Protein Fasting: Science or Clever Marketing?

How to separate nutrition research from social media trends

If you spend any time on social media, you’ll know that nutrition trends come and go at an incredible speed. Every few months there seems to be a new “secret” to better health.

A new superfood. A new supplement. A new protocol promising to unlock weight loss, longevity or optimal health.

Recently, I’ve been asked by several people about something called “protein fasting.” Perhaps you’ve seen it too. The idea is being promoted as a weight loss tool and claims to create ‘effortless calorie deficits’, muscle preservation and appetite and cravings control.

And I completely understand why this sounds appealing.

I’d like to start by saying that fasting is primarily not a weightloss tool. The science behind fasting and longevity is genuinely fascinating. Researchers are exploring how periods without food, or reduced intake of certain nutrients, can influence some of the body’s most important biological pathways.

But there is an important distinction we need to make:

Interesting science does not automatically equal proven nutrition advice.

And this is where things can become confusing. Because a real area of scientific research can sometimes be taken, simplified, repackaged and turned into something that sounds much more certain than the evidence actually shows. So today, I want to unpack the science behind fasting, explain what we actually know, and answer the question:

Is protein fasting a genuine nutrition strategy or is it clever marketing?

First, what exactly is fasting?

Fasting simply means a period of time where we are not consuming food. Humans have been fasting for thousands of years, often for cultural or religious reasons. More recently, researchers have become interested in fasting as a potential tool for metabolic health and longevity.

There are several different approaches that fall under the umbrella of fasting:

Intermittent fasting Periods of eating followed by periods of fasting. For example, a 14–16 hour overnight fasting window.

Time-restricted eating A type of intermittent fasting where food intake is limited to a specific window during the day.

Prolonged fasting Longer periods without food, usually lasting 24 hours or more. (This is the ‘OG’ way of fasting and what scientists and nutritionists usually mean when they use the term). 

Fasting-mimicking diets Specific dietary approaches designed to mimic some of the biological effects of fasting while still providing some nutrients. (Personally I’ve found this approach helpful for myself and in clinic).

The research around these approaches is fascinating. But it is also nuanced. Fasting is not a magic switch that suddenly turns on “health mode.” Our bodies are far more complex than that.

What actually happens inside the body when we fast?

One of the reasons fasting has become such a hot topic in longevity research is because it appears to influence some fundamental biological pathways involved in aging and cellular health.

You may have heard terms like:

• AMPK

• mTOR

• autophagy

  
  

These names can sound intimidating, but the concepts are actually quite intuitive. Think of your body as an incredibly intelligent, constantly adapting system. When nutrients are readily available, your body is in a state of growth and building.

It uses energy to:

• build new tissues

• create proteins

• repair muscle

• support hormones

• maintain everyday functions

  
  

One of the key pathways involved in this “growth mode” is called mTOR.

mTOR is essentially a nutrient sensor. When we eat protein, particularly amino acids like leucine, it signals to the body: “We have resources available. It’s a good time to build and repair.” And this is a very good thing.

We need mTOR. It helps us build muscle, maintain strength, recover from exercise and repair tissues. Especially as we age, maintaining healthy mTOR signalling is incredibly important. 

However, during periods of fasting or reduced nutrient availability, the body shifts priorities.

Another pathway called AMPK becomes more active. AMPK acts almost like an energy sensor. It helps the body become more efficient with its resources and encourages processes linked with cellular maintenance. One of the most fascinating processes researchers study is autophagy. The word literally means “self-eating,” but that makes it sound far more alarming than it is! Autophagy is essentially the body’s recycling system.

It helps identify old, damaged or unnecessary cellular components and break them down so they can be reused. Imagine a spring clean inside your cells. This is one reason scientists are interested in fasting and its potential role in healthy aging.

So fasting really is about recycling ‘tidying up’ not about weight loss. In my clinical and personal experience weight loss caused by fasting is usually temporary as we cannot fast forever and our bodies are great at getting back to their ‘preferred weight’ once we start eating normally again. In other words it is not a sustainable weight loss strategy. 

So where did “protein fasting” come from?

This is where things become interesting. The concept of combining fasting with protein supplementation comes from a very real challenge in nutrition science:

How do we create the potential benefits of fasting while making it easier, more sustainable and less likely to result in muscle loss?

Traditional fasting means abstaining from calories for a set period of time. However, one of the challenges with longer fasting periods or significant calorie restriction is that the body does not only use stored fat for energy.

It can also break down muscle tissue. And this matters. Because maintaining muscle is one of the most important predictors of healthy aging. Especially as we get older, losing muscle mass can affect:

• strength

• mobility

• metabolic health

• insulin sensitivity

• independence

  
  

So the idea behind approaches like “More Protein Fasten (MPF)” is understandable:

Could consuming a low-calorie protein shake during a fasting window help people maintain muscle and reduce hunger, while still capturing some of the benefits associated with fasting?

It is an interesting question. But it also raises another important question:

If you are consuming protein and calories during your fasting window, are you still actually fasting? The answer is NO

The difference between fasting and calorie restriction

This is where science becomes important. Fasting is not simply “eating fewer calories.” The reason researchers are interested in fasting is because the absence of nutrients creates specific changes in the body. When we fast, insulin levels typically decrease, nutrient signalling changes and pathways involved in cellular maintenance become more active. This is part of what makes fasting physiologically different from simply eating less.

When we consume a protein shake, even a relatively low-calorie one, we introduce amino acids into the body. And protein is a powerful biological signal.

Remember mTOR?

The pathway involved in growth, repair and muscle building? Protein, particularly the amino acid leucine, activates mTOR. And that is exactly what we want when we are trying to build and maintain muscle.

But it also means the body is receiving the message: “Resources are available. Let’s switch back into a fed state.”

This is not a bad thing. In fact, it is incredibly beneficial. But it is NOT fasting. 

So does protein during a fast “break” the fast?

From a physiological perspective, a true fast means a period without energy intake, where nutrient-sensing pathways respond to the absence of food.

A protein shake changes that environment.

It provides:

• calories

• amino acids

• a signal of nutrient availability

Therefore, while a protein-supported fasting approach may still create a calorie deficit and may help some people lose weight, it is not at all the same biological state as a traditional fast.

And this distinction matters because the marketing can sometimes blur these two ideas. A strategy can be useful without needing to call it something it isn't.

Is protein fasting bad?

So we've established that there is no such thing as 'protein fasting' and it is in fact a made up term. Essentially what people are referring to is an extreme calorie deficit where all food is replaced with 2 protein shakes (obviously sold by them...). However, this is where I want to be very clear: I am not saying that anyone who uses protein shakes is doing something wrong. Nor am I saying that intermittent fasting has no place. Nutrition is not black and white.

For some people, a structured approach that includes protein shakes may help with:

• appetite control

• convenience

• increasing protein intake

• reducing overall calorie intake

  
  

And for someone who is currently consuming a poor-quality diet and struggling with overeating, this may feel like a helpful first step. But my concern is when a commercial product takes a complex area of nutrition science and turns it into the next “secret” to health. 

And importantly, replacing real food with shakes comes with its own issues. 

The problem with relying on shakes instead of real food

Protein powders and supplements can absolutely have a place. I use and recommend supplements when there is a clear reason for them. But they should support a nourishing diet. Not replace it.

Whole foods provide far more than protein.

They provide:

• fibre to support the gut microbiome

• polyphenols that support cellular health

• vitamins and minerals

• healthy fats

• thousands of beneficial plant compounds we are still learning about

  
  

A shake can provide protein. But it cannot replicate the complexity of a whole-food meal. And this is especially important when we talk about healthy aging. Because the goal is not just weight loss. The goal is a body that remains strong, resilient and healthy for decades.

The bigger conversation: what are we actually nourishing ourselves with?

There is another part of this conversation that I think is important. Even if we put the debate around fasting aside, I believe we need to ask a bigger question:

What kind of food culture are we creating?

Increasingly, we are being encouraged to believe that health comes in the form of powders, sachets and highly engineered products designed to taste like desserts.

And I understand the appeal.

Convenience matters. Life is busy. A product that promises to make health easier is naturally attractive. But as a nutritionist, I always come back to the same principle:

Food is not just about calories, protein grams or hitting a target. Food is information for the body.

The quality of what we consume matters. Many highly processed nutrition products rely on ingredients such as artificial sweeteners, flavourings, thickeners, emulsifiers and other additives to create a particular taste and texture.

While these ingredients may be considered safe within regulated limits, we know for a fact that they can negatively impact our gut health (see my recent interview with gastroenterologist Dr Giulia Enders!). 

A protein shake can be a useful tool when there is a genuine need for convenience or additional protein. But I do not believe it should replace a relationship with real food. Additionally if we do choose a protein shake let’s opt for one with minimal natural ingredients rather than a 100s of artificial additives!

The foundations of health have not changed:

Colourful plants.

Quality protein.

Healthy fats.

Fibre-rich foods.

A diverse, nutrient-rich diet.

Meals that nourish not only our bodies, but also our enjoyment of food. 

So when clever marketing companies invent new concepts, always ask yourself if they align with the above and ask yourself if what is being offered is truly sustainable.