L-cystine is the chief sulfur-containing compound in protein. It is found in eggs, meat, dairy products, and whole grain foods. L-cystine is the disulfide form of L-cysteine, which means it consists of two molecules of L-cysteine—this is why it is occasionally referred to as dicysteine—bonded together at their sulfur groups.  Inside cells, L-cystine is metabolized into L-cysteine, which, along with glutamine and glycine, is used to make an important detoxification and antioxidant molecule called “glutathione”[1]. L-cysteine availability limits the rate of glutathione production (it is thought to be rate-limiting) [1]. Supplying L-cystine allows the body to restore intracellular glutathione levels when demand has been increased or under circumstances when it is lower (such as older age or increased toxin exposure) in tissues throughout the body (including the brain, liver, and muscles). L-cystine promotes glutathione-related antioxidant defenses, which helps protect cells and mitochondria against free radicals, cell membrane damage, damage from metals and toxins, and other oxidative stress-related and aging issues. L-cystine plays important structure and function roles in immune cells, skeletal muscle, and connective tissue. Hair and skin contain approximately 10–14% cystine by mass.*


Supports the production of glutathione *

Supports antioxidant defenses *

Supports healthy immune responses *


L-Cystine is non-GMO and gluten-free.


Average dietary intake of L-cystine has been estimated to be about 1000 mg/day [2]. L-cysteine is generally considered to be dose-dependent (see Qualia Dosing Principles) in the range it’s most commonly used (between 70-700 mg a day). Our main goal with L-cystine is to augment the supply of dietary precursors to make glutathione, rather than to try to fully replace average L-cystine intake. Because we use L-cystine in combination with other ingredients with which it may have complementary effects, we opted to use a serving at the lower end of the supplemental range.*


Supports antioxidant defenses*

Supports glutathione levels* [3–5]

Supports healthy mitochondrial ROS production* [6]

Support brain glutathione levels* [7]

Supports neuroprotection (secondary to boosting glutathione and antioxidant defenses)* [7–10]

Supports gut glutathione levels* [11]

Supports healthy immune system function*

Supports general immune health* [3,12–15]

Supports post-exercise immunity* [13,14,16]

Supports innate immunity* [13,14,16]

Supports adaptive immunity* [3,13,16,17]

Supports healthy  immune signaling* [3–5,17,18]

Supports healthy immune cell function* [13,14,16,17]

Supports healthy antibody responses* [3,17]

Supports T cell glutathione production* [19]

Supports gut immune signaling and barrier function* [11,20]

Complementary ingredients*

L-Theanine in support of general immune health* [3,12–17,21]

Glycine in support of glutathione synthesis* [22,23]

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.


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[2]Institute of Medicine, Food and Nutrition Board, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Subcommittee on Interpretation and Uses of Dietary Reference Intakes, Subcommittee on Upper Reference Levels of Nutrients, Panel on the Definition of Dietary Fiber, Panel on Macronutrients, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, National Academies Press, 2005.

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[13]S. Murakami, S. Kurihara, N. Koikawa, A. Nakamura, K. Aoki, H. Yosigi, K. Sawaki, M. Ohtani, Biosci. Biotechnol. Biochem. 73 (2009) 817–821.

[14]S. Kawada, K. Kobayashi, M. Ohtani, C. Fukusaki, J. Strength Cond. Res. 24 (2010) 846–851.

[15]S. Kurihara, T. Hiraoka, M. Akutsu, E. Sukegawa, M. Bannai, S. Shibahara, J. Amino Acids 2010 (2010) 307475.

[16]S. Murakami, S. Kurihara, C.A. Titchenal, M. Ohtani, J. Int. Soc. Sports Nutr. 7 (2010) 23.

[17]S. Kurihara, S. Shibahara, H. Arisaka, Y. Akiyama, J. Vet. Med. Sci. 69 (2007) 1263–1270.

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[19]T.B. Levring, M. Kongsbak, A.K.O. Rode, A. Woetmann, N. Ødum, C.M. Bonefeld, C. Geisler, Oncotarget 6 (2015) 21853–21864.

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[23]D. Nguyen, J.W. Hsu, F. Jahoor, J Clin Endocrinol Metab 99 (2014) 169–177.