Food and diet

Chocolate good... says choc maker

Dark chocolate cuts levels of stress hormones and rebalances other body chemicals, according to the Daily Mail. The Daily Express also featured the claim that chocolate reduces the risk of heart disease and high blood pressure and improves brain function.

The research behind these reports was commissioned by Nestlé. Researchers gave 30 healthy people 40g of dark chocolate a day for 14 days. They examined changes in metabolism and chemicals that are reportedly related to stress. The study’s methods have numerous limitations, including its small number of participants, short study period and selection of only young, healthy people to take part. Also, while the researchers measured levels of the “stress” hormones in urine, they did not directly look at changes in the participants’ stress levels.

By itself, the study is insufficient to provide evidence that dark chocolate has any benefits or effects on stress, psychological or mental health, or cardiovascular health.

Where did the story come from?

This research was conducted by Francois-Pierre J Martin and colleagues from Nestlé Research Center in Switzerland, and Metanomics GmbH in Germany. No external sources of funding were reported for this study. The study was published in the peer-reviewed Journal of Proteome Research.

Newspaper reports have mostly focussed on details from the presentation and conclusion of the study. However, the articles did not discuss the numerous limitations of this research. For example, while the Daily Mail mentions that the researchers work for Nestlé, it does not mention the small number of participants or the fact that effects were only measured over 14 days.

What kind of research was this?

This was a non-randomised experimental study in 30 individuals, which looked at the metabolic response to consuming 40g of dark chocolate a day for up to 14 days. The researchers particularly looked at how participants’ initial anxiety levels may affect changes in chemical measures related to stress.

This research model had a number of methodological flaws, including the small number of participants, the very short follow-up period and the lack of randomised groups. As such, only limited conclusions can be made from its results.

The trial could have been improved by randomising a larger number of people with equivalent anxiety levels to consume either dark chocolate or a placebo (if possible) and considering longer-term clinical effects (such as stress levels, weight gain and changes in cardiovascular health) over a longer follow-up period. The effects should also have been assessed by a researcher who was blinded to which group each participant had been assigned to.

What did the research involve?

The study recruited 30 ‘healthy and free living’ young adults: 19 women and 11 men, aged 18 to 34. The researchers excluded people who smoked, drank excessively, were overweight or obese, were on a diet or had medical disorders (including metabolic or eating disorders).

A validated psychological questionnaire was used to classify participants as having either low or high anxiety traits. According to the questionnaire, there were nine high-anxiety women, 10 low-anxiety women, four high-anxiety men and seven low-anxiety men.

Participants did not eat any chocolate in the eight days before the trial. They then received 40g of dark (74% cocoa) Nestlé chocolate a day for 14 days. They ate 20g mid-morning and 20g in the afternoon. On days one, eight and 15, the researchers took blood and urine samples. Metabolic changes following chocolate consumption were assessed using a number of different laboratory tests.

The study only analysed blood and urine samples, and did not provide any indication of the effects of chocolate consumption on the participant’s health, psychological status or wellbeing. This was another of the study’s limitations. It is also not known what other factors may have differed between the study participants, for example intake of other food and drinks or activity levels during the study period. In any case, the study’s duration was too short for questions about longer-term effects, such as cardiovascular disease or psychological changes, to be answered.

Stress hormones and energy levels in those who had higher anxiety at the start of the study were perceived to approach normal levels following chocolate consumption. However, as a psychological assessment was not performed at the end of the study, it is not clear whether these metabolic changes produced meaningful clinical differences.

What were the basic results?

The researchers noted that those who had higher anxiety traits initially demonstrated differences in energy metabolism in the body, hormone metabolism and microbe activity in the gut. Following dark chocolate consumption, there was a reduction in stress hormones excreted in the urine (cortisol and catecholamines) and reduced difference in energy metabolism and gut microbial activities in all participants.

How did the researchers interpret the results?

The researchers say that their study provides “strong evidence that a daily consumption of 40g of dark chocolate during a period of two weeks is sufficient to modify the metabolism of free living and healthy human subjects”. They say that these changes, seen after only two weeks, had “potential long-term consequences on human health”.


This study has numerous methodological flaws, and when considered in isolation does not provide any evidence that dark chocolate has benefits or effects on stress, psychological or mental health, or cardiovascular health.

  • Although the researchers refer to their study as “randomised” in their report, there does not appear to be any control group, so it is unclear exactly what they mean by this term.
  • The trial involved a small sample of 30 people. The effects of chocolate were assessed in even smaller subgroups of people with different anxiety traits.
  • All the people in this study were healthy young adults who were not overweight or obese, did not drink excessively or smoke, and did not have conditions such as diabetes. These results cannot be applied to people who are older, unwell or have less healthy lifestyles.
  • While the researchers observed changes in metabolism or stress hormones, it is not definite that chocolate consumption was responsible for this. For example, being part of a trial situation and removing participants from everyday life may have caused this effect. Additionally, other measures that may have played a role in the metabolic changes, such as diet and physical activity, were not reported.
  • A follow-up period of 14 days is far too short to make any conclusions about how long-term daily consumption may affect stress, mental health, cardiovascular health or weight gain. The researchers themselves do not state that dark chocolate has any of these effects.
  • As the trial was conducted by the food manufacturer and confectioner Nestlé, the researchers may have had a vested interest in promoting the positive results of their trial.

Although this study provides little evidence to show that daily consumption of chocolate promotes mental or cardiovascular health, chocolate may still be enjoyed as part of a balanced diet. However, chocolate (including dark chocolate) is high in fat and calories and should be eaten only in moderate amounts.

NHS Attribution