In recent years, there has been an increasing interest towards the beneficial health properties of chocolate (specifically dark chocolate and cocoa). The benefits are rather numerous actually, ranging from enhanced glucose metabolism, increased fatty acid oxidation, decreased cortisol levels, cardioprotective properties, improved platelet functions, and others.[1,2]
That being said, it should be clarified that this article is not about to tell you that eating a Twix bar or some other sugar-laden chocolate candy bar is a great way to reap the benefits of dark chocolate; if you’re getting your hopes up for an excuse to raid the candy aisle on your next trip to the grocery store, you won’t find it here.
Nevertheless, eating dark chocolate has its merits, it just needs to be the right type of dark chocolate. It appears that the benefits associated with dark chocolate consumption stem largely from the cocoa content (which is a rich source of polyphenols). Let’s take a deeper look at what polyphenols are, what scientific research says about the specific polyphenols found in dark chocolate/cocoa, and how they can serve to better your physique and overall health.
Polyphenols represent a subclass of organic (carbon-based) molecules that are found primarily in plants (i.e. they’re phytochemicals), and they may be more specifically categorized as lignans, tannins, flavonoids, and stilbenes.
Their distinguishing characteristic from other organic molecules is the presence of multiple phenolic groups, thus the name polyphenol. Moreover, certain polyphenols may exhibit antioxidant properties in humans by neutralizing free radicals and fighting inflammation.
Theaflavin-3-gallate: A specific species of polyphenol found in black tea
With that rudimentary understanding of what polyphenols are on a molecular level, let’s move onto the primary topic at hand–that being how they actually benefit us. As aforementioned, the health benefits derived from polyphenols in dark chocolate are rather ubiquitous.
For starters, research seems to suggest there is a significant reduction in cardiac-associated disturbances in individuals who ingest more dark chocolate/cocoa vs. those with limited chocolate/cocoa intake (see figure 1). 
Figure 1: Relative risks for cardiovascular disease, heart failure, and stroke in adults with higher levels of chocolate consumption compared with lower levels
The above findings may be tied into the improvement of the lipid profile of individuals who consume polyphenol-rich chocolate, as was found in the Mellor et. al study.  The study found that as the polyphenol content of chocolate increases, so does the change in high-density lipoprotein (HDL) cholesterol.
Furthermore, the same study showed that low-density lipoprotein (LDL) cholesterol - also referred to as “bad” cholesterol - decreased by nearly 10% in subjects consuming polyphenol-rich chocolate. However, the change in other lipid biomarkers was not significant.
Another curious finding in a 2010 study by Allgrove et. al was the enhanced mobilization of free fatty acids during exercise in individuals who consumed polyphenol-rich dark chocolate vs. a polyphenol-less control chocolate bar (see figure 2).
Figure 2: Free-fatty-acid (FFA) concentration before and after exercise, M (SD) DC =
dark chocolate; CON = control; Pre-Ex = preexercise; Post-Ex = postexercise; Post-Exh = postexhaustion.
Lastly, a study by Fyfe et. al in 2009 found a rather significant lowering of fasting blood glucose levels in overweight individuals treated with both 500mg and 1000mg doses of polyphenols derived from dark chocolate (see figure 3).
Figure 3: Capillary fasting glucose (FG) levels at baseline (week 0), and at the end of 1 and 2 weeks of each of the polyphenols doses. Group 1 received 1000 mg polyphenols dark chocolate (weeks 1–2) followed by 500 mg polyphenols dark chocolate (weeks 4–5). Group 2 received 500 mg polyphenols dark chocolate (weeks 1–2) followed by 1000 mg polyphenols dark chocolate (weeks 4–5).
Note that there appears to be a dose saturation effect around 500mg, given that the sequence of administration in each group didn’t change fasting glucose levels. Read on to learn just how much dark chocolate/cocoa you should ingest per day given this data.
The studies cited herein generally treated individuals with enough dark chocolate/cocoa to provide roughly 500-1000mg of polyphenols. The amount of chocolate needed to reach said amounts of polyphenols will vary based on it’s inherent cocoa content.
For example, if you look at many chocolate bars in the supermarket they will have a “X% cocoa” notation on the label. Essentially, the higher the percentage of cocoa, the more polyphenol-rich the chocolate is. I tend to be a purist and usually just buy pure cocoa powder since it’s significantly lower in calories than dark chocolate bars (especially those with added sugar, cream, butter, etc.)
For practicality purposes, and to provide a more tangible quantity for the readers, we will need to do some quick math: 
-Baba et. al found that 66g of pure cocoa contained roughly 2730mg of polyphenols, thus, on a per gram basis, pure cocoa powder provides roughly 41-42mg of polyphenols.
So to achieve the nominal polyphenol doses (500-1000mg/day) that most studies utilized, one would need to consume about 12.5 to 25 grams of pure cocoa per day. This is actually a rather small amount of cocoa, and the good news is cocoa powder is cheap and can be utilized in a variety of recipes (or just simply added to certain foods/liquids).
Given the data cited herein, it is likely that the polyphenols found in dark chocolate/cocoa accelerate fat loss by enhancing glucose metabolism, reducing calorie intake, and normalizing cortisol secretion throughout the day. These polyphenols also have significant positive effects on HDL and LDL cholesterol, which is great for cardiovascular function and longevity.
That being said, the research on dark chocolate/cocoa polyphenols is still relatively up-and-coming; the exact species of polyphenols and underlying mechanisms of how they affect our health are subject to further investigation. Nevertheless, don’t overlook the power of this food source for getting leaner and staying healthy.