Are you still fasting if you have a protein shake?

January 2, 2026 •

4 min read

According to a 2020 survey by the International Food Information Council, intermittent fasting was the most popular diet of the year. This rise in popularity has many people questioning what is and isn't allowed during the fasting window, leading to a common question: Are you still fasting if you have a protein shake? The short answer is no, and here’s why you should save your shake for your eating window.

Quick Summary

Consuming a protein shake during a fasting window breaks the fast because it contains calories and triggers an insulin response. This metabolic shift stops key fasting processes like autophagy and fat burning. Protein shakes can, however, be a beneficial part of your daily nutrition when consumed during your designated eating period to support muscle maintenance and satiety.

Key Points

Caloric Intake Ends a Fast: Any food or beverage with calories, including protein shakes, will technically break a fast.
Insulin Response is the Key: The consumption of a protein shake triggers an insulin response, which pulls your body out of a fasted, fat-burning state.
The '50-Calorie Rule' is a Myth: The belief that a small number of calories won't affect a fast is largely unsupported by scientific research.
Timing is Everything: To benefit from both fasting and protein shakes, consume your shake only during your designated eating window.
Maximize Your Eating Window: Use protein shakes strategically after workouts or as a high-protein snack to help with satiety and muscle maintenance during your feeding period.
Understand Your Fasting Type: Depending on your goals, a "clean fast" involves zero calories, while a "dirty fast" allows for a minimal, specified caloric intake.

The Science Behind Why a Protein Shake Breaks a Fast

When you consume any food or beverage containing calories, your body's metabolic state shifts out of a fasted mode. The primary goal of many fasting protocols, particularly intermittent fasting, is to keep insulin levels low, allowing the body to use stored fat for energy in a process called ketosis and to initiate cellular repair, known as autophagy.

Protein shakes are not calorie-free. Most contain anywhere from 100 to several hundred calories, depending on the ingredients. This caloric intake, along with the amino acids present in the protein powder, stimulates an insulin response. Once insulin is released, your body switches back to using glucose for fuel, effectively ending the fast. This metabolic flip-flop prevents your body from maximizing the benefits associated with a fasted state, such as increased fat burning and cellular cleanup.

The “50-Calorie Rule” and Why It's Misleading

There is a common misconception online about a “50-calorie rule,” which suggests that consuming fewer than 50 calories during a fast will not break it. This idea is largely unsupported by scientific research and is based on anecdotal evidence. Experts, such as registered dietitian Scott Keatley, emphasize that technically, any amount of calories will break a fast. While a very small number of calories may have a minimal effect on your metabolism, it is safest to aim for zero caloric intake during your fasting window, especially if you are pursuing benefits like autophagy. For those seeking strict fasting benefits, the only way to be sure is to consume only water, black coffee, or plain tea.

Fasting Protocols: Clean vs. Dirty Fasting

For those who find a strict zero-calorie fast challenging, some people practice a concept known as "dirty fasting".

What is Clean Fasting?

Zero Calories: Absolutely no caloric intake during the fasting window.
Allowed Drinks: Water, black coffee, and plain tea.
Goal: To maximize metabolic benefits, fat burning, and autophagy by keeping insulin levels completely suppressed.

What is Dirty Fasting?

Minimal Calories: Allows for a very small amount of calories, sometimes up to 50, often from fats or non-caloric sweeteners.
Possible Additions: A splash of milk in coffee, or bone broth.
Goal: To make fasting more sustainable and reduce hunger, though it may compromise some benefits like autophagy.

Most research on the benefits of fasting, such as improved blood sugar control and weight loss, has been conducted on protocols with zero caloric intake during the fasting period. The effect of dirty fasting is less studied, and it remains a personal choice based on your goals and sustainability.

A Comparison of Protein Sources for Fasting Protocols


Protein Source	Impact on Fasting Window	Best Time to Consume	Benefits for Fasting	Drawbacks for Fasting
Whey Protein	Breaks the fast due to calories and insulin response.	During eating window.	High-quality protein, quick absorption for muscle recovery.	Not fast-friendly; can be high in sugar or other additives.
Casein Protein	Breaks the fast due to calories.	During eating window, often at night.	Slow-digesting, promotes longer-lasting satiety, supports muscle maintenance.	Caloric; must be consumed during eating period.
Collagen Protein	Most forms contain calories and will break a fast, though some hydrolyzed versions might be less impactful.	During eating window.	Supports joint, skin, and hair health; can be easily mixed.	Caloric; impact on insulin depends on form and other ingredients.
Hemp/Pea/Rice Protein	Breaks the fast due to calories.	During eating window.	Plant-based options, often high in fiber; good for satiety.	Caloric; must be consumed during eating period.
Clear Whey Protein	Still contains calories and breaks a fast.	During eating window, post-fast recovery.	Fast-digesting, light, low-carb/low-fat options available.	Caloric; not suitable for the fasting window.

How to Strategically Use Protein Shakes with Your Fasting Plan

Instead of viewing protein shakes as a compromise to your fast, consider them a powerful tool to be used during your eating window. Properly timing your shake can help you achieve your goals more effectively.

Breaking Your Fast with a Shake

Some people prefer to break their fast with a protein shake. It can be a light, satisfying way to ease back into eating, and the high protein content can help curb hunger and prevent overeating at your next meal.

Using a Shake for Post-Workout Recovery

If you time your workout near the end of your fast, a protein shake is an excellent post-workout option to kickstart muscle repair and recovery immediately after entering your eating window.

Supplementing Your Protein Intake

For those with high protein goals, especially active individuals, a protein shake is a convenient way to meet your daily protein needs without consuming excessive calories from whole food sources.

Conclusion

In summary, consuming a protein shake during your fasting window will break your fast. While tempting to stretch the rules, doing so can compromise the very metabolic benefits you're aiming for. The key is strategic timing: use your protein shake as a valuable tool during your eating window, whether for post-workout recovery, a satisfying snack, or to simply meet your daily protein targets. By understanding and respecting the rules of your fasting protocol, you can use protein shakes to enhance your health journey rather than derail it.

(https://www.performancelab.com/blogs/multi/how-many-calories-break-a-fast)

Frequently Asked Questions

If you accidentally drink a protein shake during your fast, you will break the fasted state. The best approach is to get back on track with your fasting schedule and not dwell on the mistake, as consistency is more important than perfection.

No, you should wait until your eating window to have a protein shake. If your workout happens during your fasting window, have your shake immediately after your fasting period ends to aid with muscle recovery.

Both whey and plant-based proteins contain calories and will break a fast. The source of protein is less important than the caloric content and subsequent insulin response it triggers.

No, any protein powder containing calories will break a fast. You should avoid all caloric products, including protein powders, during your fasting window.

During a clean fast, you can only have calorie-free beverages such as water, black coffee, and plain tea. It's best to avoid all sweeteners and additives.

For muscle gain, ensuring adequate protein intake is crucial. While a fast can be beneficial, consuming protein during your eating window is essential. Protein shakes can be a convenient way to achieve this, and timing them around your workouts during the eating period is recommended.

Yes, collagen protein powder contains amino acids and calories that will cause an insulin response and break a fast, even if it's a 'lighter' option. It should be consumed during your eating window.