Published Jan 15, 2025
Last month, I suddenly saw headlines everywhere proclaiming that “Dark Chocolate Could Help You Ward Off Type 2 Diabetes” or excitedly stating, “How Sweet! A Daily Dose of Dark Chocolate May Cut Your Risk of Diabetes!”. And when even the New York Times asked whether “Dark Chocolate Could Reduce Your Risk of Diabetes?”, I figured I gotta look into this some more.
It turned out that the flood of headlines was all based on a new observational study, so we’ll review that one first and consider it in the context of the entire available evidence to see whether there is any substance to the hype.
Dark Chocolate Intake is Associated With a Lower Risk of Type 2 Diabetes
Early last month, a new paper was published based on three large cohorts: the Nurses Health Study, the Nurses Health Study 2, and the Health Professionals Follow-Up Study. Overall, they had data from 111,654 participants from these three cohorts. None of the participants had diabetes at baseline, and they were observed for more than 40 years. During that time, their dietary intakes and other lifestyle factors were assessed every four years. This analysis investigated whether chocolate consumption was related to the risk of developing type 2 diabetes during the follow-up period. Quite an impressive study: very large, very long follow-up, and repeated assessments of dietary intakes.
The key finding was that milk chocolate consumption was not associated with type 2 diabetes risk. However, dark chocolate consumption was associated with type 2 diabetes risk. Specifically, those participants who consumed the most dark chocolate, five servings per week or more, a serving being one ounce or 30 g, had a significantly lower risk of developing type 2 diabetes during the follow-up period. In fact, a 21% lower risk in those consuming five servings per week or more. Or expressed differently, per one serving of dark chocolate per week, the risk was reduced by about 3%. This is certainly a very meaningful difference in risk, and this was after adjustment for other type 2 diabetes risk factors as well as potential confounding factors. What are confounding factors? Well, I am glad you asked; we’ll get to that in a minute.
What does a finding like this mean? Let’s look at some of the media articles written about this study. It seems that some people immediately came to the conclusion that this study proves that dark chocolate causes us to have better glucose tolerance and a lower risk of type 2 diabetes.
I certainly don’t agree with that conclusion. This was a good study, with adequate analyses, but it’s just one study, and every study has limitations. If there is one thing that is clear from nutrition research and science in general, it is that even the best studies sometimes produce results that are at odds with each other.
One potential concern in an observational study like this is what we call residual and unmeasured confounding. Let me explain.
Imagine that this had been a randomized controlled trial where the 110,000 participants had been randomized to either consume no dark chocolate, or varying amounts of dark chocolate over 40 years. That would have been a much stronger study design because in that case, we could be certain that dark chocolate consumption was the only difference between the groups. This is not the case in an observational study. In fact, in this study, people who ate the most dark chocolate seemed to have a generally healthier lifestyle than those who did not eat any dark chocolate. This is sometimes called the healthy user bias. For example, people eating more dark chocolate had a higher level of physical activity and an overall healthier diet than those who didn’t. Now, scientists can try to measure such additional factors, and adjust for them statistically, but the concern remains that they may have been unable to reliably measure these other factors well, or maybe there are factors they didn’t even think of measuring. So what remains is a risk that we call residual or unmeasured confounding, with confounding meaning ‘confusion of effects’. For example, if an exposure (in this case, dark chocolate consumption) is strongly associated with an overall healthier diet, then we can adjust for that healthier diet effectively only if we can reliably measure all components of it. With the food frequency questionnaires (FFQs) that are used for most large cohort studies, this is unfortunately not the case. Research shows clearly that while FFQs are quite good at measuring the intakes of regularly consumed foods (beer, coffee, tea, milk, bread, chocolate), they are very poor at assessing the intake of foods that people tend to eat only occasionally or seasonally, such as leafy greens, squash, sweet potato, or pies. It is therefore possible that an adjustment for the healthfulness of the diet, as in this study, is incomplete because some components were not reliably assessed. Also, the data don’t rule out the possibility that those who ate the most dark chocolate also had other behaviors that lowered their risk of type 2 diabetes. For example, maybe people who eat more dark chocolate meditate regularly, and maybe it’s the meditation that actually lowers the risk of type 2 diabetes. So when we see the association between dark chocolate and a lower risk of type 2 diabetes, we may think that eating dark chocolate CAUSES us to have a lower risk, but what really may be going on is that the meditation that dark chocolate eaters more often engage in is what actually causes the risk to be lower. I am not saying this is the case here, but with an observational study, we can never be 100% certain that something like that is not going on.
Another concern with this particular study was that the observed association between dark chocolate intake and type 2 diabetes risk wasn’t the same in all three cohorts. What we really hope is that if we find an association in an observational study like this, this association is reflective of a cause-effect relationship. In other words, that there is a real biological effect that causes people who eat more dark chocolate to have better glucose tolerance and that this causes the risk of type 2 diabetes to be lower. If that were the case, we would expect a similar association in each of the three cohorts. Type 2 diabetes risk was reduced in participants consuming the most dark chocolate compared to those consuming no or very little dark chocolate by 21% if we consider all participants. If we look at the Nurses Health Study by itself, however, that figure was a reduction by 1%, which was not statistically significant. In the Nurses Health Study 2, the reduction was 20%, which also was not statistically significant, and in the Health Professionals Follow Up Study, the reduction was 51%, which was statistically significant (see figure below). Now, that makes me a bit uneasy, because without the Health Professionals Follow-Up Study, this investigation would still have had a lot of participants, but it would have been fully null, meaning it would not have detected an association between dark chocolate consumption and type 2 diabetes risk. So, again, I am not saying this invalidates this study, but these are two reasons why we should just hold our horses a bit and not jump to any kind of conclusion.
So, yes, an observational study does provide some insights, but I would consider this more as hypothesis-generating. This means these results should be used to hypothesize that dark chocolate may reduce type 2 diabetes risk. But by itself, this study does not give me a high degree of confidence that this is definitely the case.
So what we need to do is look for other studies on this topic.
Other Observational Evidence
We do have data from two additional cohorts.
For one, the Multiethnic cohort, with about 152,000 participants, also found that those eating the most chocolate had a lower risk of type 2 diabetes. In that study, they did not differentiate between dark chocolate and milk chocolate; they looked at all chocolates together, and found that those eating chocolate at least four times per week and an average of 10g or more per day had the lowest risk of developing type 2 diabetes.
A second study was much smaller, with about 1,000 participants. Over 30 years of follow up, those who consumed the most chocolate had the lowest risk of developing type 2 diabetes.
Taken together, we have three observational studies that describe lower risks of type 2 diabetes in people eating the most chocolate. The data are a bit inconclusive in that two studies did not differentiate between dark and milk chocolate, but I’d say together they do strengthen the hypothesis that there may be something about chocolate that could be beneficial for glucose tolerance.
Now, why do I say “beneficial for glucose tolerance”? You may argue that none of these studies looked at glucose tolerance per se. That’s correct. These studies looked at type 2 diabetes risk. But if someone develops type 2 diabetes, this means by definition that they have elevated blood glucose levels. And elevated blood glucose levels only occur if someone is glucose intolerant. And we further know that in the context of type 2 diabetes, glucose intolerance means that the person is insulin resistant and has some degree of beta-cell dysfunction, meaning they cannot make enough insulin for their reduced level of insulin sensitivity. If any of this is unclear, check my blog post about the regulation of blood sugar, in which I explain all of this in detail.
It is clear that if dark chocolate or chocolate in general indeed lowers the long-term risk of developing type 2 diabetes, in other words, if there is a cause-effect-relationship underlying the association observed in these studies, then we should be able to see an effect of eating chocolate on glucose tolerance, insulin resistance, and beta-cell function in a short-term randomized controlled feeding study.
Let’s have a look if we have any evidence for this.
The Effect of Dark Chocolate on Glucose Tolerance: Randomized Controlled Trials
We indeed have data from four randomized controlled trials in which investigators assessed the impact of dark chocolate on measures of insulin resistance. Three of these had an almost identical study design, so let’s talk about these first.
In these three studies (Grassi et al.; Hypertension 2005; Grassi et al.; American Journal of Clinical Nutrition 2005; Grassi et al.; Journal of Nutrition 2008), people with insulin resistance were randomized to one of two groups. One group first consumed one bar of dark chocolate daily for 15 days, along with their normal diet. After a short wash-out period where they didn’t consume any chocolate, they then consumed one bar of white chocolate with the same calorie content, also for 15 days. Fasting blood was drawn at the end of each period to measure insulin resistance (see figure above). The second group also completed both phases, but in reverse order. We call this a randomized controlled cross-over design, and it’s a particularly strong study design because we can compare each person’s insulin resistance on one diet to their insulin resistance on another diet. And because people were randomized here, we can ensure that everyone consumed dark chocolate in the dark chocolate phase, not just people who otherwise have a healthy lifestyle as seems to be the case in the observational study discussed above.
Now, what were the results?
In all three studies, as measured by HOMA-IR, insulin resistance was substantially and significantly LOWER after participants had consumed the dark chocolate. The size of that effect was quite consistent and pretty impressive, with an average reduction in HOMA-IR between 0.8 and 1.0 units in each of the three trials.
In one of the studies where the researchers also measured insulin secretion in response to an oral glucose tolerance test, beta-cell function was also improved in response to eating dark chocolate compared to white chocolate. In that study, the investigators also observed an improvement in glucose tolerance, such that the blood glucose response was consistently lower in response to drinking a beverage sweetened with 75g of pure glucose after participants had completed the 15-day dark chocolate phase.
One limitation of these studies was that they were pretty short, with each intervention period just lasting 15 days. Fortunately, we have data from another trial in which participants were randomized to either a group consuming dark chocolate, 27g or about an ounce per day, or a non-chocolate control group with the same number of calories, for one year. In that study, participants had type 2 diabetes and insulin resistance at baseline. As in the shorter trials, HOMA-IR was reduced significantly in participants after they had consumed the dark chocolate.
These are pretty interesting and very consistent results, strongly suggesting that there is something in dark chocolate that is not in white chocolate that improves insulin sensitivity, beta-cell function, and glucose tolerance. Let’s discuss what that may be.
What Could It Be About Dark Chocolate that Improves Insulin Sensitivity and Glucose Tolerance?
OK, what is the difference between dark chocolate and white chocolate? Both types of chocolate typically contain cocoa butter, which is essentially just a fat, and both typically contain some sugar, even though white chocolate usually has a lot more sugar than dark chocolate. White chocolate also contains milk, which is usually absent from dark chocolate. Now, it’s certainly possible that if we compare dark chocolate and white chocolate that a difference in insulin resistance emerges because white chocolate makes insulin resistance worse, for example due to it’s high sugar content. However, none of the study suggests this. Instead, the data suggest that insulin resistance is unchanged in participants when they consume white chocolate, and improves in participants when they consume dark chocolate, i.e., they suggest a beneficial impact of dark chocolate.
The biggest and most obvious difference is that dark chocolate contains the non-fat parts of the cocoa bean, in the form of cocoa liquor or cocoa powder. And these contain a wide range of very interesting substances that deserve to be discussed briefly.
In general, the substances of interest belong to a very large family of chemicals in plant foods that we often call secondary plant compounds. These are simply molecules that the plant makes that are not strictly necessary for its most basic functions. To us, secondary plant compounds are relevant because many of these have various effects on the human body.
A particular group of these secondary plant compounds are phenols, which include polyphenols. There are lots of other secondary plant compounds, but for now, let’s focus on polyphenols. You have probably heard of these. We find these polyphenols in many foods, such as olive oil, fruits and vegetables, black and green tea, red wine, and … cocoa beans. Again, there are a number of different classes of polyphenols, but particularly interesting ones in cocoa are called flavonoids, and within flavonoids, cocoa has some called flavanols. And it’s the very high flavanol content that many researchers believe could be responsible for the anti-diabetic effect of cocoa and dark chocolate.
Now, earlier I shared that dark chocolate reduces insulin resistance and improves beta-cell function in randomized controlled trials, leading to improved glucose tolerance. How could flavanols in cocoa be responsible for this? Well, there can be numerous different causes of insulin resistance and beta-cell dysfunction, but one commonality that is almost always seen in insulin resistant tissues and also in dysfunctional beta-cells is what we call mitochondrial dysfunction. Mitochondria are the power plants of each cell in which the cell converts fuel, such as glucose and fat, to the chemical form of energy, ATP. We observe that this process doesn’t work properly in insulin-resistant muscle cells and insulin-resistant liver cells. And similarly, in beta-cells that are unable to secrete enough insulin, we also often find such mitochondrial dysfunction. Another common observation in these cells is what we call oxidative stress, characterized by an accumulation of reactive oxygen species, or ROS. ROS formed in mitochondria is probably normal to some degree and may even play a regulatory role in the cell. However, when ROS production exceeds the antioxidant capacity of the cell, ROS molecules can accumulate and damage cell components, including the mitochondria from which they originate. And this is where flavanols from cocoa may come into play. Flavanols are potent antioxidants, and they may be able to deal with reactive oxygen species and remove or reduce their damaging effects on the mitochondria. Or, another way to think about this: cocoa-derived (and other dietary) flavanoids may help re-establish a healthy ROS balance in the cell. And that would be expected to improve mitochondrial function.
So while no study has ever shown that this is the exact mechanism through which dark chocolate consumption improves glucose tolerance, it’s one potential mechanism that is supported by several indirect pieces of evidence, including:
- Flavanoids are potent antioxidants
- Cocoa flavanols are potent antioxidants
- Reactive oxygen species play a role in mitochondrial dysfunction
- Flavonoids, including flavanols, protect against mitochondrial dysfunction in a variety of conditions
In addition to antioxidative effects, we also have data suggesting that cocoa flavanols have anti-inflammatory effects. Given that inflammation can also be a cause of insulin resistance, this is another plausible mechanism through which cocoa could improve insulin sensitivity. Thus, while questions remain, we do have at least two potential mechanisms that could explain beneficial impacts of cocoa and dark chocolate on glucose tolerance and type 2 diabetes risk.
Well, if this is indeed how dark chocolate improves glucose tolerance and reduces type 2 diabetes risk, then we should be able to see similar effects from consuming cocoa in other forms, for example, as a beverage. And we can also isolate cocoa flavanols, and see if these in isolation affect glucose tolerance. Let’s have a look at whether we have studies testing this.
The Effect of Cocoa or Cocoa Flavanols on Insulin Resistance: Randomized Controlled Trials
We indeed have data from six published randomized controlled trials in which participants consumed a cocoa beverage, usually specifically picked to have a high flavanol content, compared to a control beverage with the same calories but low flavanol content. And indeed, in three of these, we do see reductions in insulin resistance in participants consuming the high-flavanol cocoa beverages. The three other studies did not detect an effect of drinking a cocoa beverage on insulin resistance. In one of these, however, participants were perfectly insulin-sensitive, with an average HOMA-IR of around 1.5 at baseline. And the problem is that even the most skilled mechanic cannot fix a car that is not broken, so I would argue that any effect the cocoa may have on insulin resistance cannot manifest in people who are not insulin resistant. This is consistent with the study by Gonzales-Garrido et al. (second study mentioned above), which also detected reductions in insulin resistance only in participants who were insulin resistant at baseline, but not in participants who were insulin sensitive at baseline. Another study also did not detect an effect of flavanol-rich cocoa on insulin resistance, but that study was extremely short, with participants drinking cocoa only for 5 days, which may simply have been too little time to make a difference. So we only have one study that was conducted in insulin resistant people who consumed flavanol-rich cocoa for a sufficiently long enough period of time (4 weeks, in this case) that did not detect a reduction in insulin resistance. In this study, participants consumed a flavanol-rich cocoa beverage or a low-flavanol cocoa beverage daily for 4 weeks, a design similar to that of other studies that did detect differential changes in insulin resistance. It is therefore unclear why this study did not detect an effect.
We also have data from one study in which participants were randomized to receive a small amount of cocoa powder, about one teaspoon per day, as a supplement, compared to a placebo pills. Insulin resistance was substantially reduced in the cocoa group, from a HOMA-IR of 4.7 to 3.8, which is clinically significant, with no change in the placebo group. However, that difference was not statistically significant, which I attribute at least partly to the very small sample size of this trial. With just 17 or 18 participants in each study group, this trial may have been underpowered to detect this differential change in insulin resistance. I therefore consider this trial inconclusive, because it was likely underpowered.
The data on the effects of cocoa, when consumed as a beverage, are not conclusive. However, when considered together, they still suggest that in insulin-resistant people, cocoa when consumed as a beverage regularly over more than just a few days, seems to induce similar improvements in insulin sensitivity as we’ve seen with dark chocolate.
The Effect of Cocoa Flavanols on Type 2 Diabetes Risk: Randomized Controlled Trials
And lastly, we do have data from a long-term randomized controlled trial in which participants were given either isolated cocoa-flavanols in a pill every day, or a placebo pill. This study was called the COSMOS trial, and included 18,381 older men and women who were free of diabetes at baseline, and they were followed for three and a half years. The study outcome here was the development of type 2 diabetes.
So this was the largest of the randomized controlled trials, and it didn’t look at insulin sensitivity, beta-cell function, or glucose tolerance, but directly at who did and did not develop type 2 diabetes. And considering how consistent the observational studies and randomized controlled trials we have discussed so far have been, I think it’s fair to say that many researchers expected that the cocoa flavanol supplement would lower type 2 diabetes risk. Unfortunately, it didn’t. The researchers detected no effect, not even a little hint, that isolated cocoa flavanols had any impact on type 2 diabetes risk.
Summary and Conclusions
Now, what do we make of this? Well, I’d say that the data on dark chocolate and cocoa are so consistent that I would conclude that eating dark chocolate or consuming cocoa as a beverage does improve insulin sensitivity in people who are insulin-resistant. And possibly also improve beta-cell function, resulting in improved glucose tolerance and a lower risk of type 2 diabetes.
Why the COSMOS trial was null and did not detect any effect of isolated cocoa flavanols remains unclear. Maybe there is more to cocoa than flavanols? If the COSMOS trial had shown a reduction in type 2 diabetes risk with cocoa flavanol supplementation, the story would have been very conclusive: cocoa in the form of a beverage or dark chocolate improves glucose tolerance and reduces type 2 diabetes risk through the action of the flavanols in cocoa. Now that this trial was soundly negative, showing no effect whatsoever, the story is less conclusive, and we remain a little unclear about what it is about dark chocolate and cocoa that exert their effect on glucose tolerance.
As a take-away message, I would still conclude that if you like dark chocolate or cocoa as a beverage, it would seem fine to enjoy these regularly. Just keep in mind that eating or drinking huge amounts every day doesn’t necessarily mean a bigger effect. My suggestion would be to have, say, 2-4 squares of dark chocolate per day, with a cocoa content of at least 80%. I think that dark chocolate could have a particularly positive effect on your diabetes risk and your health in general if it replaces other types of desserts or snacks higher in sugar and fat, such as cookies, cakes, or donuts. If you prefer cocoa as a beverage, that may also be a good option, but remember to keep the sugar content to a minimum. Cocoa beverages, particularly commercial ones, are often very rich in added sugar.
Also note that cocoa powder used in beverages is often alkalized, in what is called Dutch processing or Dutching. Notably, the flavanol content of cocoa is notably reduced by Dutching, so aim to use cocoa powder that has not undergone this process.
Now, why do I recommend only 2-4 squares per day and not more? Didn’t I mention that several trials used 1 bar of dark chocolate per day for a couple of weeks? Yes, sure, and I think in those studies, a high dose was probably needed to detect an effect in a relatively short-term study. For long-term, potentially, lifelong consumption, I think there are a few other things we should consider. For one, it’s important to consider that the sugar cocoa is often consumed with is not great in larger amounts, particularly if you are concerned about blood glucose control. Also, dark chocolate is a food with a high energy density, and as I have explained before, a high energy density means that chocolate has low satiety score per calorie. Specifically, most dark chocolates have about 600 kcal per 100 g, which is very high. In other words, eating too much of such a food could easily lead to an overconsumption of calories and weight gain, and that’s certainly not great for diabetes prevention or health in general. And lastly, cocoa can contain a high concentration of heavy metals such as lead and cadmium, and it is very high in the antinutrient oxalate. For heavy metals and also oxalates, our bodies can probably manage small amounts, but going overboard and having a bar of dark chocolate or more every day because you now think of chocolate as a superfood could carry some risks. To me, it does seem likely that enjoying a reasonable amount of 2 squares to no more than 4 squares per day regularly is maybe a sweet spot for your enjoyment of the food with some health benefits while keeping potential risks at a minimum.
Thanks for reading, and take care!
P.S.: Lastly, a huge thank you to the first patrons of this website and the associated YouTube channel. If you would like to support the creation of our free evidence-based content, please consider becoming a patron over at Patreon.com. Being a patron comes with some perks, such as being able to directly ask me questions about the blog posts in live calls or a chat community.
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3 Responses
Mario, I really appreciate your videos. I have type 2 diabetes and your information has been useful to me as I navigate the management of my disease. I have a question. How do I interpret or take action from things that “reduce the risk of type 2 diabetes”. It doesn’t directly say it will benefit me. Can you provide some insight?
Hi Miriam,
Generally, interventions that reduce the risk of type 2 diabetes would usually be expected to improve glucose tolerance by either improving insulin sensitivity or beta-cell function (insulin secretion). And these interventions may quite plausibly have these same effects in patients who already have type 2 diabetes. In all cases, of course, it is important to keep your doctor informed and discuss any major changes to your diet or lifestyle with him or her; that is critical particularly if you are on diabetes medications, which may need to be adjusted should your glucose tolerance improve.
Cheers
Mario
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