A wacky study in some ways, but the results give another great reason to eat dark chocolate ("It's for my vision") if the results hold up with more research. Unfortunately the milk chocolate used did not have the same beneficial effects for vision (it just had no effect). But still to be determined is how long does the small increase in vision acuity and contrast sensitivity last after eating dark chocolate, and does it really mean anything in the real world? The beneficial results in vision were found about 2 hours after eating the dark chocolate, so it is unknown how long the beneficial effect lasted.
The good news is that in the study 30 healthy adults were randomly assigned to different groups, it was "double blind" (no one knew who got what to eliminate bias), it was rigorously done, they used real chocolate available to everyone (72% Cacao Dark Chocolate bar vs Crispy Rice Milk Chocolate bar - both from Trader Joe's), and the researchers had no ties to industry. The researchers concluded eating a "dark chocolate bar improves the ability to see low- and high-contrast targets, possibly owing to increased blood flow", but they don't know what this means for real life.
As the researchers point out - there were 8 times more flavanols in the dark (316.3 mg) vs milk chocolate bar (40 mg). Their thinking was clear in the study's introduction: "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 and cognition." Wow.
The big question: How much dark chocolate did they eat to get these beneficial effects? Answer: 47 grams (or 1 serving, 280 calories) of the 72% cacao dark chocolate. From Medical Xpress:
It may not replace prescription glasses, but a few bites of dark chocolate might offer a slight and temporary bump up in vision quality, new research suggests. Heart-healthy compounds in chocolate called flavanols appeared to sharpen eyesight for a group of 30 healthy young adults in the new study. The observed change in vision was small, but significant. However, the study authors stressed that it's too early for ophthalmologists to recommend chocolate as medicine for the eyes.