What is Cysteine?

Cysteine is a non-essential amino acid, one of the building blocks of life. It comprises molecules which combine to form proteins that help to promote and maintain the body’s health.  Cysteine is known as a ‘non-essential’ amino acid because your body is able to produce some of what you need. Additionally, you take in cysteine from certain food sources we will discuss later.

Cysteine is the main protein found in your nails, skin and hair. It also plays a vital role in making collagen, the structural protein found mainly in the connective tissue within your body. The production of collagen slows with ageing, so keeping adequate levels of cysteine becomes even more important.

How We Create Cysteine

Our bodies create cysteine naturally but need adequate amounts of folate, vitamin B6, and vitamin B12 to do so. It is important that we absorb these nutrients from our diet with healthy food sources including beans, lentils, spinach, bananas, salmon and tuna.

Food Sources of Cysteine 

L-cysteine itself is available to us via many dietary sources including animal protein, eggs and dairy products which all contain significant levels of l-cysteine. 

Specific foods with cysteine-producing properties include:


  • Chicken/Turkey
  • Red Meat
  • Fish/Seafood
  • Eggs
  • Yoghurt
  • Cheese
  • Wholegrain Products (i.e., Bread and Cereals)
  • Sunflower Seeds


Everyone, including vegetarians and vegans, should be aware that good vegetable sources of l-cysteine include:

  • Broccoli
  • Brussel Sprouts
  • Cauliflower
  • Garlic
  • Granola
  • Sprouted Lentils
  • Chickpeas
  • Oats
  • Onions
  • Peppers
  • Tofu
  • Tempeh
  • Brown Rice
  • Quinoa
  • Sunflower Seeds
  • Sesame Seeds
  • Pistachios 

Cysteine Deficiency

As cysteine is made within the body and found in high-protein foods, deficiency in Western diets is rare. However, vegetarians and vegans may be at a higher risk of cysteine deficiency depending on the quality of their diet and supplementation.

Aside from people who choose a specific diet, deficiency of l-cysteine is most likely to occur in those who are unable to biosynthesize it, such as the elderly and infants. 

People with malabsorption issues and metabolic syndrome may also be unable to synthesize the necessary amounts of l-cysteine to maintain good health. An l-cysteine deficiency may result in a compromised immune system, which can cause slow recovery from injury and ailments. 

More physical signs that you may need to top up your l-cysteine levels include brittle nails, acne, skin lesions, fatigue, hair loss and a decrease in muscle mass.

Cysteine Supplements

Cysteine can be taken as a supplement in the form of an acetylated compound of the amino acid known as N-acetylcysteine or NAC. N-acetylcysteine (NAC) is a supplement that contains the amino acid cysteine in a modified form. While it is not typically used to directly treat cysteine deficiency, it may be useful in certain conditions where cysteine synthesis or utilisation is impaired.

Cysteine’s strong antioxidant and anti-inflammatory properties are essential to our immune system, amongst many other things, cysteine:

  • Helps to detoxify the body and aids the functioning of the immune system. 
  • Alleviates symptoms of some mental health conditions.
  • Reduce substance use cravings.
  • Improves the functioning of the lungs by decreasing inflammation and breaking up mucus.
  • Improves fertility in men by reducing oxidative stress which damages the body’s reproductive cells.
  • Reduces oxidative stress damage to the heart which will lower the risk of types of heart disease.
  • And, while human-based research is in its early stages here, it is thought that by decreasing inflammation in fat tissue cysteine/NAC may reduce insulin resistance, thereby improving blood sugar regulation.

Cysteine’s Role in Creating Glutathione

Another important role of NAC is that once converted into L-cysteine after ingestion, this, in turn, is converted into glutathione, a very powerful antioxidant involved in fighting free radicals, maintaining tissue strength and supporting immune function.

Cysteine is converted to cystine, which is then transported into cells. Within the cell, cystine is reduced back to cysteine, which can then be used to synthesize glutathione. The glutathione can then help reduce antioxidant stress, support protein synthesis and detoxify the body.

Supplementing Cysteine

As with any form of supplement, it is a sensible precaution to consult your GP to assess whether taking extra NAC could be beneficial to your health.

If you choose to supplement your diet, the accepted daily recommendation is between 600-1,800mg of NAC.  It can be taken orally as a liquid, powder or aerosol spray. 

Unfortunately, the body does not absorb NAC too well and for people who are deficient and in need of an assured uptake it is better to have it administered intravenously.

IVBOOST UK includes cysteine in its Immunity Booster drip which its expert medical team can administer to patients with a compromised immune system.