Vascular Health Glossary
ADMA (Asymmetric Dimethylarginine)
Also: endogenous NOS inhibitor
ADMA is a naturally occurring amino acid that blocks the enzyme endothelial nitric oxide synthase (eNOS). When eNOS is blocked, the endothelium produces less nitric oxide, which reduces the blood vessel's ability to dilate and increases arterial stiffness.
Elevated ADMA is an independent predictor of cardiovascular events. Normal fasting plasma ADMA is below 0.75 micromol/L. Levels above 1.0 micromol/L are associated with measurably reduced flow-mediated dilation (FMD). ADMA is not included in standard lipid panels or annual physicals.
ADMA is cleared primarily by the enzyme DDAH (dimethylarginine dimethylaminohydrolase). Compounds that upregulate DDAH activity or compete with ADMA for eNOS binding are the primary targets of the GRN nitric oxide protocol arm.
Arterial Stiffness
The loss of elasticity in large arteries, particularly the aorta and carotid arteries. Healthy arteries expand when the heart pumps, buffering the pressure wave. Stiff arteries transmit that pressure wave directly to smaller vessels and organs, including the kidneys, brain, and coronary arteries.
Arterial stiffness is measured clinically using pulse wave velocity (PWV). It increases with age and accelerates with elevated blood pressure, inflammation, oxidative stress, and glycocalyx degradation. It is not routinely assessed in primary care.
Citrulline (L-Citrulline)
An amino acid found in high concentrations in watermelon. In the body, L-citrulline is converted to L-arginine in the kidney, bypassing the intestinal and hepatic first-pass metabolism that limits oral L-arginine absorption. This makes L-citrulline a more effective substrate for nitric oxide production than L-arginine supplements.
A 2012 study by Ochiai et al. (Journal of Cardiovascular Pharmacology) found that 5.6g/day of L-citrulline for 8 weeks significantly improved brachial artery flow-mediated dilation in subjects with mild arterial stiffness. The effective range in published literature is 3–6g per day of L-citrulline or L-citrulline malate.
Cluster Protocol
GRN Labs identifies 8 vascular risk clusters based on patterns of co-occurring markers from the vascular age assessment. A cluster groups markers that tend to move together and respond to the same protocol arm. For example, the Blood Pressure and Blood Flow cluster addresses both elevated resting blood pressure and reduced circulation through overlapping mechanisms: glycocalyx repair, nitric oxide production, and magnesium-dependent smooth muscle relaxation.
The cluster approach prevents the protocol from prescribing a generic supplement list. A user with elevated blood pressure but no ED markers receives a different protocol than a user with ED symptoms and normal blood pressure, even if both have similar vascular ages.
Endothelium
A single layer of cells lining the interior of every blood vessel in the body. The endothelium is not passive plumbing. It is an active endocrine organ that produces nitric oxide, regulates inflammation, controls coagulation, and signals to smooth muscle cells to relax or contract.
Endothelial health is the primary target of the GRN protocol. The reason: most of the measurable vascular decline in men between 35 and 60 occurs at the endothelial level before it shows up as structural plaque or as an event detectable on imaging.
Endothelial Dysfunction
The state in which the endothelium is impaired in its ability to produce nitric oxide, regulate inflammation, and respond appropriately to blood flow. It is considered the earliest measurable stage of cardiovascular disease, appearing before arterial plaque, before elevated calcium scores, and before elevated LDL becomes clinically significant.
Endothelial dysfunction is typically measured by flow-mediated dilation (FMD). An FMD below 7% in the brachial artery is generally considered impaired. It is not included in standard annual blood work.
eNOS (Endothelial Nitric Oxide Synthase)
The enzyme within endothelial cells that synthesizes nitric oxide from L-arginine. eNOS activity is the primary regulator of vascular nitric oxide production. It is stimulated by blood flow (shear stress), by compounds that increase L-arginine availability (citrulline, rhamnan sulfate), and by antioxidants that prevent eNOS uncoupling.
eNOS uncoupling occurs when the enzyme loses its cofactor tetrahydrobiopterin (BH4) under oxidative stress, causing it to produce superoxide instead of nitric oxide. This creates a self-reinforcing cycle of inflammation and reduced vasodilation. Reducing oxidative stress is therefore a prerequisite for effective NO augmentation.
FMD (Flow-Mediated Dilation)
A non-invasive ultrasound measurement of endothelial function. A blood pressure cuff is inflated around the upper arm for several minutes to restrict blood flow, then released. The resulting surge in blood flow causes shear stress on the endothelium, which responds by producing nitric oxide and dilating the brachial artery. The percentage by which the artery widens is the FMD score.
FMD is considered the gold standard for assessing endothelial health in research settings. Values above 7% are generally considered normal; values below 4% indicate significant endothelial impairment and correlate with increased cardiovascular event risk in longitudinal studies. A 1% improvement in FMD is associated with a 13% reduction in cardiovascular risk (Inaba et al., 2010, Journal of the American College of Cardiology).
The GRN vascular age calculator uses validated proxies for endothelial function in the absence of direct FMD measurement. The 90-day re-test measures functional change using the same proxy markers.
Glycocalyx
A thin gel-like layer of proteoglycans and glycoproteins coating the inner surface of every blood vessel. The glycocalyx is 0.5 to 4.5 micrometers thick and serves multiple critical functions: it acts as a mechanical barrier that prevents blood cells from adhering to the vessel wall, it regulates how shear stress signals reach endothelial cells to stimulate nitric oxide production, and it controls inflammatory molecule access to the endothelium.
The glycocalyx degrades with age, elevated blood sugar, oxidative stress, and chronic inflammation. Once degraded, it can be partially restored through specific interventions. Rhamnan sulfate, a polysaccharide derived from the marine algae Monostroma nitidum, is the most studied compound for glycocalyx restoration in peer-reviewed literature.
Glycocalyx degradation is not measured in standard blood panels. Products designed to restore it are a relatively recent clinical focus.
Nitric Oxide (NO)
A signaling molecule produced by the endothelium that causes vascular smooth muscle cells to relax, widening blood vessels and lowering blood pressure. Nitric oxide also inhibits platelet aggregation, reduces inflammation, and prevents the adhesion of white blood cells to the vessel wall.
Nitric oxide production declines with age. Men in their 40s and 50s produce roughly 50% less nitric oxide than men in their 20s. This decline contributes to arterial stiffness, elevated blood pressure, and reduced penile blood flow. The vascular mechanism of erectile dysfunction is fundamentally a nitric oxide insufficiency in the penile vasculature.
Nitric oxide has a half-life of seconds in biological tissue. The relevant variable is not nitric oxide itself but the sustained activity of eNOS and the clearance of ADMA, which determines how much NO the endothelium produces continuously.
Pulse Wave Velocity (PWV)
The speed at which a pressure wave travels through the arterial system, measured in meters per second. It is the most widely validated clinical measure of arterial stiffness. A faster pulse wave indicates stiffer arteries. In healthy young adults, aortic PWV is typically below 7 m/s. Values above 10 m/s are associated with significant cardiovascular risk.
PWV increases by approximately 1 m/s per decade in healthy aging and accelerates under conditions of chronic hypertension, metabolic syndrome, and endothelial dysfunction. It can be measured non-invasively using carotid-femoral tonometry or validated wrist-based devices. Unlike FMD, PWV is now included in some cardiovascular risk stratification protocols at preventive cardiology practices.
Pycnogenol
A standardized extract from the bark of the French maritime pine (Pinus pinaster). Pycnogenol contains a mixture of procyanidins, catechins, and phenolic acids that collectively stimulate eNOS activity, reduce ADMA levels, and inhibit the enzyme that breaks down nitric oxide (phosphodiesterase-5).
A 2010 RCT by Enseleit et al. (European Heart Journal) found that Pycnogenol at 200mg/day for 8 weeks improved FMD in subjects with stable coronary artery disease. A 2019 study by Stanislavov et al. demonstrated improved erectile function scores in men with mild-to-moderate ED at 120mg/day combined with L-citrulline. The combination shows synergistic effects on NO bioavailability.
Rhamnan Sulfate
A sulfated polysaccharide extracted from the green alga Monostroma nitidum. It is the most studied compound for glycocalyx restoration in published human and cell-culture research. Rhamnan sulfate mimics the heparan sulfate chains of the native glycocalyx, reattaching to denuded glycocalyx sites on the endothelium.
Research by Nincheri et al. (2021, Marine Drugs) demonstrated that rhamnan sulfate at 100mg/day restored glycocalyx thickness in endothelial cells damaged by inflammatory stimuli. Cardiovascular outcomes data in humans remains in early stages. The GRN protocol includes it in the glycocalyx repair arm for users with markers consistent with endothelial degradation.
Vascular Age
An estimate of the biological age of the arterial system, distinct from chronological age. A 48-year-old man with a vascular age of 58 has arteries that are behaving as if they belong to a 58-year-old, based on measurable functional and risk markers.
Vascular age is calculated using validated risk equations derived from large population studies, most notably the Framingham Heart Study and the SCORE2 cardiovascular risk model. The inputs vary by model but typically include blood pressure, lipid markers, smoking status, diabetic markers, and functional indicators of arterial stiffness.
The GRN vascular age calculator uses 15 markers across multiple domains. Its primary value is not the absolute number but the gap it quantifies between biological and chronological age, and the change in that gap between initial assessment and the 90-day re-test. A narrowing gap is objective evidence that the protocol is working.
Vascular Biomarker
Any measurable indicator of vascular health or arterial aging. Vascular biomarkers fall into two categories: structural (calcium score, intima-media thickness, carotid plaque) and functional (FMD, PWV, ADMA, endothelin-1, nitric oxide metabolites).
Standard annual physicals focus primarily on metabolic biomarkers (LDL, HDL, triglycerides, fasting glucose) that correlate with vascular risk but do not directly measure vascular function. Functional vascular biomarkers provide earlier signal, often 10 to 15 years before structural changes are detectable on imaging.
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