Bioavailability of 9-cis isomers and their role in dark adaptation

This study investigates the profound effect that the stereochemistry of beta-carotene has on retinal absorption and subsequent visual performance, particularly the speed of dark adaptation.

Methodology

A double-blind, placebo-controlled crossover trial was conducted over 16 weeks with 40 healthy volunteers.

Participants were divided into three groups:

1. Placebo (MCT oil)
2. Synthetic All-trans beta-carotene (15mg/day)
3. Natural Dunaliella salina extract containing an isometric mix, predominantly 9-cis beta-carotene (15mg total carotenoids/day)

Key Findings

1. Plasma Kinetics: The 9-cis isomer exhibited a significantly prolonged half-life in plasma compared to the all-trans isomer.
2. Macular Pigment Optical Density (MPOD): The natural extract group showed a 12% increase in MPOD from baseline, whereas the synthetic group showed a negligible increase of 2%.
3. Dark Adaptation Recovery: When subjected to a standard rod-bleach protocol, the natural extract group recovered baseline scotopic sensitivity an average of 4.5 minutes faster than the placebo group.

Conclusion

The superior bioavailability of the 9-cis stereoisomer makes it a highly effective candidate for early nutritional intervention to support visual health, specifically in its role as a precursor to 11-cis-retinal in the visual cycle.