripe old ages because they spent a lot of money on fancy supplements or diet programs.

The Bottom Line

Eating well doesn’t need to be so complicated. There is probably limited benefit to following this FMD protocol, and I would be particularly critical of paying such a huge sum of money for a few small packages of food each month. Instead, eat a well-balanced nutritious diet most days out of the month (if you’re able), and spend that $250.00 on something better.

Overall, the dietary recommendations in the Longevity Diet aren’t awful ones, especially when compared to some of the other popular diets of the moment (like the carnivore diet). However, it’s still more extreme than most people need to go, and there is very little evidence that limiting protein will have the health benefits they claim. It also turns eating into a TON of work when someone has to worry about what they eat, when they eat it, and find the time to incorporate these 5-day FMDs into their life every month.

Are you feeling a little overwhelmed with all of the nutrition information out there? Do you find yourself spending a lot of time stressing about what the best diet is? Check out my thoughts here, Is Your Healthy Diet a Little Too Healthy?

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About the Author:

Stephanie Hnatiuk is a Registered Dietitian, Personal Trainer, and Diabetes Educator based in Winnipeg, Manitoba. She believes good health doesn’t need to be complicated, and that you can always have your cake and eat it too.