Can I Take Glycine with NMN/NR?

Why People Stack Glycine and NMN

Glycine and NMN target overlapping wellness goals (sleep, cellular repair, healthy aging) through unrelated biochemical routes. That overlap drives the popularity of stacking them, but it also raises a fair question: do they interfere with each other?

Glycine's Role

Glycine is the simplest amino acid and a precursor to glutathione, creatine, and collagen. A 2023 review in Amino Acids confirmed its role as an inhibitory neurotransmitter in the brainstem and spinal cord, modulating NMDA receptor activity and lowering core body temperature to promote sleep onset 1. In a double-blind crossover trial (N=11), 3 g of glycine before bed reduced subjective fatigue and improved sleep efficiency measured by polysomnography 2.

NMN and NR Basics

NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) are both NAD+ precursors. NMN enters cells via the SLC12A8 transporter identified in murine intestinal tissue 3, while NR uses equilibrative nucleoside transporters. Once inside the cell, both feed the NAD+ salvage pathway. A 12-week RCT (N=30) of NMN at 250 mg/day in healthy older men showed increased blood NAD+ metabolites and improved muscle insulin sensitivity 4. NR at 1,000 mg/day raised whole-blood NAD+ by approximately 60% in a pharmacokinetic study of 12 healthy adults 5.

Pharmacokinetic Interaction Analysis

The core question: does one compound change how the other is absorbed, distributed, metabolized, or excreted? Available evidence says no.

Absorption

Glycine is absorbed in the jejunum and ileum primarily through PAT1 (SLC36A1) and the IMINO transporter (SLC6A20) 6. NMN absorption relies on a distinct transporter, SLC12A8, expressed in the small intestine 3. Because these are separate transport systems, taking both supplements simultaneously should not create competitive inhibition at the intestinal brush border. Any two compounds taken together in large boluses can slow gastric emptying. A 30-minute separation eliminates this minor concern.

Metabolism and Clearance

Glycine does not undergo cytochrome P450 metabolism. It is metabolized by the glycine cleavage system in hepatic mitochondria 7. NMN is phosphorylated to NAD+ through the NAMPT/NMNAT enzyme cascade in the salvage pathway, bypassing CYP450 entirely 8. Neither compound is a CYP inhibitor or inducer. No competitive metabolic interaction exists between them.

Pharmacodynamic Overlap: Sleep

Both glycine and NAD+ metabolism influence sleep, but through parallel, non-competing pathways. This is the area where the two supplements are most complementary rather than conflicting.

Glycine and Thermoregulation

Glycine promotes sleep by activating NMDA receptors in the suprachiasmatic nucleus, which dilates peripheral blood vessels and drops core body temperature by approximately 0.3°C 2. This thermoregulatory shift mimics the natural pre-sleep temperature decline. A separate trial (N=19) confirmed improved subjective sleep quality and reduced next-day sleepiness at the 3 g dose 9.

NAD+ and Circadian Regulation

NAD+ is a cofactor for SIRT1, a deacetylase that regulates BMAL1 and CLOCK, the core circadian transcription factors 10. Raising NAD+ through NMN or NR supplementation may strengthen circadian oscillation amplitude. In aged mice, NMN restored disrupted circadian gene expression in the liver 10. These mechanisms do not antagonize glycine's thermoregulatory pathway. If anything, stronger circadian signaling plus improved sleep onset temperature regulation could be additive.

The Methylation Question

This is the one area where theoretical concern exists, and it deserves a careful look.

How Both Compounds Draw on Methyl Groups

When NMN is converted to NAD+ and subsequently used, one byproduct is nicotinamide (NAM). The body clears excess NAM by methylating it to N1-methylnicotinamide via the enzyme NNMT, consuming S-adenosylmethionine (SAMe) as a methyl donor 11. Glycine also consumes methyl groups: the synthesis of creatine from glycine and arginine uses roughly 40% of all SAMe-derived methyl groups in the body 12.

Practical Significance

In theory, high-dose NMN (1,000 mg+) combined with supplemental glycine (3 to 5 g) could increase total methylation demand. A 2020 mouse study showed that very high NMN doses depleted hepatic methyl donors when dietary methionine was restricted 11. Translating rodent dosing to humans is unreliable, but the principle matters for people who already have low methyl-donor status (vegetarians with low B12 intake, individuals with MTHFR polymorphisms).

Mitigation

Support methylation with adequate dietary methionine, folate (400 mcg/day), B12 (2.4 mcg/day), and betaine. The Endocrine Society's nutritional recommendations for older adults provide these baseline targets 13. Monitoring homocysteine levels every 6 to 12 months is reasonable for anyone stacking multiple methylation-demanding supplements.

Glycemic Effects

Glycine and Blood Glucose

A systematic review and meta-analysis of 29 studies found that glycine supplementation (3 to 5 g/day) modestly improved fasting glucose and HbA1c in subjects with metabolic syndrome 14. The proposed mechanism involves glycine stimulating GLP-1 secretion and improving insulin signaling in skeletal muscle.

NMN and Insulin Sensitivity

The Yoshino et al. RCT (N=25 postmenopausal women with prediabetes) demonstrated that NMN at 250 mg/day improved skeletal muscle insulin sensitivity measured by hyperinsulinemic-euglycemic clamp, though it did not significantly lower fasting glucose 4. A separate 12-week trial of NR 1,000 mg/day in obese men (N=40) found no significant change in insulin sensitivity or fasting glucose versus placebo 15.

Combined Effect

Both compounds trend toward improved or neutral glycemic effects. No antagonism has been documented. Individuals taking metformin or sulfonylureas should monitor blood glucose for the first 2 weeks when adding either supplement, per standard clinical practice when introducing agents that could alter insulin signaling.

Collagen Synthesis: A Synergistic Angle

Glycine constitutes roughly one-third of collagen's amino acid content. Supplemental glycine (5 g taken with 50 mg vitamin C, 60 minutes before exercise) doubled collagen synthesis rate measured by the PINP biomarker in an 8-subject crossover study at the Australian Institute of Sport 16. NAD+ supports the activity of poly(ADP-ribose) polymerases (PARPs) involved in DNA repair during tissue remodeling 8. While no trial has tested glycine plus NMN specifically for connective tissue repair, the mechanistic rationale for combination is stronger than the rationale for interference.

Dose and Timing Recommendations

Standard Dosing

| Supplement | Typical dose | Timing | |---|---|---| | NMN | 250 to 500 mg/day | Morning, with or without food | | NR | 300 to 1,000 mg/day | Morning, with or without food | | Glycine (sleep) | 3 g | 30 to 60 minutes before bed | | Glycine (collagen) | 5 to 15 g | 60 minutes before exercise, with vitamin C |

Separation Strategy

Taking NMN in the morning and glycine at bedtime creates natural dose separation of 12+ hours. This schedule aligns with both compounds' intended effects: NAD+ peaks support daytime energy metabolism, while glycine's thermoregulatory action supports nighttime sleep. No evidence requires separation for safety, but it makes physiological sense.

When to Consult a Clinician

Talk to your prescriber before combining glycine and NMN/NR if you:

• Take anticoagulants (glycine at high doses may have mild antithrombotic properties in animal models)
• Have chronic kidney disease (both compounds are renally cleared)
• Are on clozapine (glycine modulates NMDA receptors, which could theoretically interact with antipsychotic pharmacodynamics) 17
• Take antihypertensives (glycine's vasodilatory effects at 3 g+ doses could add to blood pressure lowering)

Monitoring Protocol for the First 8 Weeks

A practical monitoring plan when starting glycine and NMN together:

Weeks 1 to 2

Track sleep onset latency and total sleep time daily using a log or wearable device. Note any GI symptoms (bloating, loose stools). If you have diabetes or prediabetes, check fasting glucose every 2 to 3 days.

Weeks 3 to 4

Assess subjective energy, cognitive clarity, and exercise recovery. Compare to your baseline from before supplementation. Reduce glycine dose to 1.5 g if vivid dreams or morning grogginess appear (reported anecdotally, not in controlled trials).

Week 8

Optional lab panel: fasting glucose, HbA1c, homocysteine, and liver enzymes (AST/ALT). Homocysteine above 12 mcmol/L may suggest increased methylation demand 13. Discuss B-vitamin supplementation with your clinician if elevated.

What the Evidence Does Not Yet Show

No human RCT has directly tested glycine plus NMN or NR as a combination. The safety assessment here is built from independent pharmacokinetic profiles, separate clinical trials of each compound, and mechanistic reasoning. A 2024 narrative review of NAD+ precursor safety in GeroScience called for combination-supplement trials but noted no signals of harm from polysupplement use in existing cohort data 18. The FDA has not evaluated NMN as a dietary supplement following its exclusion from the supplement definition in late 2022, though enforcement discretion has allowed continued market availability 19.

Fasting homocysteine above 12 mcmol/L after 8 weeks of combined use warrants adding methylfolate (400 to 800 mcg/day) and reassessing at 12 weeks.