The Health Benefits of L-Cysteine

Milk eggs and legumes on a table

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L-cysteine is a semi-essential amino acid found naturally in the human body. Abundant in protein-rich foods, L-cysteine is also sold as a dietary supplement, sometimes just called cysteine.

Cysteine, along with the amino acids glutamine and glycine, is a building block of the powerful antioxidant glutathione. The body can make cysteine from the amino acids methionine and serine, however, if these are in short supply, supplementing with L-cysteine can fill the gaps.

L-cysteine is found in many foods including meat, dairy products, eggs, nuts, seeds, and legumes. It is also abundant in protein powders used in weight-loss and body-building shakes and smoothies.

The amino acid may provide a variety of health benefits. In alternative medicine, L-cysteine is touted as a natural treatment for:

In addition, it is said to enhance lung health in people with chronic obstructive pulmonary disease (COPD), and can also help prevent colon cancer, boost sports performance, and promote detox.

Despite the many purported health uses for L-cysteine, a 2018 literature review published the journal Molecules noted the amino acid's effectiveness is still unclear and further research is needed.

Health Benefits

Research on the effects of L-cysteine supplementation is limited, however, the amino acid shows some benefits for certain health conditions. Here's a look at several key findings from the available research:

Diabetes

Research suggests that L-cysteine may aid in the treatment of diabetes by lowering blood sugar, reducing insulin resistance, and preventing blood vessel damage.

A 2012 literature review published in the Canadian Journal of Physiology and Pharmacology found that cysteine-rich whey protein improves glucose metabolism in people and animals with type 2 diabetes. However, the study authors note that more research is needed.

An earlier study published in Free Radical Biology & Medicine, found diabetic rats treated with L-cysteine experienced a significant decrease in blood-sugar levels and insulin resistance. It also appeared to inhibit blood vessel inflammation, a key contributor to heart disease among diabetics. While the study was based on animals and not humans, the amino acid shows promise for those with diabetes.

Colitis

A 2009 study from the Dutch journal Biochimica et Biophysica Acta suggests that L-cysteine shows promise in the treatment of colitis (a type of inflammatory bowel disease). In tests on pigs, scientists found that L-cysteine may help reduce colitis-associated inflammation of the gastrointestinal tract.

Free Radicals

L-cysteine may help prevent exercise-induced overproduction of free radicals, a process shown to contribute to oxidative stress. In an experiment involving 10 male basketball players, the authors of a 2007 study published in Clinical Chemistry and Laboratory Medicine determined that one week of supplementation with L-cysteine helped boost antioxidant capacity and reduce free radical production.

Possible Side Effects

Although little is known about the safety of long-term use of L-cysteine supplements, there's some concern that taking L-cysteine in combination with certain medications (such as prednisone and other drugs that suppress the immune system) may increase the potency of those medications and trigger adverse effects.

The safety of supplements in pregnant women, nursing mothers, children, and those with medical conditions or who are taking medications has not been established.

Dosage and Preparation

L-cysteine is available as a dietary supplement in capsule and powder form. It is often found in protein powders, including whey- and plant-based proteins. There is no standard recommended dosage amount. Follow the recommendations on the supplement label.

What To Look For

Widely available for purchase online, L-cysteine supplements are sold in many natural-foods stores, drugstores, and stores specializing in dietary supplements.

Dietary supplements are largely unregulated. To ensure the safety and quality of any supplement, look for an independent third-party seal on the label, such as U.S. Pharmacopeia, NSF International, or ConsumerLab. The label should not make any health promises that it can treat or cure a disease, according to U.S. Food and Drug Administration guidelines.

If you follow a Kosher diet, check the label for the certified Kosher symbol as not all L-cysteine supplements are Kosher.

Other Questions

I've heard NAC touted as a beneficial supplement. Is L-cysteine the same thing?

N-acetyl-L-cysteine (NAC) is similar to L-cysteine, but not exactly the same. Chemically speaking, NAC is an acetylated variant and precursor of the amino acid L-cysteine. While L-cysteine is found in many food sources, NAC is not—it is only available through supplementation. The purported health benefits of NAC are also different than the benefits of L-cysteine. NAC show promise for treating psychological disorders, such as obsessive compulsive disorder, addiction, and trichotillomania.

Is L-cysteine kosher?

Whether or not L-cysteine is kosher depends on where it comes from. L-cysteine is abundant in human hair and, in the past, supplement manufacturers extracted the amino acid from hair collected at barbers and salons.

L-cysteine derived from hair is not kosher, however, most companies no longer use hair. Instead, many L-cysteine supplements are made from chicken feathers. As long as the feathers are removed using kosher methods, the L-cysteine will be as well. Check the label for the kosher symbol to be sure.

Some protein powders and supplements contain L-cysteine derived from whey protein. Whey is dairy, so kosher rules regarding dairy apply to these products.

A Word From Verywell 

Due to the limited research, it's too soon to recommend L-cysteine supplements as a treatment for any condition. It's important to note that self-treating a chronic condition—especially a serious illness such as COPD or cardiovascular disease—and avoiding or delaying the use of standard care can have serious health consequences. If you're considering the use of L-cysteine supplements, consult your physician first to weigh the potential risks and benefits.

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