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This is a summary and explanation of:
Sokolov et al. (2013). Chocolate and the brain: Neurobiological impact of cocoa flavanols on cognition and behavior. Neuroscience & Biobehavioral Reviews, 37 (10), 2445-2453.
The following information is a very brief overview of many findings. This paper presents some good insight into how effects of cocoa flavanols on the brain are currently being studied, and reviews an array of papers on the issue. A more detailed look into the studies mentioned, with references, can be found in the bibliography of the resource above.
Keep in mind, many of these studies observed benefits from ingesting "high-flavanol" cocoa. The levels exceed what you would normally find in even the darkest chocolate bar on store shelves today. However, it's still interesting to see how the flavanols contained in chocolate and cocoa interact with the body.
What are flavonoids?
Let's begin with the term antioxidants, which are an array of molecules found in foods that are currently being researched and debated. The reason they are such a hot topic is that antioxidants may fight disease and aging. There are many sources that will explain what an antioxidant is in more detail, but here I'll tell you how antioxidants and cocoa flavanols fit into what will be looked at here.
Antioxidant is an umbrella term for many different kinds of molecules. In this case, we are discussing polyphenols, which is a term given to the tannins found in cocoa. Tannins are also found in tea, coffee, red wine, and give the characteristic dark colour and dry feeling in our mouth. More specifically though, we are talking here about flavonoids, a category of polyphenols. Even more specifically, we are talking about flavanols (not to be confused with flavonols), a subgroup of flavonoids. Flavanols are found in many foods, but in regards to cacao we are concerned with two specific molecules we will be dealing with, namely catechin and epicatechin.
To summarise, Epicatechin and catechin are flavanols, which belong to a group called flavonoids. Flavonoids fall under the category of polyphenols, which are (not always, but in this case) antioxidants.
Dark chocolate contains high levels of flavanols. Cocoa beans are composed of up to 20% flavonoids, flavanols being the most common. There have been many more studies that appear to link cardiovascular health with the flavanols found in chocolate. Here, Sokolov et al. review many studies associated with how flavanols in chocolate may be beneficial to cognition, memory, and overall brain health. This area of research has promise, but not enough work has been completed yet to confirm or fully explain many of these benefits.
How do flavanols impact the brain?
Sokolov et al. explains how flavanols act on the brain in 2 ways:
1. Flavanols can pass through the blood-brain barrier (as seen in animal studies) and interact directly with the neuron cells of the brain. They trigger gene expression and protein synthesis in these neuron cells, which help the neuron create proteins that either a) protect neurons or b) help communicate with other neurons. This in turn promotes: new brain cell growth, and neuron function & brain connectivity.
2. Flavanols improve blood-flow and promote new blood vessel growth in the brain and sensory system. Flavonoids help produce nitric oxide, which inhibits inflammation caused by atheromatous plaque adhesion molecules. As well, this improved blood flow supplies more oxygen and glucose to the neurons, and removes wastes products from the brain.
Epicatechin, a flavanol found in chocolate, is detectable in the blood 30 minutes after intake, peaks 2-3 hours after intake, and begins to decrease over few more hours after peaking. Sokolov et al. suggest that if flavanols can indeed penetrate the blood-brain barrier, and accumulate in the brain regions related to learning and memory, it could have a positive effect on cognition.
The Research Reviews
1.0 Neuroprotective action of cocoa flavanols in aging and neurological disease.
This section looks at how cocoa flavanols protect the brain in aging individuals, and those with diseases such as Alzheimer's, dementia, Parkinson's, and stroke. These diseases are associated with lower levels of blood flow to the brain, and cacao flavanols have been shown to improve this blood flow.
1.1 Rozan et al., (2007) fed rats ACTICOA powder, which is specifically manufactured cocoa with very high levels of flavanols, much greater than you would find through more traditionally manufactured chocolate and cocoa. The rats fed ACTICOA had reduced free radical production after heat exposure, improved cognitive abilities, and better able to reach the goal in a Morris water maze. Bisson et al., (2008) had similar results on cognition with aged rats and ACTICOA.
1.2 Fernandez-Fernandez et al. (2012) worked with mice who were bred to have the human equivalent of the human Alzheimer's disease. They fed them the LMN diet, a diet high in polyunsaturated fatty acids and polyphenols from dried fruits and cocoa). Once the amyloid plaques formed (which are proteins that build up around the neurons, blocking and preventing cells in the brain from communicating), they were fed this diet for 5 months, and began being tested after 3 months of the LMN diet. They saw reversed behaviour effects of the aging and "Alzheimer's" mice. When the mice were fed the diet before the plaques developed, they noticed a decrease in the peptides (Amyloid beta, AB, which are main components of amyloid plaques), which suggests delayed amyloid plaque formation.
1.3 Arendash et al. (2007) also fed mice a polyphenol and omega-3 rich diet, but didn't see the "Alzheimer's" mice benefit, except for some behavioural actions such as maze entries and open field activity.
1.4 Nurk et al. (2009) looked at cognitive performance and flavonoid intake from chocolate, wine and tea on elderly Norwegians. Chocolate, wine or tea consumers had significantly better test scores, and lower prevalence of poor cognitive performance. Individuals who consumed all three foods had the best test scores and the lowest risk of poor test performance. The results were also dose-dependant, with the effect most pronounced for wine, and then chocolate.
1.5 Letenneur et al. (2007) did a 10 year study on flavonoid intake from multiple foods, and found better cognitive performance and better evolution of performance over time, especially in those whose intake of flavonoids was greatest.
1.6 Sorond et al. (2008) noticed an increase in mean blood flow velocity in the middle cerebral artery by 8% in the 1st week and 10% in the 2nd week after consuming high-flavanol cocoa. This increased blood velocity can in turn benefit patients suffering from dementia and stroke, where lack of sufficient blood flow leads to such conditions. Flavanols act directly on the walls of the blood vessels in the brain, allowing the blood vessels to vasodilate, improving oxygen delivery to surrounding tissues.
There isn't any proven association between intake of antioxidants and Alzheimer's disease. However, we know the breakdown of blood flow and oxygen delivery throughout the brain, leads to mild cognitive impairment, which may then lead to Alzheimer's. Since flavanols interact directly with the blood vessels of the brain, and can pass the blood-brain barrier and interact with neurons, they may slow down the transition from mild cognitive impairment to Alzheimer's, but more research needs to be done here.
1.7 Commenges et al. (2000) did a trail with 1367 patients 65 and older, where 66 developed dementia. Although the flavonoids they consumed were not from cocoa, their results showed that antioxidant flavonoid intake was inversely related to risk of dementia.
1.8 Desideri et al. (2012) looked at individuals with mild cognitive impairment, who consumed either 990 mg, 520 mg, or 45 mg of cocoa flavanols. Those who consumed 990 mg and 520 mg required less time during their cognitive tests. Verbal fluency was significantly better for the 990 mg group than the 45 mg group. The 990 mg and 520 mg groups also exhibited decreased insulin resistance, blood pressure, and lipid peroxidation (peroxidation is the breakdown of the lipids that make up the cell membrane).
1.9 Buijsse et al. (2010) found a strong inverse correlation between chocolate consumption and stroke risk.
1.10Rautiainen et al. (2012) found dietary antioxidants had a 17% stroke risk reduction in disease-free women, and a 45% risk reduction of hemorrhagic stroke in women with a history of disease.
2.0 Neuromodulation of cognition, mood, learning, and memory.
2.1 Crews et al. (2008) saw a higher pulse rate on older adults who ate 27g of dark chocolate daily for 6 weeks. No effects on blood pressure or cognitive variables were observed.
2.2 Scholey et al. (2010) fed healthy young adults high-flavanol drinks (994 mg and 520 mg), and after performing cognitive and rapid visual processing tests, noticed better performance on both tests on those who took high-flavanol drinks as opposed to the control.
2.3 Field et al. fed 35 g of high-flavanol (720 mg) chocolate to healthy young adults, and observed improved visual contrast sensitivity (reading numbers with varying luminance), working memory for location choice reaction time, and the time required to detect direction of coherent motion.
2.4 Pase et al. (2013) had individuals take ACTICOA (high-flavanol) dark chocolate daily for 30 days. Cognition appeared to be unaffected, but after 30 days, the high-dose group (500 mg) self-rated themselves with improved calmness compared to the placebo. This suggests cocoa flavanols may have some positive effects on individuals suffering from clinical anxiety and depression.
2.5 Francis et al. (2006) observed a single acute dose of 450 mg of cocoa flavanol increased cerebral blood flow, again suggesting potential treatment of vascular impairment such as dementia and stroke.
It's important to note that flavanol content of chocolate and cocoa vary to a great degree depending on the variety of cacao bean, the origin of where it grew, and how the chocolate or cocoa was processed and manufactured. This variability in flavanol content may be responsible for the mixed outcomes of studies.
We can see that cocoa flavanols can have an array of effects on the brain, blood vessels, and behaviours associated with cognition. For this reason there is promise that cocoa flavanols, whether at natural levels or manufactured high-flavanol levels, may have benefits to diseases associated with the breakdown of neuron and blood vessel systems in the body.
As well, how we take chocolate (consuming dark chocolate with milk results in reduced antioxidant capacity), and other constituents of cocoa (such as tryptophan), also affect to what degree we benefit from cocoa flavanols.