This 2020 sheep study measured red cabbage anthocyanin concentrations:

“Study aim was to determine whether strongly bioactive hydrophilic red cabbage anthocyanins cross the blood-cerebrospinal fluid barrier (blood-CSF barrier) and whether there is a selectivity of this barrier towards these compounds.

The blood-CSF barrier, apart from the vascular blood-brain barrier, is the second important barrier. Despite very tight connections between endothelial cells of blood vessels of the choroid plexus, blood-CSF barrier allows selective passing of substances from blood to CSF, which is considered as a medium actively involved in transport of information to nerve cells.

Uncharged, lipophilic, and small-sized substances (≤ 600 Da) can cross the brain barriers without major obstacles thanks to diffusion. The rate of these substances’ penetration into brain tissue is directly proportional to their lipid solubility, and inversely proportional to particle size. Hydrophilic substances require special carriers.

The average percentage level of native anthocyanins over the whole experiment was almost 39.5%, while their metabolites constituted just over 60.5%. However, the proportion of native forms vs. metabolites did not develop identically:

1. Early term (0.5-4 hrs) was distinguished by native derivatives (> 76%).
2. Second period (4.5 h) had a similar contribution of native anthocyanins (49.85%) and their metabolites (50.15%).
3. Third interval (5.0-10 h) more than 87% of anthocyanins were metabolites.

For comparison, a human experiment showed only one period with maximum blood plasma anthocyanins concentration (2 h) after red cabbage consumption.

Only one of 17 native anthocyanins found in blood plasma was detected in CSF. Eleven of 17 metabolites found in blood were identified in CSF.

Due to their hydrophilic nature and considerable size (≥ 611 Da), there seems to be no possibility to use diffusion for permeation of red cabbage anthocyanins through the blood-CSF barrier. These pigments may pass through this barrier only by the use of special carriers. Other mechanisms of anthocyanins permeation through blood-CSF barrier cannot be eliminated.

Two maximal values of total anthocyanins concentration appeared in both blood and CSF. When the pool of cyanidin compounds available in blood became depleted, the decline of total anthocyanin concentration in CSF was also noted.

Nonacylated cyanidin derivatives penetrated the blood-CSF barrier, but acylated cyanidin derivatives did not. A significantly higher proportion of cyanidin sulfate forms in CSF (31%) compared to blood plasma (9%).

Further targeted studies are needed to determine which paths of permeation via blood-CSF barrier are actually responsible for anthocyanins passing, as well as what mechanisms are present during these processes. In addition, it is worth remembering that low molecular weight compounds formed mainly by colonic microbiota are very important metabolites of anthocyanins, and could be relevant in the context of permeation through brain barriers.”

https://pubs.acs.org/doi/10.1021/acs.jafc.0c03170 “The Blood–Cerebrospinal Fluid Barrier Is Selective for Red Cabbage Anthocyanins and Their Metabolites” (not freely available)

Don’t understand why this study hasn’t been cited even once. These researchers’ methods could be performed with broccoli and other red cabbage compounds.