Fasting Tracker

Quick answers

Does a protein shake break a fast?

Yes, clearly. Protein raises insulin, and whey more than most. Have your shake in your eating window

Protein isn't fasting-friendly, but it matters

Protein is what helps you keep your muscle while you lose fat, and it's easy to fall short on when you eat in a shorter window. Gains from strength training level off at about 1.6 g of protein per kilo of body weight a day 3: about 110 g a day for someone who weighs 70 kg.

So the answer isn't to skip the shake. It's to move it into your eating window, ideally within a few hours of a workout. Your body can use a large serving: after exercise, a 100 g dose of protein kept building muscle for more than 12 hours 4.

Training while fasting

A 30-minute run or a gym session is fine on an empty stomach; for sessions longer than about an hour, eating beforehand improves performance 5. With strength training and enough protein, people following 16:8 kept their muscle and strength 6,7.

Fitting protein around your fast

  1. Schedule strength sessions near your eating window.
  2. Break your fast with a protein-rich meal.
  3. Use a shake inside your window if you struggle to reach your target.
  4. Spread protein across your meals; four servings worked a little better than two big ones 8.

Sources

  1. Nilsson M, Stenberg M, Frid AH, et al. Glycemia and insulinemia in healthy subjects after lactose-equivalent meals of milk and other food proteins: the role of plasma amino acids and incretins. Am J Clin Nutr. 2004;80(5):1246-53. doi.org/10.1093/ajcn/80.5.1246
  2. Holt SH, Miller JC, Petocz P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr. 1997;66(5):1264-76. doi.org/10.1093/ajcn/66.5.1264
  3. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. doi.org/10.1136/bjsports-2017-097608
  4. Trommelen J, van Lieshout GAA, Nyakayiru J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Rep Med. 2023;4(12):101324. doi.org/10.1016/j.xcrm.2023.101324
  5. Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476-1493. doi.org/10.1111/sms.13054
  6. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. doi.org/10.1186/s12967-016-1044-0
  7. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. doi.org/10.1093/ajcn/nqz126
  8. Areta JL, Burke LM, Ross ML, et al. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J Physiol. 2013;591(9):2319-31. doi.org/10.1113/jphysiol.2012.244897

A note on health

Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.