Does drinking a protein shake break intermittent fasting?


Intermittent fasting has become an increasingly popular diet and lifestyle approach for weight loss, health, and longevity. The central premise of intermittent fasting is alternating between periods of fasting and eating. Many protocols involve daily 16-8 fasting, where you fast for 16 hours of the day and eat during an 8 hour window. However, there is debate around whether consuming certain foods or beverages during the fasted state breaks the fast and diminishes the benefits. One common question is whether drinking protein shakes or supplements breaks your fast.

What is Intermittent Fasting?

Intermittent fasting involves cycling between periods of fasting and eating. Some popular approaches include:

  • 16/8 method: Fast for 16 hours per day and restrict eating to an 8 hour window, such as 12pm to 8pm. This is the most popular type of intermittent fasting.
  • 5:2 diet: Fast or dramatically reduce calorie intake for 2 non-consecutive days per week but eat normally the other 5 days.
  • Alternate day fasting: Fast every other day, typically limiting intake to about 25% of energy needs on fast days.
  • Periodic fasting: Fasting for 1-2 complete days per week or month.

The proposed benefits of intermittent fasting include:

  • Weight loss and fat burn – fasting stimulates lipolysis.
  • Insulin sensitivity – fasting decreases insulin resistance.
  • Cellular repair – fasting activates autophagy and mitochondrial gene expression.
  • Longevity – fasting may extend lifespan by optimizing energy metabolism.
  • Cognitive function – alternate day fasting improved learning and memory in animal studies.

During the fasted state, the body is in a catabolic mode and taps into fat stores for energy. Consuming calories during this time may blunt some of the metabolic benefits. But will a protein shake interrupt the fasted state?

Do Protein Shakes Have Calories and Macronutrients?

Protein shakes and supplements come in different forms, but most contain some combination of:

  • Protein – typically whey, casein, soy, egg, beef or plant-based proteins
  • Carbohydrates – often as sugars, fibers or milk-based lactose
  • Fats
  • Vitamins and minerals
  • Other ingredients like cocoa, probiotics, creatine, etc.

So in almost all cases, protein supplements provide calories and macronutrients that need to be digested and metabolized. A typical serving may contain:

  • 100-200 calories
  • 20-40g protein
  • 5-30g carbs
  • 0-5g fat

While protein powder mixed with only water may be low in calories, adding ingredients like milk, nut butters, berries, bananas and oils increases the calories substantially.

Do Protein Shakes Spike Insulin?

Insulin is the hormone released by the pancreas in response to rising blood sugar levels after eating carbohydrates. It allows cells to take up glucose from the blood for energy or storage.

One proposed benefit of fasting is lowering insulin levels. Spiking insulin may inhibit lipolysis and shift the body from fat burning to glucose burning mode.

Protein stimulates insulin secretion, though not to the same extent as carbohydrates. Whey protein has been found to increase insulin levels, but not as high as sugary drinks [1]. The insulin response can vary based on the amino acid composition. Leucine, found in whey protein, is particularly insulinogenic.

While protein alone induces a moderate insulin response, combining it with carbohydrates can further spike insulin and glucose. Many commercial protein shakes contain sugar and other fast-acting carbs.

So most protein supplements will provoke at least a modest insulin reaction, breaking the true fasted state. Exceptions are supplements made with zero carbs and minimal insulin-spiking amino acids.

Can Protein Shakes Activate Muscle Protein Synthesis When Fasting?

Muscle protein synthesis (MPS) is the process of building new muscle tissue by synthesizing muscle proteins. It is stimulated by exercise and protein consumption.

During fasting, extended lack of amino acid intake and insulin can blunt MPS. This leads to concerns about muscle loss during intermittent fasting.

Research shows that consuming 20-30g of protein can maximally stimulate MPS after exercise [2]. Taking protein supplements during the fast may help counteract deficits in MPS.

However, other studies found no increase in MPS when subjects consumed whey protein after an overnight fast, due to decreased insulin sensitivity [3]. More research is needed on activating MPS with protein when fasting.

Will a Protein Shake Break Your Fast?

Based on the available evidence, most experts agree that protein shakes will technically break your fast by spiking insulin and providing calories that need to be metabolized.

However, moderate protein consumption does not appear to be detrimental for intermittent fasting. Potential benefits include:

  • Increased satiety – protein is very filling.
  • Support muscle retention.
  • Provide needed nutrients.
  • Enhance exercise performance and recovery.

Protein alone will have a minimal impact on blood sugar levels compared to carbohydrate-rich foods or beverages.

So having a protein shake is unlikely to sabotage the major objectives like accelerating fat burning, lowering insulin, and activating cellular repair processes.

Here are some guidelines on protein intake to minimize impacts on intermittent fasting benefits:

  • Keep protein under 30g per serving.
  • Choose a pure protein supplement without added sugars or carbs.
  • Consume earlier in the eating window to allow metabolic processes to return to a fasted state.
  • Limit to within 1-2 hours pre or post workout on exercise days.
  • Avoid alongside high carb meals.

Best Protein Powders for Intermittent Fasting

When selecting a protein powder to use during intermittent fasting windows, consider these factors:

Lean Proteins

Whey, casein, egg white protein, and collagen peptides have minimal carbs or fat. Lean proteins help limit calories and insulin stimulation.

No Added Sugars

Avoid protein powders with sucrose, dextrose, maltodextrin or other fast-digesting carbs. Look for low carb or keto-friendly shakes.

High in Leucine

Leucine is the amino acid with the greatest impact on muscle protein synthesis. Whey protein is naturally highest in leucine.

Unflavored or Lightly Flavored

Steer clear of mixes with syrups, juices or excessive flavor additives that add sugars. Lightly flavored vanilla or chocolate can curb cravings.

Mixable Only With Water

Shakes designed to mix well with just water avoid adding milk calories.

Low to Moderate Serving Size

Stick to 20-30g protein portions to minimize insulin response.

Here are some top protein powder picks suitable for intermittent fasting:

Protein Powder Key Attributes
Isopure Zero Carb Whey Protein Isolate 25g protein, 0g carbs, 110 calories per serving
Quest Protein Powder 24g protein, 1g net carbs, 110 calories per serving
Ancient Nutrition Bone Broth Protein 20g protein, 0g carbs, 120 calories per serving
Perfect Keto Collagen Peptides 15g protein, 0g carbs, 60 calories per serving
Garden of Life Raw Organic Protein 22g protein, 3g carbs, 110 calories per serving

Should Protein Be Avoided When Fasting?

While protein shakes may not align with strict definitions of fasting, there are compelling reasons to allow moderate protein intake:

  • Satiety – Protein is by far the most satiating macronutrient. Having a protein shake can curb hunger and prevent overeating when the fast ends.
  • Muscle retention – Consuming 20-30g of protein helps maximize MPS rates to counteract muscle breakdown during fasting periods.
  • Nutrient intake – Protein powders provide amino acids that may be lacking from fasting diets.
  • Exercise performance – Protein around workouts helps stimulate MPS for recovery and growth.
  • Metabolic advantage – Some research shows higher thermogenic effect of protein meals supports greater fat burning.

Avoiding all protein intake may not be advantageous for these reasons. As long as carbohydrate intake remains low, the metabolic benefits of intermittent fasting largely persist even with protein shakes.

Potential Drawbacks of Protein Shakes When Fasting

There are some potential downsides associated with excessive protein intake during fast periods:

  • Could inhibit ketosis by providing glucose precursors.
  • May reduce longevity benefits if amino acids activate mTOR pathway.
  • Risk of extra calories if using high fat/carb protein sources.
  • Possible GI issues like bloating, gas, diarrhea when breaking fast.
  • Increased hunger later on after insulin-stimulating effect subsides.
  • Kidney stress if consuming very high protein amounts consistently.

To alleviate these risks, intake should be kept moderate at 20-30g per shake, limit surrounding carb intake, and allow metabolic processes to return to fasted state after intake.

The Bottom Line – Will Protein Shakes Break Your Fast?

Here is a summary of the evidence on whether protein shakes interrupt intermittent fasting:

  • Most protein powders contain calories and macronutrients that require digestion, absorption, and metabolism.
  • Protein stimulates insulin secretion, though not as potently as carbohydrates.
  • Pure protein alone may not strongly activate MPS mechanisms during fasting periods.
  • Protein technically breaks the fasted state, but will not disrupt major benefits if carbohydrates are limited.
  • 20-30g protein portions are recommended to provide satiety, fuel workouts, and assist muscle protein synthesis without pronounced insulin spikes.
  • Avoid protein alongside high carb meals for optimal insulin regulation.

In conclusion, having a protein shake will break your fast by providing calories and spiking insulin. However, moderate protein consumption can be built into intermittent fasting plans to support satiety, muscle retention, exercise performance and nutrient intake without sabotaging the core objectives like fat burning. Stick to pure proteins and limit intake to 20-30g per serving for best results. Time consumption appropriately around workouts and eating periods.


[1] West, D. W., Burd, N. A., Coffey, V. G., Baker, S. K., Burke, L. M., Hawley, J. A., … & Phillips, S. M. (2011). Rapid aminoacidemia enhances myofibrillar protein synthesis and anabolic intramuscular signaling responses after resistance exercise. The American journal of clinical nutrition, 94(3), 795-803.

[2] Churchward-Venne, T. A., Cotie, L. M., MacDonald, M. J., Mitchell, C. J., Prior, T., Baker, S. K., & Phillips, S. M. (2014). Citrulline does not enhance blood flow, microvascular circulation, or myofibrillar protein synthesis in elderly men at rest or following exercise. American Journal of Physiology-Endocrinology and Metabolism, 307(1), E71-E83.

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