Glycine and NAC

What are Glycine and NAC?

Glycine and N-acetylcysteine (NAC), taken together as GlyNAC, provide two amino-acid precursors used by the body to synthesise glutathione. Glutathione is a major intracellular antioxidant involved in redox regulation, peroxide detoxification and protection against oxidative damage. Human ageing studies have reported lower glutathione synthesis alongside reduced availability of glycine and cysteine, providing the biological rationale for studying these two precursors together. (Sekhar et al., 2011; Kumar et al., 2021; Kumar et al., 2023)

Glycine is a simple, non-essential amino acid. The body can synthesise it, but glycine is also obtained from protein-containing foods and is abundant in collagen-rich proteins such as gelatin. Beyond its role in protein synthesis, glycine contributes to the formation of several biologically important molecules, including glutathione, creatine and porphyrins. (Razak et al., 2017)

N-acetylcysteine is an acetylated derivative of the amino acid cysteine. It can act as a cysteine source for glutathione synthesis and has a long history of clinical use because of its mucolytic and antioxidant-related properties. In the context of GlyNAC research, its main relevance is that it helps provide cysteine, one of the substrates required to make glutathione. (Mokhtari et al., 2017)

Glutathione itself is formed from three amino acids: glutamate, cysteine and glycine. In the first step of synthesis, glutamate-cysteine ligase combines glutamate with cysteine to form γ-glutamylcysteine. Glutathione synthetase then adds glycine to form glutathione. This two-step pathway explains why the availability of both cysteine and glycine can matter when glutathione synthesis is constrained. (Sekhar et al., 2011)

For wider context on how redox balance, mitochondrial function and other cellular pathways fit within Longevita's healthy ageing science, explore the Longevita ingredient library.

Below, we review the age-related changes that led researchers to study GlyNAC, followed by the human evidence for glutathione status, oxidative stress, mitochondrial function, inflammation, physical function and cognition.

Age-Related Glutathione Decline and Precursor Limitation

Ageing is associated with changes in cellular redox balance and a greater burden of oxidative stress. In human research comparing older and younger adults, older participants showed lower red-blood-cell glutathione concentrations and impaired glutathione synthesis. The investigators linked this deficit, in part, to reduced availability of the precursor amino acids glycine and cysteine. (Sekhar et al., 2011)

In the same study, providing cysteine and glycine for two weeks increased glutathione synthesis and concentrations in the older participants and reduced measures of oxidative stress. These findings established an important mechanistic point: at least in the population studied, age-related glutathione deficiency was not simply an irreversible consequence of ageing, but was responsive to precursor availability. (Sekhar et al., 2011)

Later GlyNAC studies extended this concept beyond glutathione concentration alone. A 24-week pilot study and a subsequent 16-week randomised clinical trial evaluated a broader set of ageing-related outcomes, including mitochondrial fuel oxidation, inflammation, insulin resistance, endothelial function, genotoxicity and physical performance. These studies are central to the modern GlyNAC evidence base, although their sample sizes and populations must be considered when interpreting the findings. (Kumar et al., 2021; Kumar et al., 2023)

Systemic Healthy-Ageing Effects of GlyNAC

1. Glutathione Synthesis and Redox Balance

The most direct rationale for GlyNAC supplementation is glutathione synthesis. NAC provides a source of cysteine, while glycine supplies the final amino acid incorporated into glutathione. This is particularly relevant when precursor availability becomes limiting, as was observed in older adults with lower glutathione synthesis. (Sekhar et al., 2011; Mokhtari et al., 2017)

Glutathione exists predominantly in its reduced form, GSH, and can be oxidised to glutathione disulfide, GSSG, during reactions that neutralise reactive species. The balance between reduced and oxidised glutathione is therefore closely tied to cellular redox status. Glutathione also acts as a substrate for glutathione peroxidase enzymes that reduce hydrogen peroxide and lipid peroxides. (Sekhar et al., 2011; Lizzo et al., 2022)

In the 2023 double-blind randomised trial in older adults, 16 weeks of GlyNAC supplementation corrected glutathione deficiency and improved several measures of oxidative stress, whereas the isonitrogenous alanine placebo did not produce the same pattern of change. This supports the idea that precursor repletion can influence both glutathione status and downstream redox measures in older adults. (Kumar et al., 2023)

Not every controlled study has produced the same result across an entire study population. In a 2022 randomised trial involving 114 healthy older adults, the primary circulating-glutathione outcome did not differ significantly from placebo overall. A post-hoc subgroup with higher oxidative stress and lower baseline glutathione showed a significant increase in glutathione synthesis. This finding highlights that baseline redox status may influence who responds most clearly. (Lizzo et al., 2022)

Direct oral glutathione has also been investigated, but older controlled research reported limited changes in systemic oxidative-stress biomarkers with standard oral supplementation. Providing precursor amino acids represents a different strategy: rather than supplying glutathione itself, it provides substrates that cells can use for endogenous glutathione synthesis. (Allen & Bradley, 2011; Sekhar et al., 2011)

2. Mitochondrial Function and Fuel Oxidation

Mitochondria depend on tightly controlled redox conditions to produce energy efficiently. Oxidative stress can interfere with mitochondrial enzymes, membranes and fuel oxidation, while dysfunctional mitochondria can in turn generate additional reactive oxygen species. This relationship makes mitochondrial function an important part of the GlyNAC ageing hypothesis. (Kumar et al., 2021; Kumar et al., 2023)

In the 2023 randomised clinical trial, older adults showed impaired mitochondrial fatty-acid oxidation compared with younger participants at baseline. After 16 weeks, GlyNAC supplementation was associated with improvement in mitochondrial fuel oxidation together with changes in molecular regulators of mitochondrial energy metabolism. (Kumar et al., 2023)

NAC may also contribute indirectly to cellular antioxidant defence through cysteine availability and redox-sensitive signalling. Reviews of NAC describe links with glutathione replenishment and Nrf2-regulated antioxidant systems, providing a mechanistic bridge between precursor availability and cellular stress responses. (Mokhtari et al., 2017; Mokra et al., 2023)

For comparison with another Longevita ingredient centred on mitochondrial energy and oxidative protection, see the Coenzyme Q10 ingredient page.

3. Inflammation, Insulin Resistance and Endothelial Function

Chronic low-grade inflammation is a recognised feature of ageing biology and is closely interconnected with oxidative stress and mitochondrial dysfunction. Rather than representing a single pathway, these processes can reinforce one another across tissues and metabolic systems. (Franceschi & Campisi, 2014; López-Otín et al., 2013)

The 2023 GlyNAC trial reported improvements in selected inflammatory markers alongside changes in glutathione deficiency and oxidative stress. The same study also reported improvements in measures of insulin resistance and endothelial function in older adults after 16 weeks of supplementation. (Kumar et al., 2023)

An earlier 24-week pilot study from the same research group reported a similar cluster of changes, including improvements in inflammation, insulin resistance and endothelial dysfunction. Because that pilot study was open-label and small, it is best treated as supportive rather than definitive evidence; the randomised trial provides stronger evidence for the direction of effect in the studied population. (Kumar et al., 2021; Kumar et al., 2023)

These findings do not establish GlyNAC as a treatment for inflammatory, metabolic or vascular disease. They show that supplementation influenced several biomarkers and physiological measures that are relevant to ageing biology in the older adults studied. (Kumar et al., 2023)

4. Genomic Stability and Oxidative Damage

Reactive oxygen species can damage DNA as well as lipids and proteins, which is one reason oxidative stress is linked to genomic instability in ageing biology. Genomic instability is also recognised as one of the hallmarks of ageing. (López-Otín et al., 2013)

In the 2023 randomised GlyNAC trial, supplementation was associated with reductions in selected measures of genotoxicity in older adults. The earlier 24-week pilot study reported a similar direction of change. These findings suggest a possible relationship between improved glutathione-redox status and lower oxidative injury to genetic material, but they do not demonstrate that GlyNAC directly repairs DNA or prevents age-related disease. (Kumar et al., 2021; Kumar et al., 2023)

5. Physical Function, Muscle Strength and Mobility

Physical function is one of the clinically meaningful domains explored in GlyNAC research. Measures such as gait speed, grip strength and six-minute walk distance are commonly used to assess mobility and functional capacity in older adults. (Kumar et al., 2023)

In the 16-week randomised trial, GlyNAC supplementation was associated with significant improvement in gait speed and grip strength, with a non-significant trend toward greater six-minute walk distance, whereas the placebo group did not show the same improvements. These functional findings occurred alongside improvements in glutathione status, oxidative stress and mitochondrial measures. (Kumar et al., 2023)

The earlier 24-week pilot study also reported improvements in muscle strength and walking performance. Its open-label design and small sample size limit generalisability, but the direction of change is consistent with the later randomised study. (Kumar et al., 2021; Kumar et al., 2023)

6. Cognitive Function

Cognition has been explored in GlyNAC research, although the evidence is less developed than the glutathione and oxidative-stress data. In the 24-week open-label pilot study, older adults showed improvements in selected cognitive tests, including the Montreal Cognitive Assessment, trail-making measures and verbal fluency. Several gains diminished during the 12-week washout period after supplementation stopped. (Kumar et al., 2021)

A 2024 open-access conference abstract from the same research group reported additional cognitive and metabolic observations from GlyNAC research in ageing. Because conference abstracts generally provide less methodological detail than full peer-reviewed trial reports, these findings should be considered preliminary and interpreted alongside the published clinical studies. (Sekhar et al., 2024)

At present, these studies suggest a possible relationship between GlyNAC supplementation and selected cognitive outcomes in older adults, but larger randomised trials with cognition as a primary outcome would be needed before drawing firm conclusions about cognitive benefit. (Kumar et al., 2021; Sekhar et al., 2024)

Glycine and Sleep: Evidence for Glycine Alone

Glycine has also been studied independently of NAC and GlyNAC in relation to sleep. This evidence should be kept separate from the clinical GlyNAC literature because the intervention, dose and outcome are different. (Yamadera et al., 2007; Bannai et al., 2012)

In a small single-blind crossover study, 3 grams of glycine taken before bedtime was associated with improvements in subjective sleep quality and polysomnographic measures. A later study in partially sleep-restricted healthy volunteers reported improvements in next-day subjective fatigue and daytime performance after bedtime glycine. (Yamadera et al., 2007; Bannai et al., 2012)

Experimental work has proposed mechanisms involving NMDA-receptor signalling in the suprachiasmatic nucleus and changes in peripheral heat loss. However, that mechanistic evidence includes animal data and should not be treated as direct proof of the same pathway in humans. (Kawai et al., 2015)

These studies therefore support glycine as a separately researched amino acid in sleep science; they should not be used to claim that the GlyNAC combination itself has been clinically shown to improve sleep.

Clinical Evidence, Dose and Safety

The most robust GlyNAC evidence comes from a 16-week double-blind randomised clinical trial in older adults. Twenty-four older participants were randomised to GlyNAC or an isonitrogenous alanine placebo, with younger adults included as a comparison group. GlyNAC supplementation was associated with improvements in glutathione deficiency, oxidative stress, mitochondrial fuel oxidation, inflammation, insulin resistance, endothelial function and physical-function measures. (Kumar et al., 2023)

The 24-week pilot study that preceded it reported a similarly broad pattern of improvements and included a 12-week withdrawal period during which several biochemical, physiological and cognitive gains diminished after supplementation stopped. The open-label design and small sample size mean that these observations are hypothesis-generating rather than conclusive evidence that ongoing supplementation is required. (Kumar et al., 2021)

The larger 2022 randomised study of 114 healthy older adults provides an important counterpoint: it did not find a significant improvement in its primary glutathione outcome across the whole group, although a post-hoc subgroup with higher oxidative stress and lower glutathione showed increased glutathione synthesis. Taken together, the available trials suggest biological activity while also indicating that response may depend on baseline status, study design and outcome selection. (Lizzo et al., 2022; Kumar et al., 2023)

Both glycine and NAC have been extensively studied in humans. Glycine was well tolerated at the 3-gram bedtime dose used in the cited sleep studies. Oral NAC has a long history of clinical use and generally favourable safety characteristics. (Yamadera et al., 2007; Bannai et al., 2012; Mokhtari et al., 2017)

In the 16-week GlyNAC randomised trial, the intervention was reported as safe and well tolerated in the older participants, with no safety signal identified in the study. The 2022 trial in 114 older adults likewise reported high tolerability over its two-week treatment period. (Kumar et al., 2023; Lizzo et al., 2022)

NAC can interact with some medicines. People taking prescription medication, managing a diagnosed medical condition, or who are pregnant or breastfeeding should discuss supplementation with a healthcare professional before use. (Mokhtari et al., 2017)

The time frame for observed changes differs by outcome. Early precursor studies measured glutathione and oxidative-stress changes over approximately two weeks; the randomised GlyNAC trial examined broader metabolic and physical outcomes over 16 weeks; and the earlier pilot study followed participants for 24 weeks. These study durations indicate when researchers measured outcomes and should not be interpreted as a guaranteed timeline for an individual response. (Sekhar et al., 2011; Kumar et al., 2021; Kumar et al., 2023)