Cysteine Benefits, Foods, Deficiency and Side Effects

Cysteine is a semi-essential proteinogenic amino acid. its role as a building block for proteins and its involvement in the synthesis of the antioxidant glutathione. Cysteine is chiral, and only L-cysteine is found in nature.

What is cysteine?

The body can produce cysteine from the amino acid methionine, which is an amino acid that contains sulfur and is present naturally in food. It is also found in the body and in food as cystine, an amino acid that contains two cysteines joined together. Cystine is the oxidized dimer form of the amino acid cysteine and is considered nutritionally equivalent to cysteine.

Although classified as a nonessential amino acid, in rare cases, cysteine may be essential for infants, the elderly, and individuals with certain metabolic diseases or who suffer from malabsorption syndromes. If there is enough methionine available, the human body can typically synthesize cysteine under normal physiological conditions.

What is the function of cysteine?

  • Protein Synthesis: Cysteine is incorporated into proteins during the process of protein synthesis.
  • Antioxidant: Cysteine is a precursor to glutathione, a powerful antioxidant that helps protect cells from damage caused by free radicals.

What can cysteine do for you?

  • Protect cells from free radical damage.
  • Help your body detoxify chemicals and heavy metals.
  • Help break down extra mucous in your lungs.

Cysteine Benefits, Foods, Deficiency and Side Effects-CookingEggs


Benefits of cysteine

Cysteine provides several benefits to the body, primarily due to its role as a building block for proteins and its involvement in the synthesis of the antioxidant glutathione. How does cysteine promote health? Here are some of the key benefits associated with cysteine:

Promoting antioxidant activity

As a key constituent of glutathione, cysteine has many important physiological functions. Glutathione, formed from cysteine, glutamic acid, and glycine, is found in all human tissues, with the highest concentrations found in the liver and eyes. Glutathione is a potent antioxidant, protecting fatty tissues from the damaging effects of free radicals. The antioxidant activity of glutathione is attributed specifically to the presence of cysteine in the compound.


Cysteine is a key constituent of glutathione, a compound that also plays a vital role in the detoxification of harmful substances by the liver and can chelate (attach to) heavy metals such as lead, mercury, and cadmium. It is also believed that glutathione carries nutrients to lymphocytes and phagocytes, important immune system cells.

Reducing the toxic effects of alcohol

Cysteine has been proposed as a preventive or antidote for some of the negative effects of alcohol, including liver damage and hangovers. It counteracts the poisonous effects of acetaldehyde. Cysteine supports the next step in metabolism, which turns acetaldehyde into acetic acid.

Help eliminate mucous

Cysteine also has the ability to breakdown proteins found in mucous that settle in the lungs. As a result, this amino acid may be useful in the treatment of bronchitis and other respiratory problems.

Hair and nail health

Cysteine is a component of keratin, the protein that makes up hair and nails. Adequate cysteine levels contribute to the strength and structure of these tissues.

What events can indicate a need for more cysteine-rich foods?

  • Frequent colds
  • Chronic obstructive pulmonary disease

Why do we need cysteine-rich foods?

Cysteine is an amino acid that occurs naturally in foods and which, with the help of other nutrient cofactors, can also be manufactured in the body from the amino acid methionine. Cysteine has unique functions since it is only one of two amino acids (the other is methionione) that contains sulfur. Cysteine is an important component of the antioxidant glutathione and can also be converted into the amino acid taurine.

Eating foods rich in cysteine enhances your body's usage of this important nutrient since these foods naturally contain other nutrients that act in synergy with cysteine, supporting its physiological function in your body and therefore contributing to your optimal health.


Deficiency symptoms of cysteine

Cysteine deficiency is relatively uncommon but may be seen in vegetarians with a low intake of plant foods containing methionine and cysteine. There is no known medical condition directly caused by cysteine deficiency, but low cysteine levels may reduce one’s ability to prevent free radical damage and may result in impaired function of the immune system.

What factors might contribute to a deficiency of cysteine?

The production of cysteine involves several nutrients. As a result, dietary deficiency of methionine, vitamin B6, vitamin B12, s-adenosyl methionine (SAMe), and folic acid may decrease the production of cysteine.


Side effects of cysteine

Consumption of foods containing cysteine, or its precursor, methionine, is not likely to cause toxicity symptoms. However, cysteine is a brain excitoxin that can cause damage to brain cells in susceptible individuals. Such individuals do not metabolize the amino acid correctly and, as a result, may be at risk for certain neurodegenerative diseases, including multiple sclerosis, amylotrophic lateral sclerosis, and Alzheimer’s disease.

Relative to most other amino acids, cysteine is much more toxic. High doses of oral N-acetyl-cysteine, such as those given to patients with acetaminophen (Tylenol) toxicity, can cause nausea, vomiting, and diarrhea. Intravenous administration of N-acetyl cysteine can cause allergic reactions in a small percentage of people, characterized by skin flushing, a drop in blood pressure, an irregular heartbeat, and respiratory distress. An accidental overdose of intravenous N-acetyl-cysteine has proven to be fatal.


What forms of cysteine are found in dietary supplements?

Many people take supplemental cysteine as a way to increase the level of glutathione in the body. As a dietary supplement, cysteine is available as L-cysteine hydrochloride and, more commonly, as n-acetyl-cysteine (NAC). NAC is a more water-soluble form of cysteine and, as such, is believed to be more bioavailable than oral L-cysteine hydrochloride.


Dietary sources of cystine

Cysteinyl is a residue in high-protein foods. Some foods rich in cysteine include poultry, eggs, beef, and whole grains. Below is a list of the top 10 foods highest in cystine:

Foods (per 100g)Cystine Content
Sunflower Seeds383 mg
Pork Chops350 mg
Beef345 mg
Chicken Breast336 mg
Tuna321 mg
Eggs292 mg
Cheese290 mg
Lentils118 mg
Oatmeal97 mg
Yogurt52 mg


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