Beneficial effects of dark chocolate consumption (in moderation, of course) on cardiovascular health have been reported in several studies, with this effect linked to the high levels of polyphenolic flavonoids found in dark chocolate or cocoa. But how, exactly, does flavonoid-rich dark chocolate take a bite out of cardiovascular mortality?

In research reported by Wirtz et al., the authors questioned whether this protective effect was exerted through a reduction in stress reactivity—and whether this stress-busting effect occurs in the periphery or more centrally.

Psychosocial stress, the investigators noted, has been implicated as a cardiovascular disease risk factor, presumably by inducing physiological stress responses and hyperactivation of the hypothalamus-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS). Cooking up a placebo-controlled, single-blind trial, the authors examined whether a single administration of dark chocolate (rich in the flavonoid epicatechin) buffered endocrine reactivity to acute psychosocial stress in 65 healthy men. After eating a standardized breakfast, participants received 50 grams of experimental dark chocolate (n = 31) or 50 grams of placebo chocolate (n = 34), and then underwent a psychosocial stressor (the Trier Social Stress Test, which included a mini-job interview). Saliva and blood samples were taken immediately before chocolate consumption and during the 2 hours after the stress test.

Hormones indicating a central stress effect (HPA axis hormone ACTH secreted by the anterior pituitary and SNS hormone norepinephrine released both as neurotransmitter from sympathetic nerve endings and to a lesser extent as stress hormone from the adrenal medulla) and peripheral stress effect (HPA axis hormone cortisol secreted by the adrenal cortex and SNS hormone epinephrine secreted by the adrenal medulla) were measured. Across all subjects, the stress test induced significant increases in cortisol, ACTH, epinephrine, and norepinephrine (p < 0.001 for all).

Deliciously, dark chocolate significantly blunted cortisol and epinephrine reactivity to psychosocial stress, compared to the placebo group; these associations remained true even after controlling for age, body mass index (BMI), and mean arterial pressure (MAP). Epicatechin plasma levels were, as expected, higher in the dark chocolate group. For participants with higher epicatechin plasma levels, stress reactivity of the adrenal gland hormones cortisol and epinephrine were significantly lower (p = 0.027 and p = 0.047, respectively). Again, additional controlling for age, BMI, and MAP did not significantly change these results.

In contrast, there were no associations of epicatechin levels with ACTH and norepinephrine stress reactivity, Wirtz et al. added. The stress-buffering effect of dark chocolate, it seems, was limited to the stress hormones of both major stress systems (the SNS and HPA axis) in the adrenal gland, and thus in the periphery, but not centrally. Notably, this study was designed only to look at the short-term stress-buffering effects of acute dark chocolate consumption and acute stress exposure; the clinical implications of dark chocolate consumption's protective effects against situations of chronic stress exposure are only speculative. The current findings do, however, indicate that one-time dark chocolate intake led to a peripheral stress protection. "Future cross-sectional and prospective studies are needed to replicate our findings, and to test their clinical relevance, long-term health consequences, as well as generalizability to chronic stress exposure and populations other than healthy men," the authors concluded.

Wirtz PH, Känel RV, Meister R, et al. J Am Coll Cardiol. 2014;63:2297-9.