Everything You Need to Know About Protein and Fasting Diets

Thanks to social media, fasting approaches like time-restricted eating (TRE) are gaining popularity right now. They promise rapid fat loss, a boost in energy and improved metabolic health, all without the rigmarole of plugging your calories into My Fitness Pal. All you need to do is eat within a set window, and let your body do the rest.

But is it really that straightforward? Not exactly. While fasting focuses on when you eat, your diet quality – especially your protein intake – is still an important consideration, especially if you’re plant-based. 

To help us unpack the science, we spoke with Yana Petkova, a PhD researcher working under Dr Adam Collins, Form’s Head of Nutrition. Dr Collins is also the Director of BSc and MSc Nutrition at the University of Surrey and holds a PhD in Body Composition.

“When we look at the research, there is very little focus on changes to diet quality during fasting-based diets,” says Yana. “For example, we don’t know for sure whether limiting the opportunities to eat in the day could compromise our intake of certain important nutrients, such as fibre and protein.”

“People with a lower protein intake, such as those following a vegan diet, or older adults who may have higher requirements, might be at risk of under-consuming protein if they’re following a fasting-based diet.”

Here, we asked Yana to explain what fasting actually does in your body, and how protein fits into the bigger picture…

What is fasting?

Fasting is the state we enter once the body has used up the energy from our last meal. This typically takes around 10–12 hours, which means many of us only spend a short window in this state – usually during the latter part of an overnight fast. As soon as we eat breakfast, we move into what’s known as the postprandial metabolic state, a period lasting several hours in which we absorb and utilise the energy and nutrients from our food.

With food so readily available and lifestyles becoming increasingly irregular, a growing proportion of the population fasts rarely – if at all. In fact, many people only refrain from eating for 8–10 hours overnight, meaning they may spend virtually no time in a true fasted state.

Why is fasting gaining traction?

Evidence is steadily mounting that spending even a few hours in a fasted state can be beneficial for health. During fasting, the body switches from using glucose to relying more heavily on fat as a primary fuel source. In practical terms, this means fat cells begin releasing more fat than they store -a process called lipolysis.

Fasting also appears to create ideal conditions for cellular repair, although researchers are still uncovering the precise mechanisms involved.

At a whole-body level, dietary approaches that increase time spent fasting are associated with a spontaneous reduction in calorie intake, small to moderate weight loss, improved blood pressure and enhanced insulin sensitivity.

How does protein fit into the fasting puzzle?

Protein is particularly interesting in the context of fasting because there’s still no clear consensus on what actually ‘breaks’ a fast. For example, advice varies widely on whether a dash of milk in your tea or adding pure fat to your morning coffee is acceptable when practising TRE.

Carbohydrate intake clearly raises insulin, flipping the metabolic switch from fat-burning (lipolysis) back to fat storage, and shifting the body toward glucose as its main energy source. In theory, nutrient intake that maintains lipolysis – namely, foods without carbohydrate – could help preserve some of the metabolic benefits of fasting.

Pure fat would maintain lipolysis, but it also provides an immediate energy source. For many people this isn’t particularly appealing taste-wise, and it may reduce the body’s incentive to draw on its own fat stores.

Protein has a more nuanced effect. Although it does supply energy, consuming it without carbohydrate or fat may limit its ability to disrupt the fasted state. Preliminary findings from an unpublished study under review suggest that while consuming 20 g of unflavoured plant protein caused a temporary change in fasting biomarkers, it did not cause a whole-body shift to a postprandial state.

Certain amino acids, aka the building blocks of protein, can cause a rise in insulin, but this increase is smaller than the response seen after eating carbohydrates. Animal proteins typically contain more of the insulin-stimulating amino acids than plant proteins, so plant-based protein may preserve fasting to a greater extent.

Why might I want to consider protein while following a fasting-based diet?

Fasting-based dietary approaches, such as alternate – day fasting, the 5:2 diet, and time-restricted eating (TRE) – often lead to smaller portion sizes and fewer eating occasions throughout the day. This can increase the risk of under-consuming protein, particularly in individuals who already have a lower baseline intake. Early results from an ongoing TRE study show reductions not only in calorie intake but also in fibre and protein in adults eating an otherwise unrestricted diet.

Because the body has a very limited ability to store protein, a fasted state naturally increases the breakdown of muscle tissue to supply the protein needed for essential functions. Adequate protein intake, ideally spread evenly across the day and paired with resistance training is therefore crucial for maintaining lean mass, especially during weight loss.

Protein is also the most satiating macronutrient, so including a protein-rich ‘snack’ may help curb hunger and improve adherence for those who struggle with fasting-based dietary patterns.