In June 2018 researchers from the USA published the results of their study to assess the short-term effects of dark chocolate and milk chocolate on visual acuity and contrast sensitivity. Thirty participants, average age 26 years, with no eye disease each consumed a dark and milk chocolate bar in separate sessions, with visual acuity and large- and small-letter contrast sensitivity being measured 1.75 hours after each consumption. Results showed that both dark and milk chocolate improved visual acuity and contrast sensitivity. However, when a comparison was made between dark and milk chocolate consumption, dark chocolate was significantly better at improving small-letter contrast sensitivity but only slightly better at improving large-letter contrast sensitivity.
Rabin JC et al. Effects of Milk vs Dark Chocolate Consumption on Visual Acuity and Contrast Sensitivity Within 2 Hours: A Randomized Clinical Trial. JAMA Ophthalmol. 2018 Jun 1;136(6):678-681