Why Isn’t Fasting Anabolic?

Abstaining from food or drink for health, religious, or medical purposes (a.k.a. fasting) has gained popularity as an effective weight loss and disease prevention method. (1) But what about its impact on anabolism — the process of building muscle? 

Dr. Layne Norton, a PhD in Nutritional Sciences, has explored this topic extensively. In a recent analysis, he critiqued a video claiming fasting on an empty stomach promotes muscle growth.

No research supports the idea that exercising in a fasted state helps build more muscle. In fact, the opposite is more accurate. Dr. Norton referenced two studies proving that fasting is not superior for muscle growth. (2)(3) 

[Related: Recapping 2024’s Most Notable Muscle-Building Studies]

Dr. Norton says fasting boosts human growth hormone (HGH), but only briefly. (4) “Even super-physiological levels of growth hormone increase lean mass almost completely from collagenous connective tissue,” Norton said. 

HGH doesn’t promote skeletal muscle hypertrophy, meaning it’s not anabolic. (5)(6)(7) Regarding mitochondria, some forms of exercise can enhance their production. However, no evidence suggests that high-intensity exercise increases mitochondrial density more effectively than regular exercise.

Muscle Protein Synthesis & Nitrogen Balance

Dr. Norton explains that fasting negatively impacts nitrogen balance rather than improving it. The body naturally excretes nitrogen regularly, and without an intake to replenish it, this creates an imbalance in the system.

“Nitrogen balance is the balance between the nitrogen you take in from protein and what you excrete in your urine, feces, and sweat,” Dr. Norton explained. Research on protein metabolism highlights the impact of fasting on nitrogen balance, providing evidence that supports Norton’s argument. (8)(9)

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References

1. Stockman, M. C., Thomas, D., Burke, J., & Apovian, C. M. (2018). Intermittent Fasting: Is the Wait Worth the Weight?. Current obesity reports, 7(2), 172–185. https://doi.org/10.1007/s13679-018-0308-9
2. Tinsley, G. M., Moore, M. L., Graybeal, A. J., Paoli, A., Kim, Y., Gonzales, J. U., Harry, J. R., VanDusseldorp, T. A., Kennedy, D. N., & Cruz, M. R. (2019). Time-restricted feeding plus resistance training in active females: a randomized trial. The American journal of clinical nutrition, 110(3), 628–640. https://doi.org/10.1093/ajcn/nqz126
3. Tinsley, G. M., Forsse, J. S., Butler, N. K., Paoli, A., Bane, A. A., La Bounty, P. M., Morgan, G. B., & Grandjean, P. W. (2017). Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European journal of sport science, 17(2), 200–207. https://doi.org/10.1080/17461391.2016.1223173
4. HUNTER, W. M., & GREENWOOD, F. C. (1964). STUDIES ON THE SECRETION OF HUMAN-PITUITARY-GROWTH HORMONE. British medical journal, 1(5386), 804–807. https://doi.org/10.1136/bmj.1.5386.804 
5. West, D. W., & Phillips, S. M. (2010). Anabolic processes in human skeletal muscle: restoring the identities of growth hormone and testosterone. The Physician and sportsmedicine, 38(3), 97–104. https://doi.org/10.3810/psm.2010.10.1814
6. Yarasheski, K. E., Campbell, J. A., Smith, K., Rennie, M. J., Holloszy, J. O., & Bier, D. M. (1992). Effect of growth hormone and resistance exercise on muscle growth in young men. The American journal of physiology, 262(3 Pt 1), E261–E267. https://doi.org/10.1152/ajpendo.1992.262.3.E261
7. West, D. W., Burd, N. A., Staples, A. W., & Phillips, S. M. (2010). Human exercise-mediated skeletal muscle hypertrophy is an intrinsic process. The international journal of biochemistry & cell biology, 42(9), 1371–1375. https://doi.org/10.1016/j.biocel.2010.05.012
8. Wechsler, J. G., Wenzel, H., Swobodnik, W., Ditschuneit, H. H., & Ditschuneit, H. (1984). Nitrogen balance studies during modified fasting. Postgraduate medical journal, 60 Suppl 3, 66–73.
9. Cheng, K. N., Pacy, P. J., Dworzak, F., Ford, G. C., & Halliday, D. (1987). Influence of fasting on leucine and muscle protein metabolism across the human forearm determined using L-[1-13C,15N]leucine as the tracer. Clinical science (London, England : 1979), 73(3), 241–246. https://doi.org/10.1042/cs0730241

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