Can I Take Glycine with Tresiba (Insulin Degludec)? A Women's Health Guide

What Is the Interaction Between Glycine and Tresiba?

Glycine and Tresiba do not appear to share a pharmacokinetic interaction. In plain terms, glycine is unlikely to change how quickly Tresiba is absorbed, distributed, metabolized, or cleared from your body. The concern is pharmacodynamic: both agents can lower blood glucose, and their effects may add together.

Tresiba (insulin degludec) is an ultra-long-acting basal insulin approved for type 1 and type 2 diabetes. Its flat pharmacodynamic profile and duration of action exceeding 42 hours make it more predictable than older basal insulins, but that also means any additive glucose-lowering from a supplement plays out across a long window.

Glycine is the simplest amino acid and one of the most abundant in the human body. Beyond its role in collagen synthesis and neurotransmission, it has measurable effects on glucose metabolism. A 2018 randomized trial published in the American Journal of Clinical Nutrition found that 3 g of glycine daily reduced fasting plasma glucose by approximately 5 mg/dL in adults with metabolic syndrome over 3 months. That is a small effect in isolation, but layered on top of a basal insulin, it becomes worth tracking.

How Glycine Affects Blood Glucose

Glycine appears to act through at least three mechanisms relevant to women with diabetes:

• Glucagon suppression. Glycine stimulates glucagon-like peptide-1 (GLP-1) secretion from intestinal L-cells. Higher GLP-1 blunts postprandial glucagon, which reduces hepatic glucose output.
• Insulin secretagogue effect. In pancreatic beta cells, glycine acts on glycine-gated chloride channels, which can augment glucose-stimulated insulin secretion. This mechanism only matters for women with residual beta-cell function, meaning it is more relevant in type 2 diabetes than in type 1.
• Glycogen synthesis support. Glycine is a cofactor in glutathione synthesis. Oxidative stress impairs insulin signaling, and glycine supplementation at 3-5 g/day has been shown to raise glutathione concentrations in older adults, which may indirectly improve insulin sensitivity.

None of these pathways overlap with the mechanism of insulin degludec, which binds insulin receptors directly. That is why the interaction is classified as additive rather than synergistic.

Pharmacokinetic Consideration: Protein and Injection Timing

One practical question women ask is whether taking glycine as a supplement around mealtimes affects the absorption of subcutaneous insulin. Current evidence does not support a meaningful pharmacokinetic effect. Glycine is absorbed in the gut and does not change subcutaneous depot formation or the hexamer dissociation kinetics that govern insulin degludec's ultra-long action profile. You do not need to separate them by a specific time window for pharmacokinetic reasons. The monitoring recommendation still applies, but it is about glucose response, not absorption.

Who This May Be Right For (and Who Should Be More Careful)

Not every woman with diabetes will have the same risk profile when adding glycine. Life stage matters considerably.

Reproductive Years and Menstrual Cycle Effects

If you are in your reproductive years and managing type 1 or type 2 diabetes, your insulin requirements naturally shift across your cycle. During the luteal phase, progesterone rises and reduces insulin sensitivity, so many women need more basal insulin in the 5-7 days before their period. The American Diabetes Association's Standards of Care note that menstrual cycle-related glycemic variability is a recognized clinical phenomenon, though guidance on adjusting insulin doses accordingly remains individualized.

Starting glycine in the luteal phase, when your baseline glucose is already running higher, may blunt the observable additive glucose-lowering effect. Starting it in the follicular phase, when insulin sensitivity is higher, may make the effect more apparent and raise hypoglycemia risk slightly. Time your glycine trial to match a predictable, consistent phase of your cycle so you can interpret your continuous glucose monitor (CGM) or self-monitoring data accurately.

Trying to Conceive

Women with diabetes who are actively trying to conceive need tighter glycemic targets (A1C below 6.5% is the target recommended by ACOG Practice Bulletin 201) and are often adjusting insulin doses already. Adding glycine in this period is not contraindicated based on available evidence, but the evidence base is thin and the margin for glycemic error narrows when trying to conceive. Discuss with your endocrinologist or OB before starting.

Perimenopause and Postmenopause

Estrogen loss during perimenopause and menopause reduces insulin sensitivity independently of weight changes. A 2021 review in Menopause documented that the transition through menopause increases fasting glucose and worsens postprandial glycemic excursions, meaning your Tresiba dose may need upward adjustment during this period. Glycine's modest glucose-lowering effect could become more or less visible against this background of shifting insulin sensitivity. Women in perimenopause who add glycine should increase the frequency of CGM review or glucose checks for at least the first month.

Who Should Be More Cautious

• Women with a history of hypoglycemia unawareness on basal insulin
• Women who have recently reduced their Tresiba dose and are still titrating
• Women on other glucose-lowering agents alongside Tresiba (metformin, GLP-1 receptor agonists, SGLT2 inhibitors) where an additional glucose-lowering input adds to an already complex regimen
• Women with severe chronic kidney disease, where glycine clearance may be reduced and its metabolic effects less predictable

Glycine, Sleep, and Why Women With Diabetes Ask About It

Glycine is popular as a sleep supplement. A 2012 double-blind crossover study in Sleep and Biological Rhythms found that 3 g of glycine taken before bed reduced subjective daytime sleepiness and improved sleep quality scores compared with placebo. Women are more likely to report sleep disturbances than men, and poor sleep independently worsens insulin resistance.

This creates a layered rationale: some women with diabetes start glycine for sleep and then wonder whether it will affect their glucose. The answer is that the sleep benefit and the glucose-lowering effect appear to be partially independent mechanisms. The sleep effect is attributed to glycine's action on NMDA receptors and its ability to lower core body temperature. The glucose effect is driven by the GLP-1, beta-cell, and glutathione pathways described above.

Taking 3 g of glycine at bedtime while on Tresiba means both the supplement and the basal insulin are working simultaneously through the night. Overnight hypoglycemia is the specific risk to watch. If you use a CGM, set a low-glucose alert at 70 mg/dL (or your provider's recommended threshold) for the first two weeks. If you do not use a CGM, check a fasting glucose each morning for at least two weeks after starting glycine.

Collagen Synthesis and Musculoskeletal Health in Women With Diabetes

Women with diabetes have higher rates of musculoskeletal complications, including adhesive capsulitis (frozen shoulder), limited joint mobility, and accelerated cartilage loss. Glycine is a critical substrate for collagen biosynthesis: roughly one-third of all amino acids in collagen are glycine. A 2019 trial in the American Journal of Clinical Nutrition showed that 15 g of collagen peptide (which is roughly 20-30% glycine by composition) taken with vitamin C before exercise increased collagen synthesis markers over 6 weeks.

Women taking glycine specifically for joint or tendon support are often using 5-15 g doses, which is substantially higher than the 3 g doses studied for metabolic effects. At 15 g, the additive glucose-lowering effect may be more pronounced. No trial has studied this specific scenario in women on insulin degludec, and the evidence gap is real. If you are taking glycine at these higher doses for collagen support, more frequent glucose monitoring is appropriate.

Pregnancy and Lactation Safety

This section contains mandatory safety information for women who are pregnant, planning pregnancy, or breastfeeding while taking Tresiba.

Insulin Degludec in Pregnancy

Insulin degludec carries FDA label pregnancy category B-equivalent status. Animal reproductive studies showed no harm, and it does not cross the placenta in meaningful amounts. Human data are limited compared with NPH insulin or insulin detemir, which have longer track records in pregnancy. ACOG and the American Diabetes Association recommend using established basal insulins (NPH or detemir) as first-line options in pregnant women with pregestational diabetes unless the woman was well-controlled on degludec before conception and switching would introduce destabilization. If you are already on Tresiba and become pregnant, do not switch without specialist guidance. Abrupt changes in basal insulin type during pregnancy introduce unpredictable glycemic risk.

Glycine in Pregnancy

Glycine is an endogenous amino acid and is present in normal dietary protein. At dietary levels (a few grams per day from food), there is no established harm. At supplemental doses of 3-15 g per day, no high-quality controlled data exist in pregnant women. Glycine is a precursor to heme and is involved in fetal growth signaling. Theoretical benefits exist, but the absence of controlled safety data in pregnancy means supplemental glycine cannot be recommended as routine during pregnancy. Discuss any supplement with your obstetric team.

Lactation

Insulin degludec does not transfer to breast milk in clinically relevant amounts. Insulin is a large peptide that is degraded in the infant's GI tract even if trace amounts were present. The FDA label confirms that Tresiba can be used in breastfeeding women. Your insulin requirements will likely decrease in the immediate postpartum period as the anti-insulin hormones of pregnancy fall rapidly, so expect to need dose adjustments whether or not you add glycine.

Glycine in breast milk: glycine is naturally present in human milk as a component of protein. There is no evidence that oral glycine supplementation at 3-5 g/day raises breast milk glycine to harmful levels, but no controlled lactation pharmacokinetic study has been done. Breastfeeding women should use dietary amounts of glycine (from bone broth, collagen-containing foods, or standard protein intake) rather than high-dose supplementation until more data exist.

Contraception Considerations

Insulin degludec is not a teratogen and does not require contraception for safe use. Glycine similarly carries no contraceptive requirement. The contraception conversation for women with diabetes, however, is worth flagging: ACOG recommends that women with pregestational diabetes use effective contraception and plan pregnancies deliberately given the risks of uncontrolled glycemia in early organogenesis. This is not a glycine-specific warning. It is a diabetes management priority.

PCOS, Insulin Resistance, and Glycine: Is There a Specific Women's Health Case?

Polycystic ovary syndrome (PCOS) is the most common endocrine condition in women of reproductive age, affecting 6-12% of women, and its defining feature is insulin resistance. Some women with PCOS develop type 2 diabetes and end up on insulin degludec. For this group, glycine may be a supplement with dual-purpose appeal: supporting insulin sensitivity and potentially improving sleep quality disrupted by the anxiety and mood symptoms common in PCOS.

A proposed framework for women with PCOS on basal insulin who are considering glycine:

1. Start with 2-3 g at bedtime, not the higher doses sometimes marketed for collagen.
2. Monitor fasting glucose daily for 14 days.
3. If fasting glucose drops below 80 mg/dL on two consecutive mornings, contact your prescriber about a modest Tresiba dose review.
4. If you are also on metformin (very common in PCOS-related type 2 diabetes), recognize that the additive glucose-lowering from three agents (metformin plus insulin plus glycine) may be more than the sum expected from any two.
5. Revisit the plan at each menstrual cycle phase change, since PCOS cycles are often irregular and insulin sensitivity shifts are less predictable than in women with regular ovulation.

No clinical trial has studied glycine supplementation specifically in women with PCOS on insulin degludec. This framework is derived from the mechanism data and general principles of basal insulin management, not from a head-to-head trial. That evidence gap is real, and you deserve to know it.

How to Monitor and What to Watch For

Monitoring does not need to be complicated, but it needs to be deliberate.

CGM Users

Set a low-glucose alert at 70 mg/dL (or your provider's threshold) before starting glycine. Review your overnight trace each morning for the first two weeks. Look specifically at the 2-4 a.m. Nadir and the fasting glucose at waking. If either drops more than 15-20 mg/dL below your typical values, pause glycine and call your care team.

Fingerstick Users

Check fasting glucose each morning for the first 14 days. Log the results. If you see three consecutive readings below 80 mg/dL that were not typical before starting glycine, contact your prescriber.

Symptoms of Hypoglycemia to Know

Shakiness, sweating, palpitations, confusion, and hunger are classic. Women with long-standing diabetes may have blunted hypoglycemia symptoms. If you have any history of hypoglycemia unawareness, the bar for adding any glucose-active supplement should be discussed with your endocrinologist first.

When to Stop Immediately

Stop glycine and contact your care team if you experience a glucose reading below 54 mg/dL, lose consciousness or have a seizure (call 911), or notice your hypoglycemia symptom threshold shifting.

Dosing Guidance Based on Available Evidence

The evidence base for glycine comes from studies using 3-5 g/day for metabolic and sleep endpoints. Higher doses of 10-15 g/day appear in collagen and joint studies. The glucose-lowering signal in the metabolic trials used the lower dose range. Starting at 2-3 g per day is a reasonable and evidence-consistent starting point for women on Tresiba. There is no evidence that a specific time separation between glycine and your Tresiba injection is required.

Tresiba is injected once daily at any consistent time. If you choose to take glycine at bedtime for sleep, and your Tresiba injection is also at bedtime, you will simply be adding a supplement to an existing insulin window. Monitor accordingly.

Talking to Your Care Team: What to Bring to the Appointment

Bring a list of your current Tresiba dose, your most recent A1C, your current CGM or fingerstick log, and any other supplements or medications. Ask specifically:

• "Given my current Tresiba dose and glucose patterns, is there a dose range for glycine you'd be comfortable with?"
• "Should I adjust my Tresiba dose preemptively, or wait and see what my glucose data shows?"
• "Is there any interaction I should watch for given my other medications?" (Relevant if you are also on metformin, a GLP-1 agonist, or an SGLT2 inhibitor.)

Your registered dietitian or certified diabetes care and education specialist (CDCES) is often the best first contact for supplement questions, since they bridge nutrition science and diabetes management in ways that a prescriber visit may not fully cover.

Evidence Summary and Honest Gaps

The data supporting glycine's glucose effects is real but limited. Most trials are small, short, and conducted in adults with metabolic syndrome rather than women specifically on basal insulin. No randomized controlled trial has examined glycine supplementation in women on insulin degludec. Women have been historically under-represented in metabolic supplement trials, and sex-disaggregated data on glycine's glycemic effects are absent from the published literature. What is stated here about mechanism is reasonably well-supported. What is stated about the magnitude and clinical course of any interaction is extrapolated from adjacent evidence, and you should treat it as a starting point for monitoring rather than a prediction.

The sleep benefit of glycine at 3 g before bed has the strongest trial evidence, from the 2012 crossover study in Sleep and Biological Rhythms. The collagen synthesis benefit at higher doses has supporting mechanistic and short-term trial data. The metabolic effect is the most relevant to women on Tresiba and is documented but modest in magnitude.

If you are already taking both glycine and Tresiba without incident, that is useful clinical information. It suggests the interaction, if any, has not been dramatic for you. Continue monitoring and keep your care team informed.