Holiday snacking: Experts contest results of new research linking dark chocolate to lower diabetes risk

Researchers suggest that increasing dark chocolate consumption may help reduce the risk of developing type 2 diabetes. However, experts who were not part of the research caution that due to its observational setup, it cannot determine a cause-and-effect relationship. Moreover, they highlight that the low occurrence of diabetes in people consuming dark chocolate may result in statistical uncertainties.

A new long-term US study links five weekly servings of dark chocolate to a 10% lower risk of type 2 diabetes and consuming over five servings to a 21% lower risk of developing the disease. Although the authors found no similar statistically significant association for milk chocolate consumption, they linked increased consumption of milk chocolate to excessive long-term weight gain. 

The US Harvard T.H. Chan School of Public Health researchers combined three observational studies, which included 111,654 participants without diabetes, cardiovascular disease or cancer at baseline. 

The team analyzed type 2 diabetes occurrence and chocolate intake over an average monitoring period of 25 years. Participants completed food frequency questionnaires every four years. 

“Our findings suggest that not all chocolate is created equal,” comments lead author Binkai Liu, a doctoral student in the Department of Nutrition. “For anyone who loves chocolate, this is a reminder that making small choices, like dark chocolate over milk chocolate, can make a positive difference to their health.”

However, George Davey Smith, professor of Clinical Epidemiology at the UK University of Bristol, who is not part of the research, says that the observations likely reflect residual confounding and selection bias, referring to randomized controlled trials that “consistently found no actual effect.” 

He highlights that a 3.5-year Cocoa Supplement and Multivitamin Outcomes Study (COSMOS) with 18,381 adults “failed to demonstrate any protection at all from cocoa extract supplementation in type 2 diabetes.” 

Follow-up COSMOS research suggested that cocoa supplementation may only benefit cognitive health in seniors with low diet quality, while multivitamin supplements help improve memory and slow cognitive aging. 

Dark vs. milk chocolate

Co-author Qi Sun, associate professor in the Department of Nutrition and Epidemiology, says the research team was surprised by the “clear split between dark and milk chocolate’s impact on diabetes risk and long-term weight management.” 

“Even though dark and milk chocolate have similar levels of calories and saturated fat, it appears that the rich polyphenols in dark chocolate might offset the effects of saturated fat and sugar on weight gain and diabetes. It’s an intriguing difference that’s worth exploring more.”

However, Smith suggests these results may stem from differences among consumers eating dark or milk chocolate. 

Experts note that changes between dark and milk chocolate consumption may also be due to differences among consumer groups.“As much research has shown, dark chocolate consumption is higher among individuals in higher-income groups. Studies attribute this pattern to the greater availability of premium and fine chocolate varieties in affluent markets and the appeal of dark chocolate’s perceived health benefits, which align with the lifestyle preferences of wealthier (and healthier) demographics.” 

He stresses that there “will be a large difference in many health-related factors that are not measured. This cannot be taken into account with a handful of confounders and measured with error.” 

Absolute risk?

The research, published in The BMJ,  combined data from three studies: Nurses’ Health Study (1986–2018), Nurses’ Health Study II (1991–2021) and Health Professionals Follow-Up Study (1986–2020).

The team identified 4,771 people with incident type 2 diabetes among the 111,654 participants. People who consumed at least five servings of dark chocolate weekly had a 5% to 34% lower rate of diabetes compared to people who never or rarely ate chocolate. 

Kevin McConway, emeritus professor of Applied Statistics at Open University, UK, highlights that the researchers used this range to determine an overall 21% risk reduction, which he says is linked to “considerable statistical uncertainty.” McConway is not part of the research team. 

In addition, as there were very few participants consuming over five servings of dark chocolate weekly who were diagnosed with type 2 diabetes (132 out of 4,771 people diagnosed), he says this results in “wide margins of statistician error in the risk estimates.” 

He adds that the absolute risks involved are small, detailing that eating more than five servings of dark chocolate per week would be associated with a reduction of between two and 13 diagnoses in a year in 10,000 people.  

“A reduction like that could be worthwhile because type 2 diabetes is a serious cause of ill health — but it’s not huge, and it might not exist at all because of the doubts over what is causing the risk difference. That’s why more research is needed.” 

Moreover, McConway notes that the researchers did not have good data on how much chocolate participants ate per serving, making it difficult to interpret different chocolate consumption levels. 

Eating over five weekly servings of dark chocolate is linked to a reduction of between two and 13 diabetes diagnoses annually in 10,000 people.Cause and effect

Although the researchers controlled for multiple lifestyle and dietary covariates that might confound the identified associations, they cautioned that they “cannot entirely rule out the role of confounding in our observed associations.” 

Dr. Mohammad Talaei, postdoctoral researcher at the Queen Mary University of London, UK, says: “We still cannot recommend dark chocolate to prevent diabetes until these results are replicated in other observational studies with different population profiles and verified in randomized controlled trials.”

He commends the authors for controlling for various confounders, including indicators of health-conscious behavior, like multivitamin intake, physical activity and overall diet quality. “However, in observational research, it is hard to ensure the effect of those factors is fully controlled or that unknown factors are not missed.” 

Talaei says that the study’s researchers reported a stronger association in half of the participants with a relatively healthy diet. This could imply an “unknown biological interplay between dark chocolate and other aspects of a healthy diet.” Still, dark chocolate intake could also predict total sugar intake, which produces a more precise signal in the context of a healthy diet.  

The research team concludes that longer-term randomized controlled trials, especially focusing on middle-aged participants, are needed to confirm the identified findings and explore potential mechanisms.