The Downside of Protein Supplements

by Rachel Stuck, RDN

Protein plays a critical role in a variety of bodily processes, making it seem like a no-brainer to give your diet a boost with protein shakes, powders, pills, and bars. But, before you stock your pantry with these items, it is important to understand the downside of protein and amino acid supplements.

Protein Is Made of Amino Acids

Amino acids are the building blocks of protein. There are 20 amino acids, split into two groups: essential and nonessential.1

  • Nonessential amino acids can be synthesized by the human body.
  • Essential amino acids are amino acids that the body CANNOT synthesize. Therefore, these amino acids must be included in the diet.

Research supports that amino acids, specifically a special group of essential amino acids called branch-chain amino acids or BCAA (including the amino acids leucine, isoleucine, and valine), in supplement form can promote muscle growth and recovery.2 While BCAA are found in a variety of foods—including red meat, dairy, poultry, eggs, and even lentils, soy, and nuts—supplemental BCAA are popular among bodybuilders, athletes, and everyday gym-goers. BCAA are typically included in the form of a powder or pill, but are also commonly found in protein powders and shakes as well. Athletes are quick to grab protein supplements because of their higher protein needs, and supplements are a quick and easy way to add protein. However, this might not be the best thing for the body.

BCAA have been found to support muscle growth and recovery, but research published in the journal Nature Metabolism found that excessive consumption of protein powders, pills, and shakes can actually be detrimental.3,4 The study found that excessive intake may increase appetite (leading to weight gain), negatively impact mood, and possibly even reduce lifespan. This is because over-supplementing with protein may lead to lower levels of other amino acids that are important for a variety of processes that affect appetite, emotional health, and other aspects of wellness.

The study reviewed how consuming supplemental BCAA and essential amino acids affected other important amino acids such as tryptophan and tryptophan metabolites like serotonin. The researchers found that protein supplementation led to a high amount of BCAA in the blood, which competed for and reduced the absorption of tryptophan. Reduced tryptophan absorption results in the body being unable to complete all the important processes that tryptophan is a part of, which are just as (if not more) important than muscle growth.

Why Tryptophan Is Important

Tryptophan is an important building block for a variety of processes that support the immune system, energy production, gut health, and disease prevention.8 Tryptophan is also the precursor to the hormone serotonin.5 Serotonin, most well known as the “happy chemical,” supports feelings of well-being, sleep, mood regulation, and appetite regulation.6,7 With that being said, it is clear that tryptophan is important to the body and that if it is included in the diet you will want to ensure it is being used efficiently. Researchers connected the overconsumption of protein supplements with the reduced absorption of tryptophan leading to reduced serotonin production. Reduced serotonin then of course affects how well the “happy chemical” can do its job in supporting mood, sleep, appetite, and feelings of well-being.

Interestingly, if you review the nutrition labels of most protein powders, you will find that tryptophan is commonly included in the blend along with BCAA; however, you can see how the tryptophan quickly becomes obsolete if it is competing against BCAA for absorption.

Here's a simple metaphor to clarify:

Think about the bloodstream as a subway route and amino acids as the subway cars. Typically, subway paths intersect and the timing of the subway cars must be well planned; otherwise, cars will collide and passengers will not reach their destinations. Faster cars will have an advantage while larger cars with more passengers are at a disadvantage, but both cars are equally important.

This is similar to how amino acids are shuttled through the body. The body is an intricate system of pathways that, if overwhelmed, will affect how well each amino acid travels through the body and is therefore absorbed. Think of BCAA as the high-speed subway cars and amino acids like tryptophan as the slower cars. The high-speed cars (BCAA) will get to the subway stop (absorption site) first, blocking the larger subway cars (the amino acid tryptophan). Both are important, but if they are not properly coordinated, they can quickly disrupt the system.

Is it possible to support both muscle growth and amino acid absorption?

The short answer is YES!

If you’re a protein supplementer, you might be thinking, “OK, so what do I do to support muscle growth and my serotonin levels?”

First, it is important to note that most people do not need to include protein supplements and BCAA. A diet rich in a variety of protein-dense foods will adequately support muscle mass. Including both plant- and animal-based proteins will ensure the body is getting a variety of amino acids, specifically the essential amino acids and the branch-chain amino acids.

If you are an active gym-goer, a weight lifter, an athlete, or if you just really have a tough time giving up your protein supplements, here are a few tips on how to include protein supplements while avoiding the negative impacts they can have on other amino acids.

  1. Aim to include less than one serving of protein supplements per day. That means you should include protein shakes and supplements in your diet six days per week or less. Instead of relying on protein powder to get your needed protein, plan high-quality, protein-dense meals. Remember: A protein shake pre- or post-workout will not make or break the benefit of your workout.
  2. Skip protein supplements on rest days or low-impact recovery days. Ideally, protein supplements should only be used when the muscles are under stress and being broken down by exercise. See the next tip for guidance!
  3. Redefine your post-workout refuel. A well-rounded meal offers the body the necessary building blocks to support muscle growth and recovery. A sixty-minute or longer exercise session, for example, will lead to muscle breakdown, so you would benefit from a protein boost. However, a shorter exercise session does not need to be supplemented. Instead, follow a shorter session with protein-dense foods like Greek yogurt, hard-boiled eggs, edamame, turkey lunch meat, chicken, lentil soup, or beef jerky.
  4. Avoid boring, repetitive meals. Eating the same two or three protein-rich foods every week can reduce the variety of amino acids in the diet. Aim to include a variety of proteins from both plant and animal sources weekly. Reflect on your current eating habits and consider what you can do to diversify your protein-rich foods.

Variety Is the Key

Ultimately, including a variety of protein-dense foods in your diet is the best way to support muscle growth and amino acid absorption. Aim to get a majority of protein from whole-food sources like poultry, beef, pork, eggs, dairy, seafood, fish, legumes, beans, soy, quinoa, and nuts. Understanding the quality and diversity of protein in the diet is just as important as the amount of protein consumed, especially if most of the protein is coming from protein supplements or other processed foods and snacks.

Protein supplements can fit into a balanced diet, and having a whey- or plant-based protein powder on hand is a great way to refuel or get a protein boost after a tough workout or when you are short on time. Consider adding additional nutrients and fiber to your shake by blending antioxidant-dense berries and spinach with your protein powder. This also helps to slow digestion.

How the Ixcela Test Provides Insight

The Ixcela test provides insight into the amino acids tryptophan and tyrosine. More specifically, the Ixcela test can help you determine if you need to focus on improving your protein intake or if it would be best to hold off on the protein supplements until your amino acid levels are more balanced. Because tryptophan is the precursor to serotonin and a variety of other important processes, testing your tryptophan levels with Ixcela means that you will be able to uncover the cause of your sleep struggles, sugar cravings, and emotional ups and downs.

About the Author

Photo: Rachel Stuck, RDN

Registered Dietitian Nutritionist Rachel Stuck has a background in culinary arts and nutrition counseling. Rachel takes a positive approach to nutrition: she avoids recommending restrictive diets and instead focuses on helping people choose foods that promote health and well-being. She is passionate about empowering and assisting Ixcela members as they develop their unique, gut-healthy lifestyles.

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Interested in learning more about Ixcela? Check out Ixcela’s microbiome test, personalized nutrition and fitness plans, and other tools to help you optimize your health.


  1. “The Chemistry of Amino Acids.” The Biology Project, University of Arizona, 30 Sept. 2003,
  2. Pasiakos, Stefan M., et al. “The Effects of Protein Supplements on Muscle Mass, Strength, and Aerobic and Anaerobic Power in Healthy Adults: A Systematic Review.” Sports Medicine, Springer International Publishing, 29 Aug. 2014,
  3. Shimomura, Yoshiharu, et al. “Nutraceutical Effects of Branched-Chain Amino Acids on Skeletal Muscle.” OUP Academic, Oxford University Press, 1 Feb. 2006,
  4. Solon-Biet, Samantha M., et al. “Branched-Chain Amino Acids Impact Health and Lifespan Indirectly via Amino Acid Balance and Appetite Control.” Nature News, Nature Publishing Group, 29 Apr. 2019,
  5. Schaechter, Judith D., and Richard J. Wurtman. “Serotonin Release Varies with Brain Tryptophan Levels.” Brain Research, Elsevier, 7 Mar. 2003,
  6. Jenkins, Trisha A., et al. “Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis.” MDPI, Multidisciplinary Digital Publishing Institute, 20 Jan. 2016,
  7. Curzon, G. “NYAS Publications.” The New York Academy of Sciences, John Wiley & Sons, Ltd, 17 Dec. 2006,
  8. Taleb, Soraya. “Tryptophan Dietary Impacts Gut Barrier and Metabolic Diseases.” Frontiers, Frontiers, 21 Aug. 2019,