Some peptides can reach the brain from the circulation under particular conditions, while many cross poorly. There is no reliable rule that all small peptides cross or that all peptides are excluded. Transport must be established for the exact molecule and experimental setting.

Crossing requires more than receptor binding

A peptide may bind to an endothelial receptor, be taken into the cell, be degraded, or be moved across the cell. These are different events. Detecting binding or cellular uptake is not enough to establish delivery into brain tissue.

Insulin illustrates the complexity. Experimental work has investigated saturable transport into the brain, while studies disagree about the precise role of the signaling insulin receptor in that process. A mouse study found transport persisted after endothelial-specific loss of the signaling insulin receptor or pharmacological inhibition of that receptor. This was not removal of every insulin receptor throughout the animal. The result cautions against equating receptor signaling with transport. Rhea and colleagues, 2018

What researchers need to measure

Useful studies distinguish intact peptide from breakdown products and vascular signal from tissue exposure. They examine time, concentration and transport in the relevant direction. Results from cultured cells, rodents and humans should be labeled separately rather than combined into one certainty.

A formulation or route can change exposure, but it does not make delivery automatic. Evidence that one peptide enters the brain cannot establish that another peptide, or a modified commercial version, behaves the same way.

How to evaluate a product claim

Ask for a study of the specific molecule, not merely a general paper about peptide transport. Then ask whether reaching the brain produced a meaningful outcome and what risks were assessed. Transport is necessary for some proposed mechanisms, but it is not proof of effectiveness or safety.

The scientifically accurate answer is therefore compound-specific: sometimes, by particular mechanisms, and only to the extent demonstrated by suitable evidence.