1.1 Cocoa-rich dark chocolate and its bioactive components

Cocoa derived from Theobroma cacao L., the botanical name for the cocao (cacao) tree, has been known for thousands of years and played important roles during sacred ceremonies (“the food of the gods”), but also in human nutrition and traditional medicine [1]. Particularly the beans of cocoa have been shown to be rich in a multitude of bioactive compounds, such as polyphenols including flavonoids (e.g., catechins, epicatechins) and proanthocyanins, as well as methylxanthines (e.g., theobromine, caffeine) that are responsible for health-promoting including anti-inflammatory effects [1, 2]. The concentrations of respective biologically active cocoa ingredients can vary considerably in chocolate depending on the type of bean and the way the chocolate has been processed [3]. When ingested, respective phytochemicals can interact with distinct transcription factors and enzymes and either induce or suppress their activity in several metabolic pathways resulting in a multitude of effects including production of distinct pro- and anti-inflammatory mediators [4, 5].

1.2 Chronic inflammatory immune cell responses and resulting morbidities

In general, chronic inflammation may occur when the delicate and well-orchestrated immune cell homeostasis is disturbed and shifted towards pro-inflammatory immune responses due to pathogen persistence, autoimmune responses or metabolic disorders, among others [6–8]. In turn, pro-inflammatory mediators might be persistently secreted mounting in systemic inflammatory responses that often remain low-grade, and cellular matrix changes result in fibrosis or even necrosis [9]. Several factors might contribute to the development of chronic inflammation such as sedentary lifestyle with only little to even no physical activity, and unbalanced, “unhealthy” nutritional habits (“Western diet”) [10]. The intestinal barrier that serves as an intestinal firewall for pathogens [11] may be compromised and result in a “leaky gut” with an altered gut microbiome caused by processed high-fat foods constituting only one among many other factors promoting chronic inflammatory processes [7]. Furthermore, obesity as a key feature of the metabolic syndrome is accompanied by increased visceral fat tissue mass, that produces leptin besides other pro-inflammatory mediators, and results in a down-regulation of the anti-inflammatory hormone adiponectin, in enhanced cell turnover of cells needing pronounced phagocytosis, and in an increased risk of cell hypoxia and subsequent necrosis [12–14]. The exact mechanisms underlying the bi-directional relationship between chronic inflammatory responses and metabolic disorders mounting in potential collateral damages such as cardiovascular diseases due to atherosclerosis [15, 16], type 2 diabetes mellitus (T2DM) [17], chronic kidney disease [18], and cancer [19] remain to be elucidated.

Given that the prevalence rates of these lifestyle- and chronic inflammation-associated morbidities and their often fatal complications are progressively rising all around the globe [6, 20–23], dietary interventions including supplementation with biologically active anti-inflammatory natural compounds including cocoa-rich dark chocolate may play pivotal roles as therapeutic and/or preventive measures [2, 24].

1.3 Aim

Our structured literature review aims to provide an overview of recent evidence on the health-beneficial effects of cocoa-rich dark chocolate and their therapeutic perspectives in acute as well as chronic diseases derived from studies conducted in the past 10 years.

2.1 Search strategy, inclusion and exclusion criteria

A literature review was conducted from December 18th, 2023, to January 15th, 2024, using the meta database PubMed. First, ‘dark chocolate’ was chosen as a search term to receive clinically relevant and applicable studies. The Boolean operator AND was used to combine the terms ‘dark chocolate’ and ‘inflammation’ and the filter ‘last 10 years’ was applied. This combination of terms yielded 23 results. Reviews were excluded adding the Boolean operator NOT and ‘review [publication type]’ to the search bar reducing the number of hits to 15. To figure out whether relevant studies had been missed an asterisk was added after ‘inflamm’. The final combination of terms (dark chocolate AND inflamm* NOT review [publication type]) then obtained 24 hits that were all scanned for their relevance. Thus, two additional reviews, a case report, a mere study protocol and two off-topic articles on endometriomas and new Covid treatment options were excluded. Finally, 18 studies were included in our review.

3.1 Protective effects of dark chocolate on gut barrier function

The protective effects of molecules derived from cocoa beans on intestinal barrier function were addressed by Iaia et al. [25] and Nocella et al. [26] in vitro using colorectal adenocarcinoma (CaCo-2) cells as gut mucosa model. Therefore, Iaia and colleagues [25] cultivated CaCo-2 cell monolayers to test whether the methylxanthine theobromine was able to reduce the damage induced by an oxysterol mixture known to impair mucosal cell integrity [27]. The researchers incubated cells with different concentrations of theobromine (namely, 10, 15, 20, 25 or 30 µM) as pre-treatment regimens. Subsequently, an oxysterol mix was added as an inflammatory stimulus to these pre-treated cell layers. After the oxysterol incubation period, neither theobromine concentration exerted any necrogenic effects, but even 10 µM theobromine significantly reduced the oxysterol induced cell damage. While cells that had only been incubated with oxysterols displayed severe sequelae of oxidative stress as indicated by higher levels of pro-apoptotic Bax, IL-8, and monocyte chemoattractant protein 1 (MCP-1), the theobromine pre-treated CaCo-2 cells exhibited decreased pro-apoptotic mediators, but elevated levels of the anti-apoptotic protein Bcl-xL. Furthermore, the CaCo-2 cell layers incubated only with the oxysterol mix showed a significant decrease in tight junction proteins and higher matrix metalloprotease (MMP) activity, whereas the pre-incubated layers displayed substantially higher levels of occludin, junctional adhesion molecule A (JAM-A), and lower MMP activity, indicating a less damaged cell barrier following theobromine pre-treatment [25].

With a similar experimental design, Nocella et al. [26] proved the integrity preserving effects of polyphenols by pre-treating CaCo-2 cell monolayers with a polyphenolic extract derived from dark chocolate prior to lipopolysaccharide (LPS) stimulation. While LPS application resulted in pronounced secretion of the soluble NADPH oxidase 2-derived peptide (Nox2-dp) and of hydrogen peroxide (H₂O₂) indicative for cell damage, respective oxidative stress responses were far less distinct upon dark chocolate derived polyphenol pre-treatment which was accompanied by more intact cell-cell adhesions supporting a preserved gut epithelial barrier function [26].

In the in vivo part of their study, Nocella et al. [26] tested the effects of dark chocolate intake on gut epithelial integrity and permeability in male elite versus amateur football players before and after a period of excessive exercise. Therefore, 24 male elite football players and 23 age- and sex-matched amateur players were enrolled in a randomized controlled trial (RCT). The elite players were divided into two groups and the intervention group was asked to consume 40 g of dark chocolate daily alongside a standardized training and their normal diet. When measuring the plasma levels of LPS and of the tight junction proteins occludin and zonulin before intervention, the elite athletes displayed higher markers of gut permeability compared to the amateur group. After exercise and intervention, however, the plasma levels of all tested markers had markedly increased in the amateur, but not professional athletes. In fact, after the 30-day dark chocolate intervention period, lower LPS, occluding and zonulin plasma concentrations were measured in the latter if compared to the control group suggesting a protective effect the dark chocolate derived polyphenol in preserving the intestinal barrier integrity in the professional football players [26].

3.2 Modulation of pro-inflammatory immune responses by dark chocolate

In 2016 Vongraviopap et al. [28] enrolled 25 male acne patients between 18 and 30 years of age in their clinical trial to investigate whether the regular consumption of dark chocolate had an impact on disease severity. After a 4-week intervention period, in which participants had consumed 25 g of 99% dark chocolate daily, the acne had worsened. According to the Leeds revised acne score quantifying the observed skin lesions, the intervention induced worsening was most prominent during the first two weeks and plateaued afterwards. The authors further observed that certain types of skin lesions were even worse given that scores for comedo and inflammatory papules doubled, whereas those for pustules and nodules were not affected [28].

In their study from 2017, Ioannone and colleagues addressed whether phenols extracted from dark chocolate would attenuate an artificially induced oxidative burst in leukocytes [29]. Therefore, chocolate polyphenol extracts (CPE) derived from different brands were tested for their anti-oxidant capacities. Then, neutrophils and monocytes that had been were isolated from blood drawn from 8 normal weight and from 7 obese subjects, were stimulated with phorbol 12-myristate 13-acetate (PMA) to induce oxidative bursts, treated with different concentrations of the most potent anti-oxidant CPE (ranging from 5 to 100 mg L⁻¹), and the cellular production of oxygen radicals measured. The analyses revealed that the lowest CPE concentration already led to a significant reduction of approximately 30% of ROS generation in cells derived from obese, but not normal weight subjects. Upon stimulation with higher CPE concentrations, however, the ROS reducing effects were more pronounced in stimulated leukocytes derived from obese subjects if compared to normal weight counterparts indicating that CPE constitutes a promising dietary anti-oxidant to counteract oxidative stress conditions [29].

3.3 Effects of dark chocolate on inflammatory cytokines and adipokines

Four studies addressed the effects by dietary intake of dark chocolate on pro-inflammatory mediator secretion [30–33]. In a randomized placebo controlled clinical trial, Kuebler et al. [30] tested whether dark chocolate intake could directly reduce stress reactivity and asked 65 healthy men between 20 and 50 years of age to perform a Trier Social Stress Test after consuming either 50 g of dark chocolate (verum group) or white chocolate (placebo group). Two hours after the chocolate consumption, when blood levels of the dark chocolate-derived polyphenol epicatechin were expected to be highest, the Trier Social Stress Test was carried out. Furthermore, blood and saliva samples were taken at different time points before and after the Trier Social Stress Test and tested for epicatechin, pro- and anti-inflammatory mediators, and for NF-κB binding activity in immune cells. Whereas in both groups the stress test was accompanied by increased IL-6 and IL-1β mRNA levels and enhanced cellular NF-κB binding, the increases in pro-inflammatory mediators were less pronounced in the verum if compared to the placebo group, whereas a trend towards increased anti-inflammatory IL-10 mRNA concentrations were observed in the former as opposed to the latter. Furthermore, the blunted stress reactivity observed in the dark chocolate cohort was accompanied by increased epicatechins levels in both, saliva and blood [30].

In their study from 2018, Jafarirad et al. [31] enrolled patients suffering from T2DM treated with either metformin or glibenclamide to investigate the effect of dark chocolate on serum adiponectin among other inflammatory parameters. Therefore, subjects were either allocated to a control group, that only received a lifestyle modification, or to a dark chocolate intervention group, that additionally consumed 30 g of 84% dark chocolate for 8 weeks daily. After the dark chocolate intervention, the T2DM subjects displayed significantly lower blood levels of fasting glucose, hemoglobin A1c (HbA1c), and pro-inflammatory markers such as IL-6, TNF-α, and high-sensitive CRP (hs-CRP) in addition to an overall better lipid profile if compared to the control counterparts, whereas at least a trend towards higher adiponectin levels could be observed in the verum versus the control cohort [31].

In a RCT from 2020, Eskandari et al. [32] enrolled 48 obese adolescent boys who consumed either 30 g of 83% dark chocolate or white chocolate as placebo for six weeks and had to follow a standardized jump rope exercise regimen (standard protocol, three times weekly), whereas a negative control group did not experience any intervention at all. In all intervention groups, significant reductions in the body mass index, waist-hip ratio, and fat mass index could be observed which also held true for pro-inflammatory mediators such as hs-CRP, IL-6, and pro-inflammatory adipokines leptin, resistin, RBP-4, and MCP-1 after the six weeks, whereas the anti-inflammatory adipokines adiponectin and irisin were increased. These anti-inflammatory changes were, however, most pronounced upon dark chocolate intervention, and only in this cohort decreased chimerin levels were measured. The authors concluded that dark chocolate supplementation in addition to physical exercise constituted a promising measure to dampen obesity-induced inflammatory responses in adolescent boys [32].

Another clinical trial was conducted by Ribeiro et al. in 2023, in which the authors addressed the immune-modulatory responses upon ingestion of dark chocolate by chronically diseased individuals [33]. Therefore, 59 male patients between 20 and 65 years of age with chronic kidney disease on hemodialysis were included into the study and 48 of them were asked to consume 40 g of 70% dark chocolate over 8 weeks during their dialysis session (i.e., three times per week), and a broad range of systemic inflammatory, oxidative, and metabolic markers was assessed. The results revealed that at the end of the observation period, the 35 patients that completed the study exclusively displayed significantly lower plasma TNF-α concentration if compared to the control group. The authors pointed out that even though chocolate is known to be rich in distinct minerals, blood levels of phosphorus and potassium did not rise upon consumption, suggesting that chocolate was safe to eat for patients on hemodialysis that often had to follow dietary restrictions [33].

3.4 Effects of dark chocolate on vascular function

In their randomized double-blind study from 2014, Esser et al. [34] addressed potential beneficial effects of flavanol-enriched dark chocolate (high flavanol content) in comparison to dark chocolate with a normal flavanol content on vascular function in overweight men. After 4-week consumption of dark chocolate with either 70 g high flavanol content or normal flavanol content daily, all subjects showed a similarly improved flow mediated dilation and a decreased augmentation index that were accompanied by lower soluble leukocyte, leukocyte adhesion marker, and cell surface adhesion marker counts. Whereas the authors did not observe any clinically relevant protective effects on vascular function that were directly linked to the flavanol concentrations of the ingested chocolate, plasma concentrations of the anti-oxidant epicatechin were five times higher in subjects from the high flavanol content versus normal flavanol content group. According to the subjects, taste and willingness to eat the chocolate was affected negatively by the higher flavanol content [34].

In line, a randomized study conducted by Germonpré et al. in 2017 addressed different preconditioning methods to reduce decompression sickness [35]. Specifically, the authors aimed to explore whether the formation of gas emboli and the impairment of endothelial function could be reduced by either a 30-min sauna session, a 30-min vibration session or an intake of 30 g of 86% cocoa containing dark chocolate 2 h before a dive as means of preconditioning. Therefore, nine healthy advanced divers were included and performed standard dives at least six times, three control dives, and a minimum of one dive after each intervention. Then, respective values of flow mediated dilation and number of gas emboli after each dive were compared to the mean values of their own control dives. While all divers showed a significantly reduced flow mediated dilation (mean 93.45% of the 100% set as the pre-dive value) after their control dives, both the sauna and the chocolate intervention resulted in increased and thus, improved values compared to respective pre-dive parameters. Hence, the chocolate ingestion could in fact improve vascular function by increasing flow mediated dilation [35].

In another study Skrypnik et al. found a correlation between the regular consumption of dark chocolate and a decreased peak to peak time, a marker inversely correlating with arterial stiffness [36]. For their cross-sectional study, the researchers had randomly recruited 829 Polish participants who had to complete a questionnaire on how frequent their consumption of certain foods including dark chocolate ranging from 1 (never) to 6 (twice a day or more) was. Then, the participants were subjected to anthropometric measurements and blood sample testing, in order to assess the influence of diet on the participants' arterial stiffness, lipid metabolism, liver and renal functions by comparing the “1–2 frequency” to the “5–6 frequency” consumers. The findings revealed that the high frequency group displayed higher uric acid plasma levels and decreased peak to peak time values indicative for higher arterial stiffness [36].

3.5 Reduction of blood pressure by dark chocolate

Rostami et al. [37] tested in a randomized placebo-controlled double-blind trial whether consumption of dark chocolate had an anti-hypertensive effect in diabetic patients. Therefore, 60 hypertensive T2DM diabetic patients were included who consumed either 25 g of dark chocolate (83% cocoa, 450 mg flavonoids) or 25 g of white chocolate as placebo for 8 weeks. Subjects from the dark chocolate cohort experienced more distinct reductions of both, systolic (additional minus 5 mm Hg) and diastolic blood pressures (additional minus 6 mm Hg) as well as fasting blood glucose concentrations (additional minus 3.8 mg dL⁻¹) as compared to the white chocolate group. Furthermore, dark chocolate consumption resulted in a trend towards lower HbA1c and hs-CRP levels in the verum versus placebo cohort [37].

De Jesus Romero-Prado et al. [38] addressed in a RCT whether the daily addition of 450 mg of dietary flavonoids derived from dark chocolate and other flavanol-rich foods for 6 months could further reduce blood pressure in young patients with newly diagnosed hypertension that had just started anti-hypertensive therapy with either telmisartan or captopril. In fact, after the intervention period the intake of dietary flavonoids had reduced the systolic blood pressure by an additional 20% and the diastolic blood pressure by 15%. Those individuals that had consumed dietary flavonoids alongside telmisartan also exhibited better lipid profiles and higher leptin concentrations after the intervention period [38].

In a triple-blind randomized crossover study published by Mohammadi et al. in 2022 [39], the authors did not find any statistically significant differences on cardio-metabolic indices including body mass index, hs-CRP, low-density lipoprotein (LDL), high-density lipoprotein (HDL), and total anti-oxidant capacity among others when comparing individuals from the intervention group who had consumed 5 g of dark chocolate with fermented garlic extract (650 mg, containing 0.94 mg of S-allyl cysteine) for 6 weeks and the control group that had consumed 5 g of chocolate only, neither at the end of the observation period nor when considering changes before and after intervention. The authors concluded that the dose of the applied compounds might not have been high enough to result in a more favorable outcome [39].

3.6 Reduction of fasting blood glucose due to dark chocolate

In line with the in the afore-mentioned study conducted by Rostami et al. [37] who were able to show a significant reduction of fasting blood sugar and a trend of lowered HbA1c in their intervention group after dark chocolate consumption, Jafaridad et al. found that diabetic patients who ate 30 g of 84% dark chocolate alongside a lifestyle modification presented both, lower fasting blood sugars and lower HbA1c values than the control group that had undergone the lifestyle change only [31].

3.7 Effects of dark chocolate on blood lipid profile

While several studies included analyses of the lipid profile [31, 32, 34, 37–40], only the ones published by Jafarirad et al. [31], De Jesus Romero-Prado et al. [38], and Petrilli et al. [40] found significant results.

In their before-mentioned report, Jafarirad et al. [31] showed that the consumption of dark chocolate in addition to an oral anti-diabetic medication resulted in significant decreased total cholesterol, triglycerides, and LDL, but increased HDL concentrations if compared to the control group.

In line, De Jesus Romero-Prado et al. [38] reported even more enhanced HDL levels and more prominent reduction in serum triglycerides (of approximately 30%) in the verum group with supplementation of dietary flavonoids in addition to the pharmacological anti-hypertensive treatment.

In 2016 Petrilli et al. [40] investigated the effects of chocolate and Yerba Mate phenols on several biomarkers including serum lipids in patients infected with the human immunodeficiency virus (HIV). Each of the 92 participants consumed 3 g of Yerba Mate or 65 g of dark chocolate containing 36 g of cocoa as well as the respective placebos in 4 washout separated trial periods of 15 days each. Blood samples were collected after each intervention. After the last period and compared to baseline, the serum HDL concentrations had increased exclusively in participants who had consumed dark chocolate [40].

3.8 Pain modulating effects of dark chocolate

In a recent study from 2023, Hajati et al. [41] compared different types of chocolate (i.e., 30% cocoa white, 34% cocoa milk and 70% cocoa dark chocolate) with each other regarding their effects on neurogenic inflammatory pain (i.e., on pain duration, peak pain intensity, and the pressure pain threshold). For pain induction, hypertonic saline was injected into the masseter muscle of 15 young healthy men and 15 matched women, who then had to report the pain intensity at defined time points after the injection on a visual analogue scale and the maximum pain spread on a chart that displayed different parts of the head. Subsequently, an algometer applied pressure to the injection site and subjects had to press a button as soon as this pressure turned into pain. This routine was repeated three times in three different visits, each procedure following the consumption of 3.6 g of a different type of chocolate. The analyses revealed that all types of chocolate were able to reduce both, the intensity and duration of pain and to increase the pressure pain threshold but without any effects on the peak pain intensity. The cocoa content, however, did appear to play a significant role in alleviating the induced pain. Interestingly, milk and white chocolate showed a greater pain reduction in men than in women [41].

3.9 Effects of dark chocolate on the outcome of chronical inflammatory diseases

Raguzzini et al. [42] addressed whether regular consumption of dark chocolate (i.e., 1–3 times per week) had an impact on celiac disease. Their results revealed that even though chocolate is known to interfere with nutrient absorption, the patients from the dark chocolate consuming group did not present altered erythrocyte counts, hemoglobin levels or mean corpuscle volumes. The neutrophil-to-lymphocyte-ratio and the platelet-lymphocyte-ratio were significantly lower, however, in subjects that consumed dark chocolate if compared to the control subjects. Regular dark chocolate consumers also exhibited an elevated lymphocyte-to-monocyte ratio, that is often associated with more favorable outcomes of distinct including chronic diseases [42].

In their in vivo study from 2022, Mirazi et al. [43] addressed the protective effects of dark chocolate in experimental polycystic ovary syndrome (PCOS). Therefore, female Wistar rats were divided into 4 groups, namely a healthy control group, and letrozole-induced PCOS groups with either mock, metformin (500 mg per kg per day), or dark chocolate (500 mg per kg per day) treatment for 28 days. Induction of PCOS by letrozole application resulted in decreased concentrations of the anti-oxidant enzyme superoxide dismutase (SOD) and of the protective hormones estradiol and follicle stimulating hormone (FSH). After the intervention period the rats with untreated PCOS displayed very differentiated ovarian tissues, increased pro-inflammatory cytokines such as IL-1β and TNF-α, higher levels of disease-promoting luteinizing hormone (LH) and testosterone as well as of higher malondialdehyde (MDA), a marker for oxidative stress. When compared to the untreated and metformin treated PCOS cohorts, the consumption of dark chocolate dampened PCOS disease progression in the rats as indicated by less distinct macroscopic signs of disease including fewer cystic lesions in the ovarian tissues, lower levels of pro-inflammatory cytokines such as IL-1β and TNF-α, while the concentrations of disease-alleviating parameters such as SOD, estradiol, and FSH remained higher [43].

4.1 Summary of main findings

Our literature survey revealed that dark chocolate and derivatives of its main constituent cocoa could effectively dampen pro-inflammatory including oxidative stress responses, enhance gut epithelial barrier function, modulate pain sensations and hence, alleviate a plethora of morbidities such as i.) vascular diseases including atherosclerosis, hypertension, and decompression sickness, ii.) metabolic morbidities such as obesity and T2DM, iii.) celiac disease, iv.) chronic kidney diseases, and v.) PCOS. On the other hand, dark chocolate consumption was found to worsen acne symptoms.

4.2 Health-promoting, disease-alleviating, and adverse effects of dark chocolate

Interestingly, dark chocolate challenge in vitro and in vivo did not only alleviate distinct inflammatory including vascular and metabolic morbidities [31–34, 37–40, 42] but also resulted in health-promoting effects in non-diseased subjects. Professional football players, for instance, who had added dark chocolate to their standard diet for 30 days presented with less distinct oxidative stress responses and a preserved intestinal barrier function upon physical exercise [26]. Furthermore, dark chocolate intake could reduce stress reactivity in healthy men [30] and improve vascular function in scuba divers [35]. On the other hand, a screening of more than 800 Polish subjects revealed that regular consumption of dark chocolate was associated not only with higher uric acid plasma levels that could mount in gout but also with increased parameters of arterial stiffness [36]. On the contrary, a study by Grassi and colleagues showed that cocoa consumption had a dose-dependent positive effect on vascular function given improved flow-mediated dilatation and less distinct arterial stiffness [44]. In line, cocoa-rich chocolate could positively affect arterial stiffness in postmenopausal women and reduced cardiovascular risk factors including hypertension [43] supporting the anti-hypertensive effects of dark chocolate as shown in our literature survey [37, 38].

The immune-modulatory including anti-oxidant and anti-inflammatory effects of dark chocolate consumption became also evident in metabolic disorders. For instance, in obese adolescent male subjects who were challenged with interval physical exercise, the balance between pro- and anti-inflammatory parameters including cytokines, chemokines, and adipokines shifted towards the anti-inflammatory side upon dark chocolate application [32]. In T2DM patients, not only decreased pro-inflammatory mediator serum parameters, but also more favorable blood glucose and lipid profiles could be achieved upon dietary supplementation with dark chocolate alongside general lifestyle modifications [31, 37]. The shift of the blood lipid parameters towards a less pronounced atherogenic profile could also be confirmed by studies testing the beneficial effects of dark chocolate consumption in newly diagnosed hypertensive subjects [38] and in HIV infected patients [40].

The pronounced anti-hypertensive effects of dark chocolate described by the included studies were obtained in combination with other medications given in T2DM [31] and blood hypertension [37]. In other publication addressing the blood pressure-lowering effects of cocoa or chocolate alone the obtained results were rather inconsistent. Grassi et al. [44], for instance, found significant blood pressure reductions after intake of cocoa with varying concentrations of flavanols for a week, while another study revealed that consuming chocolate daily for six months did not have any effects on blood pressure [45]. Furthermore, given that the two studies applying the lowest cocoa doses did not yield biologically relevant benefits to the study subjects [39, 41] as opposed to other studies using chocolate with higher cocoa contents strongly suggests that the anti-inflammatory effects of dark chocolate are in fact dose-dependent. Hence, differences in study design, included subjects, and applied chocolate type with specific contents in bioactive molecules including polyphenols might explain such discrepancies as reviewed by Samanta et al. [3].

Of note, dark chocolate application could also alleviate chronic inflammatory morbidities such as celiac disease [42] and PCOS [43]. PCOS is one of the most common hormonal and metabolic disorders in women in their reproductive age and can lead to metabolic syndrome, cardiovascular syndrome, and infertility [46]. In a PCOS rat model, the substitution of dark chocolate achieved a pronounced improvement of the clinical outcome compared to metformin treatment, the standard medication used to treat PCOS patients [43].

The finding that chocolate consumption could worsen acne symptoms in men [28] is further supported by other dietary intervention studies [47, 48]. In line, Netea et al. [49] reported that chocolate pre-incubation of cells resulted in enhanced secretion of pro-inflammatory cytokines such as IL-1β and TNF-α upon stimulation with opportunistic pathogens such as Propionibacterium acnes or Staphylococcus aureus [49] that reside on the skin and are known to be involved in the formation of the acneiform skin lesions [50]. Additionally, Chalyk et al. discovered that regular consumption of dark chocolate promotes the desquamation of corneocytes and skin-located populations of distinct Gram-positive bacteria [51].

Overall, no other major adverse effects were reported upon dark chocolate supplementation of the study participants' diets. Even though dark chocolate is known to be high in minerals, application to patients suffering from chronic kidney diseases did not increase the phosphorus and potassium levels in the blood [32], whereas in celiac subjects no anemia was induced despite potential inhibiting effects on the absorption of distinct nutrients from the intestines exerted by dark chocolate [42]. Notably, no significant (i.e., adverse) changes in anthropometric parameters was found upon dietary supplementation with dark chocolate even though chocolate is high in calories. However, the subjects reported that their desire to eat dark chocolate was inversely correlated to its flavanol content which may prevent from excessive consumption [34].

4.3 Conclusion and future perspectives

Given the plethora of health-beneficial and disease-alleviating effects of dark chocolate as assessed in the included studies, dietary supplementation with dark chocolate or its derived biologically active molecules such as polyphenols and methylxanthines might be promising adjunct immune-modulatory treatment options of distinct acute as well as chronic inflammatory morbidities that need to be evaluated in more detail in future in vivo including clinical studies.

4.4 Limitations

Inaccuracies in search strategy are potential weaknesses of this evaluation since “PubMed” was the only utilized data base for this literature survey and the results that were accessed using the previously described search terms were not extended by further varying search terms or other search strategies. Particularly the applied term ‘dark chocolate’ is not very specific and many studies only containing the word cocoa that may also be very clinically relevant might have been missed. Hence, it cannot be ruled out that not all studies matching the research question and including and excluding criteria were acquired. Since the search strategy was carried out by a single researcher, errors in the search cannot be excluded, even though the survey was done as carefully as possible. As already stated, direct comparisons of the results obtained from the included publications are hampered by differences in study designs including processing and composition of the applied chocolate, content in coca, and of bioactive molecules. In addition, the numbers of subjects included in the intervention studies were rather small limiting the meaningfulness of the conclusions drawn from the obtained results.