Dark chocolate's good and good for you, a study finds.

Mar 29, 2011

Field, DT, CM Williams and LT Butler. 2011. Consumption of cocoa flavanols results in an acute improvement in visual and cognitive function. Physiology and Behavior http://dx.doi.org/10.1016/j.physbeh.2011.02.013.

Synopsis by Wendy Hessler and Steven Neese


2011-0326 darkchocolate
Boz Bros/flickr



Eating a dark chocolate bar may improve visual and cognitive functions – at least temporarily. Visual sensitivity – the ability to see in difficult conditions – was enhanced up to two hours later in people who ate the chocolate as part of a study. The results highlight the potential health benefits derived from dark chocolate and suggest new avenues of research.




Flavanoids are a class of naturally occurring plant compounds being studied as dietary aids for healthy aging. Several types are found in the human diet, including the flavanols. Grapes, red wine, green tea and cocoa all contain flavanols.

There is increasing interest in the health effects – including heart and brain benefits – associated with flavanols. Exactly how they benefit health is not known. They may act as antioxidants, and they may increase the production of nitric oxide gas (Fraga et al. 2011). Nitric oxide has many actions in the body. It controls blood vessel expansion – called vasodilation – which increases blood flow and decreases blood pressure.

Cocoa beans – the building block of many chocolate treats – are naturally rich in flavanols. In general, darker chocolates contain the highest levels of these compounds. Milk chocolate and other overly processed chocolates have less because the sweetening can strip away up to 90 percent of the flavanols from the bean.

Some research suggests that eating dark chocolate provides positive benefits for the heart and might lower blood pressure (Mehrinfar and Frishman 2008). For instance, some studies find nitric oxide levels increase and blood pressure declines in people after they eat cocoa flavanols.

Few studies have tested cocoa flavanols for cognitive benefits. Those that have find specific short-term effects. In one study, blood flow to the brain increased after drinking a cocoa drink with 150 milligrams of flavanols for five days (Francis et al. 2006). Cocoa did not induce changes in reaction time nor error rate on a behavioral flexibility task in this study. A separate study did find a short-term improvement in the participants' performance of a working memory task and in their reported mental fatigue after a series of tests after they drank a cocoa drink with 520 milligrams of cocoa flavanols (Scholey et al. 2010).

The potential role for cocoa flavanols on vision has not been studied. Supporting evidence from other studies with glaucoma and ginko biloba – which is high in flavanoids – hint that the compound may affect sight by increasing blood flow to the eye.

What did they do?

Researchers from the University of Reading examined the visual and cognitive effects of cocoa flavanols. In the study, 30 18- to 25-year-olds ate a dark chocolate bar (CHOXI+) containing 773 milligrams cocoa flavanols during one test session and a white chocolate bar containing none at a second test session a week later. Two hours after eating the bar at each session, they were tested on visual, memory and reaction time tasks.

One visual task assessed the ability to read as light conditions change – called contrast sensitivity. After participants correctly read a set of numbers on a screen the light contrast was lowered to make the numbers more difficult to read. The contrast was again lowered after each correct response until neither of the digits was read correctly.

Other sets of visual tests used moving dots. Participants identified their direction and their pattern – either horizontal or random.

Cognitive skills were assessed through memory and reaction time tests. For memory, the participants were asked to identify which objects had changed locations from one screen to another. The reaction time task required participants to quickly press one of three keys associated with a number or two letters that flashed on the computer screen.

What did they find?

Two hours after eating the dark chocolate bar, the participants' visual and cognitive abilities improved when compared to those who ate the white chocolate without flavanols.

Dark chocolate eaters were about 17 percent better able to discern objects in contrasting light conditions. They were able to read numbers at lower contrast levels than those eating white chocolate. They also more quickly identified whether a series of dots was moving horizontally or in a random pattern.

For working memory, those who ate a dark chocolate bar correctly identified more of the objects that had switched places than those eating white chocolate. Their times to correctly press a key in response to letters or numbers were also shorter.

What does it mean?

Performance on visual and cognitive tasks improve after eating a single serving of a commercially available dark chocolate that contains a moderate amount of cocoa flavanols.

This is the first study to show that eating dark chocolate can result in improved performance on visual function tests. The study also supports a previous study that documents positive effects on the performance of memory tasks after ingesting cocoa flavanols. These findings also extend these positive effects to a reaction time task.

The effects on vision – as measured through contrast sensitivity – suggest cocoa flavanols have positive effects in some challenging visual conditions, such as working in a low light environment. Older adults often report a decline in contrast sensitivity. The results open up the possibility that flavanols could have a similar effect in older adults.

A short-term enhancement in working memory and reaction time also was reported. These – along with the visual improvements – suggest a general underlying mechanism. Although not measured in this study, the authors suggest the effects may be the result of increased blood flow to the eye and the brain. Cerebral blood flow is increased two hours after consuming cocoa flavanols, but effects on blood flow to the eye are not known.

Can chocolate now be considered a health food? Chocolate is still high in fat and calories. Yet a growing body of research suggests possible positive health effects of eating cocoa and chocolate products. Importantly, one to two ounces of dark chocolate can contain a significant amount of cocoa flavanols – anywhere from 100 to 200 milligrams, but these levels do vary from product to product (Miller et al. 2009).

While the study did not directly look at supplements, the results offer the possibility that cocoa flavanol supplements may be beneficial. Animal studies suggest this as well. Until appropriate research is conducted, the potential health benefits and risks of flavanol supplements remains unknown.

It is important to note that the dark chocolate bars eaten in this study also contained other ingredients – including caffeine – which were not in the white chocolate bars. A previous study showed working memory improved after drinking cocoa with the flavanols when compared to drinks with equal nutritional components – including caffeine – but without the cocoa flavanols (Scholey et al. 2010). Although the current study is only a preliminary one, the potential influence of caffeine on the outcomes of this research cannot be ruled out and need further investigation.

Based on the findings, consuming products containing cocoa flavanols may provide some visual and cognitive benefits. More work is needed to understand the extent of these improvements.

To date, the evidence for health benefits of eating cocoa products is only from short-term and observational studies. Accumulating evidence does show that dark chocolate and other cocoa products may have health benefits, but the question of how little or how much is good for you remains unanswered.


Fraga, CG, MC Litterio, PD Prince, V Calabro, B Piotrkowski and M Galleano. 2011. Cocoa flavanols: effects on vascular nitric oxide and blood pressure. Journal of Clinical Biochemistry and Nutrition 48(1):63-67.

Francis, ST, K Head, PG Morris and IA Macdonald. 2006. The effect of flavanol-rich cocoa on the fMRI response to a cognitive task in healthy young people. Journal of Cardiovascular Pharmacology 47(Supplement 2):S215-S220.

Mehrinfar, R, and WH Frischman. 2008. Flavanol-rich cocoa. Cardiology in Review 16(3):109-115.

Miller, KB, WJ Hurst, N Flannigan, B Ou, CY Lee, N Smith and DA Stuart. 2009. Survey of commercially available chocolate- and cocoa-containing products in the United States. Journal of Agricultural and Food Chemistry 57:9169-9180.

Scholey, AB, SJ French, PJ Morris, DO Kennedy, AL Milne and CF Haskell. 2010. Consumption of cocoa flavanols results in acute improvements in mood and cognitive performance during sustained mental effort. Journal of Psychopharmacology 24(10):1505-1514.




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