Propolis comprises of lipids, waxes and resins in a complex matrix with a large molecular weight, contributing to a poor bioavailability and absorption. The form of polyphenol administered (natural fruit, juice or extract) or the presence of multiple polyphenolics support synergistic effects and are important determinants of bioavailability.
Factors believed to contribute to poor bioavailability of polyphenols include digestive instability, poor transcellular efflux in intestinal cells, and rapid metabolism and excretion.
Since dietary polyphenols exist as esters, polymers or in glycosylated forms, they cannot be absorbed and must be hydrolysed by the intestinal enzymes or by the colonic microflora before absorption.
When in the intestinal system, poorly absorbed polyphenolic compounds are converted to smaller phenolic acids with improved bioavailability, aided by enzyme activity of the colonic microbiota.
As microbiota vary between people the inter-individuality in absorption and metabolism is being increasingly considered. Despite poor absorption percentages of bio-accessible phenolic compounds in propolis, the recovered amounts detected in plasma were still high due to their high initial contents compared to other food materials such as fruits and vegetables.
Once the propolis phenols reach the blood stream, their selective permeability across the blood-brain barrier and systemic elimination limit the therapeutic efficacy with regards to optimal brain function.
However, increasing evidence suggests certain propolis derivatives are capable of crossing the blood-brain barrier. Caffeic acid phenethyl ester (CAPE) is hydrolysed to caffeic acid within six hours of reaching the plasma and CAPE has recently been shown to cross the blood brain barrier at least in rats. The degree to which propolis polyphenols and metabolised derivatives are capable of crossing the blood-brain barrier depend on their lipophilicity with less polar polyphenols or metabolites (i.e., O-methylated derivatives) capable of greater brain uptake than the more polar ones (i.e., sulphated and glucuronidated derivatives).
The rate at which polyphenols are excreted in the urine appears to vary considerably between individuals. Those placed on a three-day high phenolic diet demonstrated this variable excretion with one participant excreting 8 of the 17 phenolic acids measured, another all 17. The reasons for individual variability in excretion are generally unknown, but may be related to ageing, renal function or inherent propolis properties. Ongoing research and overall consideration to the general health of consumers of propolis and the effect on renal function is warranted.