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Nutrition

Get excited about nutrition, and learn as you go with these information-packed resources on a wide variety of nutrition-centric topics! Our bistroMD experts review the importance of the macronutrients protein, fat, and carbohydrates, as well as how to make them work most efficiently for you.

Protein Fasting Pros & Cons to Know

When trying to lose weight or optimize health, a protein-restrictive diet is often not a common solution. But could there truly be benefits of protein fasting? Here’s exactly what to know.

Protein Fasting Pros & Cons to Know


Protein is typically the least vilified macronutrient of the three - fat and carbohydrate being the other two. Interesting, then, that protein fasting is a moderately popular diet these days. 

So, is this dietary approach truly safe or effective? Discover all you need to know about protein fasting side effects and protein fasting pros and cons here! 

Benefits of General Fasting

Understanding protein fasting without first understanding the science behind fasting, in general, is difficult. Perpetuated for health and religious reasons, fasting refers to an eating pattern that consists of fasting and eating windows. There are various varieties of fasting such as intermittent fasting, 12-hour fasting, and extended water fasts lasting more than 24 hours and up to multiple weeks (under medical supervision).

The biggest benefit of fasting is the induced autophagy of cells. Without food or energy, the body essentially aims to reset itself. The good cells go and clean up the old, dysfunctional cells and essentially eat them for fuel, only to then regenerate newer, better, and healthier.

Also interesting is that fasting can actually improve metabolism in the long run. Everyone has heard that lowering calories slows metabolism. While this is correct to some extent, longer fasts with little to no energy intake once again allow the body to reset itself, including metabolism.

When someone simply lowers calories, the body adapts by matching energy output to intake - aka slowing metabolism, usually by moving less. However, when absolutely no energy intake is present, the healthy cells eat the disruptive cells, and the body halts insulin secretion. This creates an environment for metabolism to reset and function more optimally after refeeding. 

This notion of cell regeneration through fasting is wonderfully conceptualized by cancer patients. Many studies now show that patients who fast through chemotherapy treatment experience minimal or much fewer side effects than those who continue eating.

Obviously, they receive ample medical oversight and are typically allowed to consume black coffee, carbonated/sparkling water, herbal teas, and electrolytes along with plain water. However, this directly opposes the common notion that the body absolutely needs nutrients to properly heal. Indeed, it seems that doing just the opposite can be extremely beneficial for certain situations.

Also popular is the concept known as intermittent fasting, with the most popular method is fasting for 16 hours overnight and eating within an 8-hour window. While this elicits similar benefits as longer fasting, it is not as effective because the body simply has less time to clear out all the bad cells. Nonetheless, consistently offering the body 16 hours of fasting is still considered healthful for various populations. Keep in mind that not everyone should or will benefit from fasting though.

All in all, fasting is an effective, results-driven healing modality that can be safely practiced with the right knowledge and guidance. Main health benefits include:

• Inducing potent autophagy
• Decreasing blood glucose production and insulin secretion
• Regenerating stem cells of the immune system
• Increased production of tumor-killing cells
• Can promote long-term weight loss
• Improves longevity and vitality

Keeping in mind the differences between complete water fasting, intermittent fasting, and food group fasting (i.e. protein fasting or the keto diet which fasts carbohydrates), discover the potential benefits and risks of protein fasting below.

What Is the Protein Fasting Diet?

Coined by Bulletproof as Bulletproof Protein Fasting, creators and proponents claim protein fasting is a powerful, but underutilized tool for peak performance. Sometimes referred to as protein cycling, the practice involves limiting protein intake to less than 15 grams one day per week. Healthy fats and carbohydrates like fruit and vegetables are encouraged.

The main benefits protein fasting touts include:

• Managing/decreasing inflammation
• Kick-starting fat loss
• Maximizing performance

If it seems a bit counterintuitive to what Google and the alike say, that's because it is. Protein is important for weight loss and for muscle growth and recovery. It is also typically considered the least inflammatory macronutrient. 

Yet, protein fasting apparently induces similar mechanisms as complete water fasts.

Bulletproof purports that protein fasting once a week induces autophagy and encourages the degradation of old cells into newer, better ones. Autophagy is the body's natural cleaning process, recycling oxidized "junk" in cells and turning them into new viable proteins and membranes.

More specifically, protein fasting rather than fat or carb fasting might be more conducive to autophagy because proteins literally do everything besides provide energy. Due to this, when the body does not detect protein consumption, it will do whatever it takes to recycle the available proteins already present within the body so they can continue doing their jobs such as repairing and communicating with other cells. And because the body is tremendously brilliant, it aims to recycle the old, less functional, oxidized proteins first.

So after a day of protein fasting and after consecutive weeks of protein cycling, the body theoretically possesses more vibrant, higher functioning cells. Proponents do reiterate that chronically undereating protein is not safe or encouraged. 

To determine if protein cycling is right for you, consider the pros and cons along with side effects.

Side Effects of Protein Fasting

Interestingly enough, there are not many side effects of limiting protein in the short term. Many people still think the body functions as a 24-hour robot. Meaning, if someone does not eat enough of XYZ, they are immediately going to become malnourished and their metabolism and body will shut down.

In fact, many cultures have and still do regularly practice fasting techniques with little to no harm. As is hopefully clear by now, regular and/or consistent extended fasters generally experience better health related to the profound power of autophagy induced by fasting. 

The side effects that many people think would happen - like muscle wasting, nutrient deficiencies, and malnutrition - usually do not occur. 

In fact, limiting protein every once in a while caters to muscle building because autophagy is necessary to build muscle mass. Limiting protein also shuts off mTOR, which is necessary for building muscle. 

It is also very difficult to become malnourished or nutrient deficient during extended water fasts (like 3-5 days long!), let alone one day of protein fasting but continuing to eat carbs and fat.

Thus, the most typically reported side effects include:

• Inability to feel completely full
• Feeling more hungry than usual
• Feeling slightly weaker in hard/intense workouts
• Feeling colder than normal

Pros of Protein Fasting

Even though there are some solid pros to protein fasting, any type of fasting is not right or beneficial for everyone. Fasting generally is not beneficial for populations that require consistent food intake to sustain health such as people with type 1 diabetes or pregnant or breastfeeding women. In addition, it is contraindicated for those with autoimmune viruses like HIV, the elderly, and those with or with a history of disordered eating/eating disorders.

Working with a professional when navigating any sort of fasting endeavor is always best, especially to ensure correct electrolyte balance. A registered dietitian or another health professional can successfully guide fasts that result in beneficial health outcomes. 

But, without further ado, here are the potential pros of protein fasting:

• Induces systemic autophagy
• Rejuvenates cells
• Spares muscle when executed correctly
• Helps build bigger, stronger muscles over time
• Promotes increased longevity
• Increases subjective willpower and self-efficacy
• May induce moderate weight loss over time (in addition to nutritious eating and healthy exercise)

Cons of Protein Fasting

Health experts suggest there are not too many cons to protein fasting. While it might sound extreme to some, it is not more extreme than those who fast for 16 hours every day. Eating less protein (>15 grams) one day of the week certainly will not harm you, let alone instantly catabolize muscles. Once again, research shows this practice helps with muscle maintenance and growth in the long run as well as extending longevity.

Thus, most of the cons are related to subjective feelings on the day of the fast only and include:

• Not feeling full enough
• Feeling weaker in workouts
• Cravings for high protein foods
• Incessant thoughts about food

Notice hair will not start falling out, nails will not break, and nutrient stores will not plummet. However, it cannot be reiterated enough that this diet could be very problematic for certain populations, as mentioned above. So, it is imperative to consult with a professional before hopping into this journey.

Furthermore, notice that weight loss is not a likely side effect although the diet is sometimes touted for its potential to kickstart that process. While it may subjectively lead to learning other helpful weight loss habits, it could also theoretically backfire. If one is eating inflammatory carbs and fats on their protein fasting day, this could result in unnecessary weight gain.

Overall Review of Protein Fasting

While much of the article may seem biased towards the benefits of fasting protein once a week, in the end, this is just another diet. It perhaps holds more merit than other fad diets, but really, it is just another form of fasting.

The induced autophagy of protein fasting is definitely its most driven benefit. However, research still seems to show that long to extended fasts with zero energy intake are more effective at inducing potent autophagy. Ultimately, this means that if seeking autophagy and are healthy enough to choose between protein fasting or longer intermittent fasting/extended fasts of 24 hours or more, the latter is the path to choose.

Conversely, protein fasting is likely easier to follow than intermittent fasting for 16 hours or doing a long water fast. One can still eat carbs, fat, and up to 15 grams of protein on their fasting day. Therefore, the risk for any acute and long-term damage is very minimal and it does not require such strong willpower or mental toughness. 

For reference, there is a much higher risk for complications like electrolyte imbalance, mood destabilization, and nutrient deficiencies with outright fasting. This especially serves true when fasts extend beyond 72 hours.

Overall, this dietary pattern receives a very average score of 2.75/5. It likely will not cause any harm, but there are also more effective dietary patterns that elicit more potent autophagy and/or help maintain proper muscle mass.

Reference:

Griffith T. Fasting and Cancer: The Science behind This Treatment Method. Healthline. Updated September 29, 2018. https://www.healthline.com/health/fasting-and-cancer.

Sarah Asay's Photo
Written By Sarah Asay, RDN. Published on February 26, 2018. Updated on July 01, 2022.

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