The Endothelial Glycocalyx

Picture the inside of an artery as a smooth pipe and you have the wrong image. The luminal surface of every endothelial cell is coated in a hydrated, carbohydrate-rich mesh — proteoglycans anchored in the cell membrane (syndecans and glypicans) carrying long chains of heparan sulfate and chondroitin sulfate, woven together with hyaluronan, and loaded with adsorbed plasma proteins including albumin, antithrombin III and extracellular superoxide dismutase.^[1] It is thin enough to have been dismissed for decades as a fixation artefact and substantial enough that, in one human study, its total systemic volume was estimated at roughly 1.5 litres in healthy adults — a plasma compartment most physicians have never accounted for.^[3]

It does four jobs. It is the real permeability barrier, which forced a revision of Starling's century-old model of fluid exchange — the Michel–Weinbaum formulation, in which the oncotic gradient that matters sits across the glycocalyx, not across the whole vessel wall.^[1] It is anti-adhesive, physically burying the adhesion molecules that leukocytes need to grab. It maintains an anti-thrombotic surface by holding antithrombin III and other regulators in place. And — the function that makes it interesting to anyone thinking about arteries and healthy longevity — it is the mechanosensor for blood flow.

This is the cleanest experiment in the field, and it is more than twenty years old. Endothelial cells exposed to steady shear stress produce nitric oxide; that is the basic reflex behind flow-mediated dilation and much of what we call endothelial function. When Florian and colleagues stripped heparan sulfate — the dominant glycosaminoglycan of the glycocalyx — off cultured endothelial cells with heparinase III, shear-induced nitric oxide production was abolished completely, at both steady (20 dyne/cm²) and oscillatory shear.^[2] The cells were not poisoned: add bradykinin, which stimulates nitric oxide through a different route, and they produced it normally.

So the glycocalyx is not a coating on the flow sensor. It is the flow sensor. Lose it and the vessel stops responding to the signal that keeps it dilated and quiet. That single result is why glycocalyx biology gets taken seriously, and it is also the hook every supplement in this category hangs its marketing on.

Chronic hyperglycaemia is the best-documented human insult. In a study of type 1 diabetes, systemic glycocalyx volume fell in a stepwise fashion: 1.5 ± 0.1 L in matched controls, 0.8 ± 0.4 L in patients without microalbuminuria, and 0.2 ± 0.1 L in those with it.^[3] Sublingual capillary glycocalyx measured 0.5 ± 0.1 µm in diabetes versus 0.9 ± 0.1 µm in controls, and plasma hyaluronan and hyaluronidase were both elevated — the layer was being actively shed, not passively worn away.

Acute insults do it faster. During coronary artery bypass grafting, the perfused boundary region — an inverse measure of glycocalyx thickness — rose from 1.81 ± 0.21 µm after induction of anaesthesia to 2.27 ± 0.25 µm within minutes of starting cardiopulmonary bypass (p < 0.0001), alongside rises in circulating syndecan-1, hyaluronan, heparan sulfate, angiopoietin-2 and thrombomodulin.^[4] And it thins with age: in adults averaging 66 years, perfused boundary region was 2.11 ± 0.14 µm in postmenopausal women versus 1.97 ± 0.13 µm in men, with body mass index a stronger predictor than sex in multivariate analysis.^[5]

Almost all human glycocalyx data comes from one technique: sidestream dark-field video microscopy of the vessels under the tongue, processed by software (GlycoCheck) that computes the perfused boundary region, or PBR — essentially how far red blood cells stray toward the vessel wall. A thick glycocalyx keeps them in the middle of the lumen, so a higher PBR means a thinner glycocalyx. It is an inference, not a photograph.

That inference is imperfect, and the CABG study said so plainly: PBR correlated only moderately with heparan sulfate, E-selectin and thrombomodulin, and only weakly with syndecan-1, hyaluronan and Tie-2. The authors concluded that no single marker captures the glycocalyx in its full complexity.^[4] Keep that in mind when a trial's headline result is a change in PBR alone.

There is a second thing to keep in mind. The physiologist who developed the sublingual PBR platform appears as an author on several of the positive supplement trials, with affiliations listed as MicroVascular Health Solutions LLC and the GlycoCalyx Research Institute — the commercial entities behind both the measurement device and one of the supplements being measured.^[13][14][15] That is disclosed, and disclosure is not misconduct. But it is exactly the situation in which an independent replication carries disproportionate weight, and we will come to the one that exists.

Arterosil HP, sold by Calroy Health Sciences, is built around MonitumRS, described on the product page as “a patented extract of rhamnan sulfate from a rare green seaweed sustainably harvested in the Pacific Ocean” — the alga is Monostroma nitidum. The dose is two capsules daily; a bottle is a 30-day supply and lists at $99. The central claim is that MonitumRS is “scientifically shown to protect and restore the endothelial glycocalyx.”^[16]

Rhamnan sulfate is a sulfated polysaccharide — structurally in the same family as heparan sulfate, which is the point of the whole idea. The literature characterises it as having antiviral, anticoagulant and antitumour activity in laboratory systems.^[9] So the mechanism is not absurd. The question is what the word “shown” is doing in that sentence.

In cell culture, it is genuinely impressive. Rhamnan sulfate enhanced endothelial barrier function, prevented LDL deposition, and maintained that barrier even in the presence of glycocalyx-degrading enzymes. It bound FGF-2, PDGF-BB and NF-κB subunits with high affinity and potently inhibited TNF-α-driven NF-κB activation in endothelial cells.^[6]

In mice, it does something real. In the same paper, ApoE-deficient mice fed a high-fat diet for four weeks and then nine more weeks with or without oral rhamnan sulfate showed reduced vascular inflammation and less atherosclerosis — with a notably stronger effect in females, including a fall in plasma cholesterol seen in female but not male mice.^[6] In a lipopolysaccharide model, 1 mg of oral rhamnan sulfate daily prevented pulmonary vascular leak, suppressed neutrophil infiltration into lung, liver and jejunum, and preserved lung glycocalyx and syndecan-4, which were lost in untreated animals.^[7] And in a 2026 cecal-ligation-and-puncture sepsis model imaged by electron microscopy, oral rhamnan sulfate restored glycocalyx continuity by 72 hours and raised seven-day survival to 55% versus 25% in controls.^[8]

In humans, there is one report, and it is six patients. Six people already enrolled in a functional-medicine cardiology prevention programme, who had carotid plaque that was not improving, added a rhamnan sulfate supplement; six months later, carotid intima-media thickness ultrasound showed reduced total plaque burden in every case, with a mean reduction of 5.55 mm.^[10] There was no control group, no randomisation, no blinding, and the report appeared in an integrative-medicine journal rather than a cardiology one.

It is worth being precise about why that design cannot answer the question. With six patients, no comparator and an operator-dependent ultrasound measure, there is no way to separate a drug effect from regression to the mean, measurement drift, or the continued effect of the drug therapy, supplements and lifestyle changes those patients were already on. A case series is a reason to run a trial. It is not a trial.

And a trial was run — but we cannot see it. A 15-participant study at the University of Texas at Austin, “Seaweed-derived Rhamnan Sulfate and Vascular Function,” tested three weeks of supplementation on flow-mediated dilation in middle-aged and older adults with at least one cardiovascular risk factor. It is registered as completed in February 2025. As of September 2026 no results have been posted to the registry and no publication appears in PubMed.^[11]

That is why a vascular supplement should be judged by its own human evidence — the same standard applied in our review of vitamin K2 and heart health — rather than by a plausible mechanism alone.

No randomised trial of rhamnan sulfate has been published. But a different glycocalyx supplement — Endocalyx Pro, a blend of glucosamine sulfate, fucoidan and hyaluronic acid — has been randomised three times, and the pattern across those three trials is the most informative thing in this whole field.

The two positive trials both include industry-affiliated authors. In 56 South-Asian Surinamese patients with type 2 diabetes in the Netherlands, three months of glycocalyx supplementation improved PBR and the overall microvascular health score and reduced urinary MCP-1, with the effect persisting at three-month follow-up.^[13] (Usefully, the same trial's fasting-mimicking-diet arm worsened PBR despite improving BMI and HbA1c — a reminder that these measures do not simply track general metabolic health.) In 57 patients recovering from mild-to-moderate COVID-19, four months of supplementation produced larger improvements than placebo across the board: PBR −6.8% versus −1.3%, pulse wave velocity −13.2% versus −3%, augmentation index −28.5% versus −2.5%, malondialdehyde −26% versus −2.9%, protein carbonyls −31.3% versus −1%, and coronary flow reserve +12.9% versus +1.6%, all p < 0.05.^[14]

The one trial with no supplement-company author on it was null. At the University of Iowa, 23 adults aged 66 ± 7 years were randomised double-blind to 12 weeks of Endocalyx Pro at 3,712 mg/day or placebo. Between groups, the supplement did not increase glycocalyx thickness in microvessels 4–25 µm (p = 0.33), 4–7 µm (p = 0.07) or 10–25 µm (p = 0.47), and did not improve flow-mediated dilation (mean ratio 1.16, 95% CI 0.77–1.74; p = 0.48).^[12] Two secondary findings were positive — flow-mediated dilation normalised to shear-rate area under the curve (mean ratio 2.41, 95% CI 1.14–4.13; p = 0.001) and capillary glycocalyx thickness in the subgroup not taking antihypertensives — and the authors are careful to call it a pilot and say further investigation is warranted.

Three qualifications, in fairness. Twenty-three people is small, and a null result in 23 people is not proof of absence. The positive trials used longer durations and sicker populations, where there may be more room to improve. And the disclosed affiliations do not make those results wrong.

But two things remain true. The only trial run without the supplement industry in the author list is the only one that failed its primary endpoints. And none of these trials tested Arterosil. Endocalyx is glucosamine sulfate and fucoidan; Arterosil is rhamnan sulfate. They are different molecules from different organisms. Citing one product's randomised data in support of another is the single most common sleight of hand in this category, and it should not survive contact with a reader who checks.

Two placebo-controlled phase 2 trials of glycocalyx supplementation are underway at Amsterdam UMC — one in treatment-resistant hypertension and one in chronic heart failure, 64 participants each, with primary completion dates in 2026 and 2027.^[17] Those will help. What would settle it for Arterosil specifically is a randomised, placebo-controlled trial of rhamnan sulfate with a pre-registered hard endpoint — flow-mediated dilation at minimum, plaque volume or events ideally. One small trial has been completed and not reported. That is the state of the art.