Cashews are high in cysteine (NAC)

N-Acetyl Cysteine (NAC)

7 minutes to read
Carly Hanna

Carly Hanna

BSc (Human Nutrition and Psychology)

Evidence Based Intermediate

In many countries, N-acetyl cysteine (NAC) is classified as a dietary supplement that enhances the production of glutathione and assists with reducing oxidative stress. In the US, however, NAC is classified as a drug/medication and cannot be sold over the counter.

This obviously limits a large number of people to benefit from the positive effects that NAC exhibits, such as 

  • Boosting our immunity system
  • Protecting our cardiovascular and renal health
  • Protecting our brain and assisting with mental health disorders
  • Boosting our performance and strength levels
  • Keeping our antioxidant levels high

If you would like to know more about NAC and if you should include it (as a supplement) in your daily routine, or increase foods that are high in cysteine, we encourage you to read the article, and make a few notes along the way.

What is N-Acetyl Cysteine (NAC)?

N-Acetyl Cysteine (otherwise known as NAC) is a supplement form and precursor of the amino acid cysteine.[1]

Cysteine is one of the twenty-two amino acids, which are the building blocks of proteins in the body. Along with the other amino acids glycine and glutamate, cysteine is the most evidence-based amino acid precursor to making glutathione, a very important antioxidant.[2]

Antioxidants can help reduce oxidative stress

By reducing free radicals, this may also help lower the risk of different types of cancer.[3]

N-Acetyl-Cysteine chemical reaction

Cysteine is found in high protein foods such as

  • Meat
  • Eggs
  • Legumes
  • Nuts/seeds

NAC is an FDA approved drug/supplement that has shown to help with chronic respiratory conditions, fertility and brain health.[1,4]

First use of NAC in clinical practice

NAC was first used for clinical benefits in the 1960’s for patients with cystic fibrosis. In the 1980’s it again started to gain research interest in antioxidant properties for conditions such as cancer.[5] From then on, NAC has been researched to help with

N-Acetyl Cysteine deficiency

Cysteine is a non-essential amino acid. This means that the body can produce cysteine on its own as well as obtain it from the diet. However an NAC deficiency can still occur. 

Important note – essential amino acids can only be obtained from the food we eat.

NAC is vital for the modulation of the glutamate system, which is the reward/reinforcement pathway of the body.[6] Both glutamate and glutathione systems are important metabolic pathways that affect physical and mental health.[6]

NAC deficiency can affect different parts of the body, including 

  • immunity
  • brain (including mental health)
  • respiratory system
  • heart
  • digestive and renal system
  • fertility
Glutathione levels and ageing
Correlation between total glutathione (GSH) level and age.

Source: Moon, J. Circulating Total Glutathione in Normal Tension Glaucoma Patients: Comparison with Normal Control Subjects. (2012)

These symptoms occur mainly through the mechanism of having depleted glutathione levels and having an increase in oxidative stress. This mechanism is explained further in detail in the following sections.

Who is most at risk of N-Acetyl Cysteine deficiency?

NAC deficiency often accompanies low glutathione levels. 

Glutathione can be low during certain periods of life, with certain lifestyle factors, and also due to genetic and medical reasons. 

The following groups are more at risk of an NAC deficiency and may need more cysteine than the general population. 

High blood sugar and NAC

Both type I and type II diabetics are at risk of having low glutathione levels, which results in higher levels of oxidative stress. This is why those who have dysregulated blood sugar, may need more cysteine in order to produce healthy levels of glutathione.[2]

RELATED — Type 1 Diabetes: Autoimmune disease that is on the rise

High blood pressure and NAC

Similar to individuals with high blood sugar, people who have high blood pressure (eg. hypertension) need more cysteine in order to produce healthy levels of glutathione. This is to balance the high levels of oxidative stress that come with having hypertension.[2] 

Effect of N-acetyl-cysteine (NAC) on systolic blood pressure
Effect of N-acetyl-cysteine (NAC) on systolic blood pressure
Source: Longerich, L. Role of aldehydes in fructose induced hypertension. Molecular and Cellular Biochemistry. (1998)
 

Elderlies and NAC levels

As we age, our body’s ability to synthesise and absorb nutrients from food decreases. This is why older people are more likely to be at risk of developing a NAC deficiency.[2]

Immunity and NAC

When our body doesn’t have enough glutathione, our ability to fight infections decreases. 

Likewise, if we have an infection, our body is likely to need more cysteine to produce enough glutathione.[2]

Poor nutrition and NAC absorption

Not consuming enough food can lead to multiple nutrient deficiencies, including NAC. Insufficient nutrition can come from lifestyle behaviours, such as disordered eating, or medical causes, such as infection or gut disorders. This is because the body’s ability to absorb nutrients is disrupted.[2]

RELATED — Understanding Eating Disorders: History, Types and Statistics

High performance exercise and NAC requirements

Individuals who enjoy a lot of exercise, whether that be occupationally, recreationally, or for elite sport are likely to have a higher energy and/or protein requirement. The increase in need for certain nutrients, such as NAC, is for the building and repairing of muscles.[2]

Alcohol and NAC absorption

Alcohol can limit the absorption of nutrients in the body, so if we drink a lot of alcohol, this can put us at risk of NAC deficiency.[2]

Different types of N-Acetyl Cysteine

There are two types of NAC

  • Cysteine which is naturally found in foods such as meat, eggs, legumes and nuts/seeds
  • N-Acetyl L-Cysteine which is in a supplement form (powder, gel, capsule or spray)

It’s best to get NAC from our diet first

Our diet can provide bioavailable nutrients that are absorbed faster into the body, and have added benefits such as accompanying macronutrients (protein, fat and carbohydrate) and micronutrients (vitamins and minerals). 

Important note – NAC in supplement form has low bioavailability. However, if you are at risk of an NAC deficiency, a supplement can be a good back-up. 

Health benefits of N-Acetyl Cysteine

There have been several studies looking at NAC’s efficacy on improving mental and physical health. Health benefits of taking this supplement are explained below.

NAC increases glutathione production

NAC’s main mechanism for health benefits is through increasing production of glutathione. Glutathione deficiency can contribute to: 

  • Alzheimer’s disease (brain cell degeneration causing memory loss)
  • Parkinson’s disease (cell damage that leads to the loss of dopamine, characterised by tremors)
  • Huntington’s disease (nerve cell damage that results in motor and memory loss)

Evidence has shown that NAC supplementation can help reduce the risk of these neurodegenerative diseases because NAC and glutathione are important for fighting cellular damage.[7]

NAC supports performance

In a study with cycling athletes, NAC infusions increased time to fatigue by 26% than controls.[8] This improved muscle performance is likely due to glutathione availability and enhanced muscle cysteine levels.

Several-human-studies-indicate-that-N-acetylcysteine-delays-muscular-fatigue-during
Several human studies indicate that N-acetylcysteine delays muscular fatigue during prolonged submaximal exercise
Source: Powers, S. Exercise-induced oxidative stress in humans: Cause and consequences. (2010)

NAC as an antioxidant against cancer

NAC is a precursor to glutathione, which is an antioxidant that has chemoprotective properties against lung cancer by helping DNA repair.[9]

The recommended safe dose of NAC for chemoprotective properties is 600mg daily.[10] There is also evidence to suggest that NAC can help reduce the risk of colon cancer.[3]

NAC helps manage HIV symptoms

A robust study showed that oral NAC can replenish glutathione levels in patients with HIV infection. Although HIV is not curable, supplementation can help manage symptoms and quality of life.[11]

NAC increases magnesium levels

Regardless of whether we have blood pressure issues or not, NAC supplementation has shown to increase magnesium levels.[2]

Increase in glutathione correlates with increase in magnesium levels in red blood cells
Increase in glutathione levels shows an increase in magnesium levels

Source: Resnick, L. Effects of Glutathione on Red Blood Cell Intracellular Magnesium. (1999)

NAC reduces oxidative stress

When there is an imbalance between free radicals and antioxidants in the body, this can lead to oxidative stress and subsequent physical and mental health problems. 

Supplementing with NAC for two weeks has shown to reduce levels of oxidative stress.[2]

NAC supports kidney and liver function

Glutathione is essential for removing toxins from the body, and NAC supplementation can help expel toxins in urine. 

NAC supplementation has shown to have positive effects on liver and kidney health, which is often why doctors give intravenous NAC to reduce kidney and liver damage.[1,2,12]

NAC improves mental health

Oxidative stress has been associated with a variety of psychiatric disorders including:

  • Anxiety – 1-2g NAC daily can help reduce subjective anxiety through glutathione activity[13]
  • Depression – 1g NAC daily can help depressive symptoms[13]
  • Bipolar disorder – 1g NAC twice daily with usual medication, lowered depressive symptoms but not mania[14]
  • Schizophrenia  – 1g NAC twice daily helped with restlessness[6,15]
  • Substance use disorder – preliminary evidence suggest that NAC can potentially help with addiction behaviour[1,12]
  • OCD (obsessive-compulsive disorders) – NAC influences the amino acid glutamine. This system is responsible  for reward and reinforcement that is often prevalent in OCD behaviour[6]
NAC and mental health disorders
N-Acetylcysteine in the Treatment of Pediatric Trichotillomania
Source: Grant., J. N-Acetylcysteine in the Treatment of Pediatric Trichotillomania: A Randomized, Double-Blind, Placebo-Controlled Add-On Trial. Child and Adolescent Psychiatry. (2013)

NAC supports immunity

Glutathione is the most abundant antioxidant in the body and is essential for the immune system. NAC supplements, compared to a placebo, have shown to increase levels of natural killer cell activity, which are the cells that are responsible for fighting infection.[1,2,16]

NAC increases the levels of natural killer cells

NAC reduces the risk of heart disease

Maintaining healthy cholesterol levels can help reduce our risk of cardiovascular disease. NAC can help reduce oxidative stress, which affects the lipids as LDL (low density lipoprotein) is associated with cholesterol.[2]

There is also evidence that NAC improves nitric oxide production (and blood flow) which can reduce our risk of a heart attack through its antioxidant properties.[1,12]

NAC stabilises blood sugar levels

Research has shown that NAC may help stabilise blood sugar, reduce the risk of Type II diabetes and improve insulin resistance for those that have PCOS (polycystic ovarian syndrome).[12]

RELATED — Diabetes: Early Signs, Causes, Types and Treatment

NAC helps maintain healthy blood pressure

NAC has shown to reduce blood pressure by 5 mmHg in hypertensive patients.[9] This is due to decreasing oxidative damage to tissues in the heart.

NAC improves fertility

Antioxidants are important for both male and female reproductive systems. 600mg/day of NAC has shown to improve semen integrity and pregnancy rate by 22%.[1,12]

NAC (N-acetyl cysteine)and Fertility
NAC and fertility and men and women
Source: Azadi, L., A Preliminary Study: N-acetyl-L-cysteine Improves Semen Quality following Varicocelectomy. (2016)
 

NAC boosts brain health

Reducing oxidative stress can lead to better DNA repair and cognitive changes that can help manage psychiatric disorders. By increasing levels of glutathione, through NAC supplementation, this can improve brain health including memory and learning.[1,2,12]

NAC improves respiratory health

NAC supplements have shown to help with lung collapse, to prepare people for lung tests and care for people with tube in their windpipe.[17]

NAC helps with cystic fibrosis via various mechanisms including ion transport, electrolyte balance and fluid content in tissues.[2]

Health claims that still need more evidence and research

NAC, chest pain and angina

When used in conjunction with other medication, such as nitroglycerin, NAC has shown to help with chest pain and angina symptoms. However, more research is needed to investigate the correlation.[17]

NAC and autism

Oral NAC supplements have shown to improve irritability in young people with autism, although no evidence supports improvement of other autistic symptoms.[17]

NAC and kidney damage

While there is plenty of evidence that supports NAC and kidney health, for individuals with kidney damage more research is needed to explore how NAC can help, which includes help with renal failure.[17]

NAC and high cholesterol

More research is needed to investigate the potential of NAC reducing levels of lipoprotein (fat in the blood). This is a low-density lipoprotein that can increase the risk of cholesterol build up in the heart arteries.[17]

NAC and cancer drug toxicity

NAC seems to prevent side effects of the cancer drug ifosfamide, but there are other drugs, such as mesna, that seem to have a better effect than NAC.[17]

Help with lung disease, flu and COVID-19 symptoms

NAC is believed to reduce flu symptoms, however, we need more evidence to support this claim.This includes reducing shortness of breath and inflammation of the airways [lungs].[17] 

NAC and COVID 19 symptoms
Effect of NAC treatment on cell-mediated immunity
Source: Puyo, CA., N-Acetylcysteine to Combat COVID-19: An Evidence Review. Dovepress. (2020)
 

Taking oral NAC doses for six months have shown to help reduce flare ups by 40% of those with moderate-severe COPD (chronic obstructive pulmonary disease).[17]

There have also been case studies that have shown that NAC supplementation can help the treatment and management of COVID-19 symptoms.[18]

Best sources of N-Acetyl Cysteine

Nutrients from food are generally more bioavailable than those in a supplement form.

As the body naturally produces cysteine, there is no specific recommended daily intake (RDI). However, based on other research, we know that the body needs a certain amount of protein a day. 

Food Source

Concentration (mg/100g)

Daily Value (DV) RDI% 

Cashews

393 mg

137%

Sunflower Seeds

383 mg

133%

Pork

350 mg

122%

Beef 

345 mg

120%

Chicken

336 mg

117%

Tuna

321 mg

112%

Eggs

292 mg

102%

Swiss Cheese 

290 mg

101%

Turkey

265 mg

92%

Lamb

252 mg

88%

Almonds

215 mg

75%

Lentils

118 mg

41%

Oats

97 mg

34%

Black Beans

96 mg

33%

Kidney Beans

81 mg

28%

Cysteine in food

The calculations in the table below are based on a 70kg person, with an RDI of 287mg cysteine a day (about 4.1mg/kg day).[19]

Daily requirements and recommended intake

As mentioned previously, NAC is a non-essential amino acid so there is no set recommended daily intake (RDI). 

However, protein requirements depend on the weight of the person (mg/kg) and also on our activity levels. 

NAC in supplement form is not highly bioavailable

This means that the amount consumed isn’t necessarily the amount that is absorbed by the body. 

This is why the requirements in the table below are higher than the RDI in the previous table.[20]

Age (years)

Male (mg NAC/kg)

Female (mg NAC/kg)

Birth to 6 months

59

59

7-11 months

43

43

1-3 years

28

28

4-8 years

22

22

9-13 years

22

21

14-18 years

21

19

19+ years

19

19

Important note – for women who are pregnant and/or are lactating, the amount of NAC per kilogram of body weight should be between 25-26 mg.

How to take N-Acetyl Cysteine

NAC can be administered intravenously or orally in a spray, liquid, or powder form. NAC has low bioavailability and is usually consumed orally with a dosage of around 600-1200mg daily.[17]

As an amino acid, NAC is water soluble, so taking with a glass of water instead of food can help absorption.

In order to process NAC more efficiently, it is recommended that NAC supplements are to be taken together with

  • Vitamin B6 (Pyridoxine) 
  • Vitamin B9 (Folate)
  • Vitamin B12 (Cobalamin)[2]

It is also recommended that NAC supplements should be taken with zinc to maximise the absorption rate.[21]

RELATED — Zinc (for immunity, skin health and libido)

Common signs and symptoms of N-Acetyl Cysteine deficiency

Symptoms of NAC deficiency and low glutathione levels can be classified as mild, moderate and severe.

Mild to moderate deficiency symptoms of NAC

Mild NAC deficiency can result in the destruction of red blood cells (hemolytic anaemia).[22]

People can also experience increased acidity in the body (metabolic acidosis) which can manifest as symptoms including

  • Nausea
  • Fatigue
  • Decreased appetite
  • Confusion
  • Heart arrhythmias
  • Headache
  • Hyperventilation (long and deep breaths).[22]

Severe deficiency symptoms of NAC

Severe deficiency can result in neurological symptoms, such as

  • Seizures
  • Psychomotor retardation (slowing down of movement and speech)
  • Intellectual disability and/or mental health condition
  • Loss of coordination (ataxia)
  • Bacterial infections (due to compromised immune system).[22]

N-Acetyl Cysteine risks and side effects

Excessive NAC slows blood clotting

Too much NAC can interfere with the body’s ability to produce blood clots. Therefore, NAC supplements can increase the risk of bleeding and bruising and also lead to bleeding disorders.[17]

NAC may reverse hyperglycemia

Individuals with diabetes usually have poor regulation of blood sugar levels. Although NAC may help with hyperglycemia (high blood sugar) it can also cause hypoglycemia (low blood sugar).[12]

Risk of NAC for people with asthma

Although unlikely, NAC could potentially increase the risk of bronchospasm, so always check with your physician before taking any supplements.[17]

NAC may cause kidney stones

Although it is rare, NAC supplements can cause kidney stones to form. This is due to the insoluble parts of NAC clotting together to form mineral build ups – which is otherwise known as kidney stones.[12]

Side effects of taking N-Acetyl Cysteine by oral supplements or nasal sprays

Oral supplements take longer to be absorbed in the body because NAC has to go through the digestive system. When taken by mouth, NAC supplements can cause

  • Dry mouth
  • Nausea
  • Vomiting and diarrhoea[17] 

Nasal sprays are absorbed faster in the body as NAC goes straight into the respiratory system. NAC supplements, however, in the form of sprays can cause

  • Swelling in the mouth
  • Runny nose
  • Drowsiness
  • Clamminess
  • Chest tightness[17]

Possible interactions with herbs and supplements

Activated charcoal, used to prevent drug poisoning, shouldn’t be taken in conjunction with NAC.[17] 

Chloroquine, a drug used to treat malaria, has its effectiveness reduced by NAC.[17]

Possible interactions with medications

Medications for blood pressure (antihypertensive drugs), in combination with NAC, can cause a drop in blood pressure. This can be very dangerous and fatal if untreated.[17]

Medications for slowing blood clotting (anticoagulants) can increase the risk of bruising and bleeding if taken with NAC.[17]

Angina medication (such as nitroglycerin) and diabetes medication dilates blood vessels and increases blood flow.[12] Taking these with NAC can increase risk of side effects including headache, dizziness and lightheadedness.[17]

Summary

Cysteine (NAC) - Health Benefits, Deficiency, and Supplementation

Key Takeaway – In this summary illustration we have outlined the most important information that you should know about n-acetyl cysteine (NAC).

Related Questions

1. What is the difference between NAC and cysteine?

N-Acetyl cysteine is a precursor/form of cysteine. Cysteine is a non-essential amino acid (building block of protein in the body).

2. What is the difference between essential and non-essential amino acids?

A non-essential amino acid can be produced by the body and also be obtained from food, whereas an essential amino acid can only be obtained from the diet/supplements, and is not produced in the body.

RELATED — What are Amino Acids and what is their role in keeping us healthy?

3. Why is NAC so important?

NAC is an important precursor to the antioxidant glutathione, which is involved in several important metabolic pathways in the body.

4. Do you need to take NAC supplements?

Unless you are at risk of a nutrient deficiency, you should always aim to get your nutrients from food first

Consider a supplement if you are of old age, have poor nutrition, do excessive exercise, have high blood sugar or high blood pressure. 

Have you tried taking NAC as a supplement? If so, have you seen any benefits? Please let us know in the comment section below.

Having passion for mental health and nutrition, Carly’s goal is to become a registered psychologist with a focus on self-care – food, exercise, and sleep. She has a special interest in various mental health disorders, plant-based diets, and the relationship between food and mood. 

Through evidence-based research, holistic approach to health and personal experience, Carly hopes to empower others’ well-being. 

Carly is a part of the Content Team that brings you the latest research at D’Connect.

References

(1) Goodson A. Top 9 Benefits of NAC (N-acetylcysteine). Healthline [Online]. 2022 Feb 11. Accessed 14/07/22. Retrieved from: https://www.healthline.com/nutrition/nac-benefits

(2) Smith A. 10 Research-Based Benefits of Glutathione and NAC (N-acetylcysteine). 2020 Aug 26. Nordic [Online]. Accessed 14/07/22. Retrieved from: https://www.nordic.com/healthy-science/benefits-of-glutathione-and-nac  

(3) Atkuri KR, Mantovani JJ, Herzenberg LA, Herzenberg LA. N-Acetylcysteine—a safe antidote for cysteine/glutathione deficiency. Current opinion in [pharmacology. 2007 Aug 1;7(4):355-9. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4540061/ 

(4) Church F, The Yack on NAC and Parkinson’s. Accessed 14/07/22. Retrieved from: https://journeywithparkinsons.com/2017/05/25/the-yack-on-nac-n-acetyl-cysteine-and-parkinsons/ 

(5) Rushworth GF, Megson IL. Existing and potential therapeutic uses for N-acetylcysteine: the need for conversion to intracellular glutathione for antioxidant benefits. Pharmacology & therapeutics. 2014 Feb 1;141(2):150-9. Retrieved from:  https://doi.org/10.1016/j.pharmthera.2013.09.006

(6) Sansone RA, Sansone LA. Getting a Knack for NAC: N-Acetyl-Cysteine. Innovations in Clinical Neuroscience. 2011 Jan;8(1):10-14. PMID: 21311702; PMCID: PMC3036554. Retrieved from: https://europepmc.org/article/PMC/3036554 

(7) Johnson WM, Wilson-Delfosse AL, Mieyal JJ. Dysregulation of glutathione homeostasis in neurodegenerative diseases. Nutrients. 2012 Oct 9;4(10):1399-440. Retrieved from: https://doi.org/10.3390/nu4101399

(8) Medved I, Brown MJ, Bjorksten AR, Murphy KT, Petersen AC, Sostaric S, Gong X, McKenna MJ. N-acetylcysteine enhances muscle cysteine and glutathione availability and attenuates fatigue during prolonged exercise in endurance-trained individuals. Journal of Applied Physiology. 2004 Oct 1. Retrieved from: https://doi.org/10.1152/japplphysiol.00371.2004 

(9) Martina V, Masha A, Gigliardi VR, Brocato L, Manzato E, Berchio A, Massarenti P, Settanni F, Della Casa L, Bergamini S, Iannone A. Long-term N-acetylcysteine and L-arginine administration reduces endothelial activation and systolic blood pressure in hypertensive patients with type 2 diabetes. Diabetes care. 2008 May 1;31(5):940-4. Retrieved from: https://doi.org/10.2337/dc07-2251

(10) van Zandwijk, N., 1995. N‐acetylcysteine (NAC) and glutathione (GSH): antioxidant and chemopreventive properties, with special reference to lung cancer. Journal of Cellular Biochemistry, 59(S22), pp.24-32. Retrieved from: https://doi.org/10.1002/jcb.240590805

(11) De Rosa SC, Zaretsky MD, Dubs JG, Roederer M, Anderson M, Green A, Mitra D, Watanabe N, Nakamura H, Tjioe I, Deresinski SC. N‐acetylcysteine replenishes glutathione in HIV infection. European journal of clinical investigation. 2000 Oct 19;30(10):915-29. Retrieved from: https://doi.org/10.1046/j.1365-2362.2000.00736.x

(12) Brennan D. Health Benefits of NAC. WebMD [Online]. 2020 Nov 5. Accessed 14/07/22. Retrieved from: https://www.webmd.com/diet/health-benefits-nac#1

(13) Slattery J, Kumar N, Delhey L, Berk M, Dean O, Spielholz C, Frye R. Clinical trials of N-acetylcysteine in psychiatry and neurology: a systematic review. Neuroscience & Biobehavioral Reviews. 2015 Aug 1;55:294-321. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/25957927/ 

(14) Berk M, Copolov DL, Dean O, Lu K, Jeavons S, Schapkaitz I, Anderson-Hunt M, Bush AI. N-acetyl cysteine for depressive symptoms in bipolar disorder—a double-blind randomized placebo-controlled trial. Biological psychiatry. 2008 Sep 15;64(6):468-75. Retrieved from: https://doi.org/10.1016/j.biopsych.2008.04.022

(15) Berk M, Copolov D, Dean O, Lu K, Jeavons S, Schapkaitz I, Anderson-Hunt M, Judd F, Katz F, Katz P, Ording-Jespersen S. N-acetyl cysteine as a glutathione precursor for schizophrenia—a double-blind, randomized, placebo-controlled trial. Biological psychiatry. 2008 Sep 1;64(5):361-8. Retrieved from: https://doi.org/10.1016/j.biopsych.2008.03.004 

(16) Richie JP, Nichenametla S, Neidig W, Calcagnotto A, Haley JS, Schell TD, Muscat JE. Randomized controlled trial of oral glutathione supplementation on body stores of glutathione. European journal of nutrition. 2015 Mar;54(2):251-63. https://link.springer.com/article/10.1007/s00394-014-0706-z#citeas 

(17) WebMD. N-acetylcysteine (NAC) – Uses, Side Effects and more. [Online]. Accessed 14/07/22. Retrieved from: https://www.webmd.com/vitamins/ai/ingredientmono-1018/n-acetyl-cysteine-nac

(18) Shi Z, Puyo CA. N-acetylcysteine to combat COVID-19: an evidence review. Therapeutics and Clinical Risk Management. 2020;16:1047. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7649937/

(19) Whitbread D. Top 10 Foods Highest in Cysteine. My Food Data [Online]. 2022 Apr 24. Retrieved from:  https://www.myfooddata.com/articles/high-cystine-foods.php#cystine-rich-foods 

(20) McAuley D. RDA and EAR recommendations for essential amino acids [Online]. 2018 Apr 21. Accessed 27/07/22. Retrieved from: https://globalrph.com/rda-and-ear-recommendations-for-essential-amino-acids/   

(21) Koozehchian, M. N-Acetylcysteine, the obscure antioxidant. Huffins Institute. 2013, Feb 19. Accessed 19 July 2022. [Online]. Retrieved from: https://www.huffinesinstitute.org/Resources/Articles/ArticleID/410/N-Acetylcysteine-the-Obscure-Antioxidant

(22) MedlinePlus. Glutathione synthetase deficiency (2015 Mar 1) Accessed 21/7/22. [Online]. Retrieved from: https://medlineplus.gov/genetics/condition/glutathione-synthetase-deficiency/#:~:text=These%20problems%20may%20include%20seizures,also%20develop%20recurrent%20bacterial%20infections

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