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More chocolate please – a single masked randomised controlled crossover clinical trial shows polyphenol flavanols improve both visual acuity and contrast sensitivity

Researchers, based at the Rosenberg School of Optometry, San Antonio, Texas, have shown that contrast sensitivity and visual acuity are significantly higher 2 hours following the consumption of dark chocolate, compared to the consumption of milk chocolate. The key ingredient – polyphenol flavanols – are thought to enhance blood flow and increase metabolic supply to the highly vascularized retina. While the study was small and conducted on participants without any ocular pathology, the application and long-term impact has yet to be shown.


Cacao flavanols found in dark chocolate have been shown to have antioxidant effects, potentially capable of suppressing and partially reversing degenerative changes in a number of diseases, including a number of age-related disorders. A natural compound capable of stabilizing or improving visual acuity in a range of ocular disorders could potentially represent a significant clinical advantage. To test the potential of flavanols in such a role, thirty (30) participants were recruited to a randomised single masked crossover clinical study. Each participant was tested approximately 1.75 hours after ingestion of a 72% cacao dark chocolate source (47 g bar; cacao, 34 g; total flavanols, 316.3 mg) or, ingestion of a milk chocolate source (40 g bar; milk chocolate cocoa, 12.4 g; total flavanols, 40 mg). At least 72 hours after the first ingestion, participants then crossed over to the other treatment. Participants underwent a comprehensive eye examination, high-contrast visual acuity testing, small-letter contrast sensitivity testing and large-letter contrast sensitivity testing.


Results showed that among the 30 participants in the study, small-letter contrast sensitivity was significantly higher after consumption of dark chocolate (mean [SE], 1.45 [0.04] logCS) vs milk chocolate (mean [SE], 1.30 [0.05] logCS; mean improvement, 0.15 logCS [95%CI, 0.08-0.22 logCS]; P < .001), while large-letter contrast sensitivity was slightly higher after consumption of dark chocolate (mean [SE], 2.05 [0.02] logCS) vs milk chocolate (mean [SE], 2.00 [0.02] logCS; mean improvement, 0.05 logCS [95%CI, 0.00-0.10 logCS]; P = .07). In addition, visual acuity improved slightly after consumption of dark chocolate (mean [SE], −0.22 [0.01] logMAR; visual acuity, approximately 20/12) and milk chocolate (mean [SE], −0.18 [0.01] logMAR; visual acuity, approximately 20/15; mean improvement, 0.04 logMAR [95%CI, 0.02-0.06 logMAR]; P = .05). While the authors of the study did not define the specific mechanism of action leading to the observed improvement in visual performance, the research report did speculate that an increase in cerebral blood flow could be a contributory factor leading to an increased bioavailability of oxygen and nutrients. The researchers concluded their study commenting that, “the highly vascularized retina, particularly the macula with its substantial projection to the visual cortex, may be most susceptible to enhanced blood flow and increased metabolic supply afforded by polyphenol flavanols in dark chocolate. The findings reported suggest that a single dose of dark chocolate improves visibility of small, low-contrast targets within 2 hours compared with milk chocolate.”