Stop Taking BCAAs If You Want the Benefits of Fasting

Don’t worry about your precious muscle. The body is wicked smart and resilient, and it will adapt accordingly.

That’s right, BCAAs (branched chain amino acids) are ranked high as one of the most commonly “incorrectly” prescribed supplement for fasting.

But have any of the bro scientists ever given you a good reason as to why you should be using BCAAs? Reasons other than “it will protect your muscle bro.”

That’s right, BCAAs (branched chain amino acids) are ranked high as one of the most commonly “incorrectly” prescribed supplement for fasting.

But have any of the bro scientists ever given you a good reason as to why you should be using BCAAs? Reasons other than “it will protect your muscle bro.”

I get it, I know where the emphasis on BCAAs comes from. It’s from programs like lean gains and carb backloading, which then trickle down to consumers, bloggers, and even coaches everywhere.

But what does science say?

To be honest, I can explain this one with one simple sentence: “BCAAs contain leucine, which stimulates mTOR, which blocks AMPK.”

OK, now what does that mean?

You Need to Understand Fasting

Let’s first understand some of the incredible benefits of fasting. For simplicity, here’s a summary, which was published in the academic journal, Obesity, entitled “Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting.”1

Now which of these is dependent on the AMPK (an enzyme, adenosine monophosphate-activated protein kinase) signaling? In other words, what will NOT happen if we block AMPK with something like BCAAs or EAAs (essential amino acids)?

  • Mitochondrial biogenesis 2
  • Synaptic plasticity in the brain 3
  • Resistance to injury and disease 2
  • Reduction in inflammation in the blood, intestines, muscles, and fat stores 4
  • Antioxidant benefits 5
  • Insulin sensitivity 2
  • Lowering of blood sugar 2
  • Stress resistance 1
  • Utilization of fat for energy (or ‘metabolic flexibility’) 1
  • Enhanced endurance 1
  • All the crazy benefits of ketones 6

That’s a lot of benefits we are missing out on.

Let’s unravel a couple of these in a bit more detail. But first, a recap:

The amino acid leucine strongly triggers mTOR (the mammalian target of rapamycin) which leads to building of new muscle. This is great! But there’s a problem, when we trigger mTOR we block one of the most beneficial signal pathways known to humans, AMPK.

Think of this as the “building things up” pathway (mTOR) and the “breaking things down” pathway (AMPK).

Breaking things down sounds bad, but biology makes mistakes and accumulates toxic garbage. AMPK—and ‘cellular autophagy’ in particular—is one of the ways the body deals with toxic cellular garbage. This single benefit is one of the reasons why scientists are so fascinated with fasting because this can potentially prevent cancer and other chronic illnesses.5, 7

One more thing, you must identify what your goals are by fasting. If you are fasting to induce a caloric deficit and thus lose weight then you probably don’t care about all this. But if you are fasting to reap the longevity and performance benefits, then you should take note.

I’m going to explain to you the three main reasons why I get the athletes I work with to fast without taking in calories or supplements like BCAAs.

Reason 1 to Be BCAA-Free

Remember that cell detox benefit I was talking about, cellular autophagy? One of the benefits of training at elevation is that it massively increases cell autophagy.8

Yeah, you want this.

Reason 2 to Be BCAA-Free

Metabolic flexibility is the ability to utilize both carbohydrates and fats for fuel. Put another way, your ability to use primarily fat (fat oxidation) at higher intensities becomes more efficient (for you exercise phys gurus, your RER remains lower at higher intensities).

This is advantageous because we can store exponentially more fat than carbs, and thus perform for longer without having to constantly refuel with sugar/carbs.

The classic example of this is endurance athletes literally shitting their pants from the absurd amount of carbs they are forced to consume mid-event. This is a major roadblock for endurance athletes increasing performance (check out the article Training the Gut for Athletes for more on this).

Limiting carb intake also improves insulin sensitivity which has too many other downstream benefits to list. But as an example, it improves your ability to get amino acids and essential minerals into your muscles after training or competition.

Reason 3 to Be BCAA-Free

Ketones. These are anti-inflammatory, antioxidant, stimulate growth of new mitochondria, and have a host of neurological benefits. The production of ketones is inhibited by supplementation of BCAAs or EAAs. That’s why protein is actually super low on a classic therapeutic ketogenic diet.

Some Thoughts on the Concern Over Losing Muscle

But what about my gains bro? Won’t I lose muscle during my fast? For the majority of athletes, this is an unrealistic fear. I have personally fasted with nothing but water and salt for up to 5 days, lifted heavy during the fast, and even hit PRs 1-3 days after ending the fast.

Even with a long fast (in the range of 24-48 hrs) there’s ‘muscle protective’ mechanisms that kick in.

HGH (human growth hormone) massively increases (this too has several benefits) during a longer fast. One study showed that this increase in human growth hormone is able to preserve lean muscle during the absence of caloric intake. Meanwhile, taking amino acids in prevents this benefit.9

Ketones, when above 2.0 mmol, literally prevent muscle breakdown. One study gave patients an inflammatory stimulus called LPS (lipopolysaccharide), which triggers muscle catabolism and inflammation. Muscle loss was prevented when ketones were increased to 2.0 mmol in the patients. Some people can achieve a level this high post-workout, or even from fasting for only 16 hours. Even more people will have ketones this high after a 24-hour fast. But again, all of this is blocked when amino acids are used.10

In simple terms, what all this means is that athletes who properly incorporate fasting without the use of BCAAs can improve endurance, recovery from training and competition, resiliency to injury, and overall health.

The Value of Fasting

The majority of the benefits of fasting only happen when we are doing just that, FASTING.

Depriving ourselves of calories and specific nutrients causes a cascade of signals that deliver these benefits. Consuming BCAAs during the fast will block these starvation signals which are what actually lead to all those incredible benefits in the first place.

Fasting is fasting. And don’t worry about your precious muscle—the body is wicked smart and resilient, and it will adapt accordingly.


1. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254–268. doi:10.1002/oby.22065.

2. Cantó C, Jiang LQ, Deshmukh AS, et al. Interdependence of AMPK and SIRT1 for metabolic adaptation to fasting and exercise in skeletal muscle. Cell Metab. 2010;11(3):213–219. doi:10.1016/j.cmet.2010.02.006.

3. Marinangeli C, Didier S, Ahmed T, et al. AMP-Activated Protein Kinase Is Essential for the Maintenance of Energy Levels during Synaptic Activation. iScience. 2018;9:1–13. doi:10.1016/j.isci.2018.10.006.

4. Youm YH, Nguyen KY, Grant RW, et al. The ketone metabolite β-hydroxybutyrate blocks NLRP3 inflammasome-mediated inflammatory disease. Nat Med. 2015;21(3):263–269. doi:10.1038/nm.3804.

5. Antunes F, Erustes AG, Costa AJ, et al. Autophagy and intermittent fasting: the connection for cancer therapy?. Clinics (Sao Paulo). 2018;73(suppl 1):e814s. Published 2018 Dec 10. doi:10.6061/clinics/2018/e814s.

6. Puchalska P, Crawford PA. Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics. Cell Metab. 2017;25(2):262–284. doi:10.1016/j.cmet.2016.12.022.

7. Ruth E. Patterson and Dorothy D. Sears, Metabolic Effects of Intermittent Fasting, Annual Reviews of Nutrition, July 17, 2017.

8. Zhang Y, Chen N. Autophagy Is a Promoter for Aerobic Exercise Performance during High Altitude Training. Oxid Med Cell Longev. 2018;2018:3617508. Published 2018 Apr 5. doi:10.1155/2018/3617508.

9. The Protein-Retaining Effects of Growth Hormone During Fasting Involve Inhibition of Muscle-Protein Breakdown. Helene Nørrelund, K. Sreekumaran Nair, Jens Otto Lunde Jørgensen, Jens Sandahl Christiansen, Niels Møller. Diabetes Jan 2001, 50 (1) 96-104; DOI: 10.2337/diabetes.50.1.96.

10. Koutnik AP, D’Agostino D., Egan. Anticatabolic Effects of Ketone Bodies in Skeletal Muscle. Trends Endocrinol Metab. 2019 Apr;30(4):227-229. doi: 10.1016/j.tem.2019.01.006.

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