Reports that dark chocolate 'improves eyesight' are ...

"Dark chocolate improves your eyesight," is the unusual headline from the Mail Online following a small trial comparing the effects of dark and milk chocolate on vision. The theory is that dark chocolate is high in antioxidant flavanols, which are touted as having many potential health benefits, including effects on the nervous system.

The US study included 30 adults who were given a bar of either dark or milk chocolate and then had their vision tested on a standard chart. The same 30 adults repeated the test 3 days later, eating the alternative bar of chocolate.

Overall, they found that vision was slightly better than normal after eating the dark chocolate: people could read at 20 metres what someone of normal vision could read at only 12 metres (20/12 vision). And after eating milk chocolate, their vision was 20/15 – still better than normal, but not quite as sharp as with dark chocolate.

Whether these minor differences between milk and dark chocolate would make any difference at all in real life, and whether they would be sustained with long-term consumption, is unclear.

There's also the question of whether these findings would be verified in other studies. It could be just a chance finding, given that it was such a small group.

As it stands, there isn't good evidence that dark chocolate will improve your eyesight.

Where does the study come from?

The study was conducted by researchers from University of the Incarnate Word Rosenberg School of Optometry in San Antonio, Texas. No sources of funding are reported and the authors declare no conflict of interest. The study was published in the peer reviewed journal JAMA Ophthalmology and is freely available to access online.

The Mail article reports that 70% of people had improved vision, but only later in the article does it reveal this was a study in 30 people who were tested 2 hours after eating. They also say it "increases blood flow that sharpens your ability to read numbers and words", which hasn't been shown here.

What kind of research was this?

This was a randomised crossover trial aiming to investigate whether dark chocolate could have any effects on vison compared with milk chocolate.

The crossover design means that people act as their own controls, receiving one intervention then the other in random order. It's often used to boost numbers for comparison, but this is still a small trial, consisting of only 30 people. It also only looked at immediate effects in the 2 hours following consumption of chocolate, so we certainly can't say that regularly consuming chocolate would improve your vision.

What did the researchers do?

The study recruited 30 healthy adults from Texas, US, who were paid to participate in the study. Two-thirds were women, with an average age of 26 years.

They were randomised to consume either a:

47g bar of dark chocolate (72%) containing 316mg of flavanols
40g crispy rice milk chocolate bar containing 40mg flavanols (8 times fewer flavanols than dark)

Almost 2 hours later they had an eye examination to measure their visual acuity (clarity of vision) on a standard wall chart, scoring the number of letters read correctly on different lines. Their vision was corrected for their latest prescription, so they were allowed to wear their glasses, for example.

After a 72-hour period they repeated the assessment after consuming the other chocolate bar.

Participants had been told the purpose of the study was to assess the possible effects of chocolate on vision, though hadn't been specifically told it was to see if the dark performed better than the milk. Assessors were aware of group assignment.

What were the basic results?

The researchers found that visual acuity was slightly better after eating the dark chocolate compared with eating the milk chocolate. Participants had an average visual acuity of about 20/12 after eating dark chocolate, meaning they read at a distance of 20 metres what a person of normal vision could read at 12 metres. This compared with a score of 20/15 after milk chocolate, which is still better than average but not quite as sharp as with dark chocolate.

Overall, 24 of the 30 participants had shown some improvement to visual acuity scores after eating dark chocolate.

What do the researchers conclude?

The researchers conclude: "Contrast sensitivity and visual acuity were significantly higher 2 hours after consumption of a dark chocolate bar compared with a milk chocolate bar" but they do rightly caution that "the duration of these effects and their influence in real-world performance await further testing."

Conclusions

This trial is of passing interest but there's little that can be concluded from it.

It's a small sample of adults from Texas. They consumed 2 bars of chocolate on 2 occasions and had their vision tested 2 hours later. Their visual acuity was marginally better after eating dark chocolate, when compared to milk chocolate, but there are limitations to bear in mind.

First, it's possible that this was a chance finding, given that the group was so small. The participants didn't know the exact purpose of the study, so they shouldn't have been "trying harder" after the dark bar. But the assessors did know, which allows the possibility that they could have been biasing the results towards the dark and assessing more favourably.

Also, the difference was small, and the participants had better-than-normal vision after eating either bar. They were reading at 20 metres what a person of normal vision could read at 15 after the milk and 12 after the dark. But whether this would translate into any noticeable real-life long-term difference is up for debate.

It is also worth noting these are only immediate effects. It's not known if it would make a real difference if the person were to continue eating dark chocolate on a regular basis.

Chocolate can form part of a healthy diet – but it's high in sugar and calories, so best eaten in moderation.