Weekend Reads | Which Is Healthier: Dark or Milk Chocolate?

Now that it's January, what are you going to do with all of that leftover holiday chocolate? Before you decide to eat it all, you might want to check out this weekend's read: the latest research on whether eating chocolate is good or bad for you. 

This report, led by researchers at Harvard's T.H. Chan School of Public Health, is not the first one to look at the question. For years, scientists, doctors, and nutritionists have debated the relative benefits and harms of chocolate consumption. On one hand, cocoa (from which chocolate is made) is full of flavonoids, a family of chemical compounds found in some plants that are known to have health benefits. A diet rich in flavonoids has been associated with lower blood pressure and reduced risk of cancer, heart attack, stroke, and Type 2 diabetes. But chocolate candy — the most common way to consume it — is also often loaded with sugar and fat. So do the benefits outweigh the harms?

That turns out to be a very complicated question, for several reasons. One reason is that chocolate comes in multiple forms, with different amounts of cocoa, sugar, and fat. Most notably, there is "milk chocolate" and "dark chocolate." Milk chocolate usually has less cocoa and more sugar; dark chocolate the opposite. So the researchers decided to measure the health effects of each separately, as well as chocolate in any form.

The team accessed the data from three long-term health studies conducted on doctors, nurses and other health professionals, dating back as far as 1986. They identified 192,000 study participants who at the start had no history of Type 2 diabetes, cardiovascular disease, or cancer, and who provided data on their consumption of chocolate; 111,000 of them additionally indicated whether it was milk chocolate or dark chocolate they ate. They used their subsequent health histories to estimate the "hazard ratio" of developing Type 2 diabetes: Compared with someone who eats chocolate rarely or never, does eating chocolate make you more or less likely to develop it?

They found that when looking at all chocolate consumption, there was little meaningful difference in outcomes between those who eat chocolate and those who don't (those who had more than five servings a week had a slightly lower risk of developing diabetes). But that result hides a very important distinction buried further down in the analysis: People who eat milk chocolate had the same diabetes risk as those who don't, but people who eat dark chocolate (a smaller group — milk chocolate is much more popular) had a substantially lower risk.

Drawing that ultimate conclusion was no easy feat, as there were many "confounding factors"; It turns out that chocolate consumption correlates with several other relevant factors. For example, people who eat milk chocolate tended to have less healthy diets and to gain weight over time, whereas dark-chocolate eaters tended to have healthier diets and maintained their weight. It's well-known that weight gain and poor diet are linked to Type 2 diabetes, so was it the milk chocolate that increased their risk, or the poor diet or weight gain? Did eating milk chocolate cause the weight gain, or is eating milk chocolate just one component of an unhealthy diet?

The researchers were required to tease out dozens of these potential confounding factors to come to a clear conclusion. Their list will give you head spins, and includes age, race and ethnicity, family history of diabetes, blood pressure, cholesterol level, postmenopausal hormone use, oral contraceptive use, level of physical activity, smoking status, alcohol consumption, quality of diet, and BMI. Fortunately, they had a very large number of study participants, so they could control for these factors individually and, in some cases, collectively to get to the heart of the matter: If everything else is the same and the only thing that is different between study participants is their chocolate consumption, who is more likely to develop Type 2 diabetes? 

Controlling for factors such as diet or obesity either didn't change the outcome or strengthened the association between eating dark chocolate and lower diabetes risk; the same when they adjusted for diet and even for the amount of flavonoids that study participants consumed from other sources. It really did appear to be the chocolate that was lowering the risk for developing diabetes — and the more dark chocolate consumed, the lower the risk (up to a point; too much of anything is unhealthy).

The researchers suggest that the likely reason for this is one particular type of flavonoid present in cocoa, called epicatechin, which has been shown to improve insulin sensitivity, protect the cells in the pancreas that manufacture insulin, and reduce inflammation. Dark chocolate has plenty of epicatechin, as does milk chocolate, but milk chocolate also contains large amounts of sugar, which often leads to insulin resistance and increased inflammation — both of which contribute to diabetes. There is enough bad stuff in milk chocolate to balance out the good stuff.

This isn't a perfect study: The study participants didn't match the racial and ethnic diversity of the U.S. population as a whole; they ate less chocolate than average Americans; and they self-reported their consumption of chocolate and other food in their diet (self-reporting of consumption tends to be biased because we all tend to hide our bad behavior). Nevertheless, it's strong evidence that we should dump the milk chocolate that Santa put in our stockings and stick to the dark chocolate, which really does seem to have some health benefits.

Chocolate intake and risk of type 2 diabetes: prospective cohort studies