Dark chocolate is good for your eyes, suggests new research.
Bars rich in cacao improves vision compared to milk chocolate that has lower cacao content, according to the study.
Within two hours of eating a bar with 72 per cent cacao, researchers found contrast sensitivity and visual acuity were 'significantly higher' than in those who had a milk chocolate bar.
But how long the improvements last needed further study but the findings suggest a 47g bar improved blood flow to the eyes helping with vision.
Study author Prof Jeff Rabin said: "Several studies suggest that dark chocolate from favanol rich cacao beans may enhance blood flow to central and peripheral nervous systems, improve cardiovascular function, and retard memory loss and other signs and symptoms of degenerative diseases, including Alzheimer and Parkinson diseases.
"The cacao flavanols in dark chocolate have antioxidant effects that retard and partially reverse degenerative changes in various diseases.
"Dark chocolate consumption also has been associated with enhanced mood andcognition.
"Short-term improvement in contrast sensitivity was observed after consumption of dark chocolate and attributed to increased blood flow, although direct evidence is lacking.
"Moreover, the effect of dark chocolate on other visual functions remains unknown."
So scientists at the University of the Incarnate Word Rosenberg School of Optometry in San Antonio enrolled 30 participants to test multiple aspects of vision, including contrast and colour perception, distraction effects, marksmanship, and visual electrodiagnosis.
The nine men and 21 women with a mean age of 26 were given a 47g Trader Joe's 72% Cacao Dark Chocolate bar and a 40g Trader Joe's Crisp Rice Milk Chocolate bar in separate sessions at least 72 hours apart.
The dark chocolate had eight times more flavanols than the milk chocolate bar.
Visual acuity and the ability to read letters of different sizes and contrast - lighter versus darker letters- were measured about two hours after eating the chocolate.
Prof Rabin said: "These results show that high- and low-contrast vision can be improved within two hours after consumption of a commercially available dark chocolate bar.
"Although the specific mechanism for visual improvement awaits further study, an increase in retinal, visual pathway, and/or cerebral blood flow could be contributory, enhancing bioavailability of oxygen and nutrients to metabolically active sites.
"Dietary bioavailability of flavonoids is influenced by coingestion of other food products, susceptibility to oxidation, and a short half-life, with plasma clearance occurring in as little as four hours.
"However, the exact duration of any improvement in vision performance was not determined in this study."
He added: "Enhancements in visual acuity and large-letter contrast sensitivity after dark chocolate ingestion were small, and the functional relevance is unclear.
"Additional testing may reveal utility in these effects.
"However, a more substantial improvement occurred in small-letter contrast sensitivity, a visually challenging task that can reveal decrements and increments in visual performance despite normal high-contrast visual acuity.
"The highly vascularised 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 improvesvisibility of small, low-contrast targets within two hours compared with milk chocolate, but the duration of this difference and clinical relevance remains uncertain."
The study was published in JAMA Ophthalmology.