How We Select Research
Every recommendation in the GRN protocol follows the same structure: mechanism first, then a published human study, then the specific recommendation. This page explains how we choose what to include and what to exclude.
The citation-first principle
A recommendation without a mechanism is a guess. A mechanism without a study is speculation. GRN Labs does not publish either. Every protocol item tells you what the compound does at the biological level, names the study that measured it in humans, and then states the specific recommendation that follows from that evidence.
If we cannot name the mechanism, cite a peer-reviewed human study with a measurable endpoint, and state a specific recommendation, the item does not appear in the protocol.
Example: how a protocol entry is structured
Mechanism: L-citrulline is converted to L-arginine in the kidneys, increasing substrate availability for endothelial nitric oxide synthase (eNOS), which produces nitric oxide (NO). Elevated NO induces vasodilation and reduces arterial stiffness by activating soluble guanylate cyclase in vascular smooth muscle.
Study: Ochiai et al. (2012, Journal of Cardiovascular Pharmacology) demonstrated that L-citrulline supplementation at 5.6g/day for 8 weeks significantly improved brachial artery flow-mediated dilation (FMD) in subjects with mild arterial stiffness. FMD improved by 1.5 percentage points versus placebo.
Recommendation: 5g L-citrulline malate, taken in the morning on an empty stomach or before exercise.
Study inclusion criteria
What qualifies
- Peer-reviewed and published in an indexed journal (PubMed, Cochrane, Embase)
- Conducted in adult humans, not animals or cell cultures, unless used to establish mechanism only
- Reports a measurable vascular endpoint: flow-mediated dilation (FMD), pulse wave velocity (PWV), blood pressure, nitric oxide bioavailability, ADMA levels, or a clinically validated functional measure
- Sample size of 20 or more participants for mechanistic studies; 100 or more preferred for dosing claims
- Intervention clearly reported: compound, dose, duration, delivery method
What does not qualify
- Animal studies used as primary evidence for a dosing recommendation
- Industry-funded studies used as sole evidence where the funding source has a commercial interest in the outcome
- Self-reported symptom improvement without an objective vascular measurement
- Studies with follow-up periods under 4 weeks for lifestyle interventions
- Preprint studies that have not completed peer review
How the vascular age model works
The GRN vascular age calculator scores 15 markers across five domains: cardiovascular history, lifestyle inputs, functional symptoms, metabolic indicators, and behavioral patterns. Each marker is weighted according to its correlation with validated vascular aging measurements, specifically FMD and PWV, which are the two most widely studied objective measures of arterial health in clinical research.
The resulting vascular age estimate is not a diagnosis. It is a reference point. Its value is not the number itself but the gap it creates: the difference between chronological age and vascular age is the variable that matters for protocol design and for measuring change at 90 days.
The 90-day re-test standard
The protocol is not complete at the recommendation stage. It completes when the user re-tests at 90 days and the gap has narrowed. This is the verification step that distinguishes the GRN protocol from a report or a supplement recommendation list.
If the re-test shows no improvement, the protocol recalibrates. A protocol that cannot fail the verification test is not a protocol. It is marketing copy.
How the protocol is updated
The GRN protocol is reviewed quarterly. Updates are triggered by new meta-analyses or RCTs published in primary vascular research journals, changes in consensus dosing guidelines, or patterns in user re-test data that suggest a protocol arm is underperforming for a specific cluster type.
When a protocol item is updated, the previous citation and the new citation are both documented in the change log. We do not silently swap out recommendations.
What we do not claim
GRN Labs is a wellness intelligence platform. We do not diagnose, treat, or prescribe. No content on this site is medical advice. The protocol is designed to improve measurable vascular markers in healthy adults. Anyone with a diagnosed cardiovascular condition, taking prescription medications, or managing a chronic illness should consult their physician before starting any new supplement or lifestyle protocol.
See our full Not Medical Advice page.
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