Discussion
Analyses of data from Tromsø7 confirm recent findings with a significant association between espresso coffee consumption and elevated S-TC. The association was significantly stronger for men than for women. This population-based cross-sectional study adds important information regarding the association between brewing methods and S-TC in men and women.
Previous studies regarding the association between espresso and serum cholesterol have shown varying results.
The randomised controlled trials (RCTs) of D’Amicis et al19 and Martini et al,20 and the cross-sectional study of Grioni et al21 found no significant association between consumption of espresso coffee and increased S-TC. However, the RCTs had a modest level of intake, with three cups of espresso daily in the intervention groups. Also, Martini et al had a small sample size divided into three groups (n=7 per group). On the other hand, Cornelis and van Dam used cross-sectional data from the UK Biobank, finding that espresso was associated with increased S-LDL cholesterol in both sexes.16 Although having a large sample size, results could be affected by a ‘healthy volunteer’ selection bias, not necessarily making results generalisable. Furthermore, Weusten-Van der Wouw et al22 estimated that every 10 mg of cafestol plus a similar amount of kahweol raised S-TC by 0.13 mmol/L. The following prediction of Urgert et al9 was that five cups of espresso daily would raise S-TC by 0.10 mmol/L.
There are several possible explanations for the results in our study. Combined intake of other coffee brews together with espresso may have caused the rise in S-TC. However, when running the analysis both with and without adjusting for combined coffee habits, the association was strengthened when adjusting for other coffee types. This suggests that espresso itself rises S-TC. The questionnaire regarding coffee in Tromsø7 had a wide definition of espresso, including coffee machines, capsules and mocha pots. Moeenfard et al investigated the variability of diterpenes in different types of espresso, finding that cafestol concentrations were 36 mg/L for mocha and 54 mg/L for espresso machines.23 For capsules, pods and vending machines, cafestol concentrations varied between 10 mg/L and 43 mg/L. Wuerges et al found similar results when testing different commercial capsule coffee in Brazil.24 This shows that diterpene levels vary within different types of espresso. In comparison, boiled coffee and filtered coffee contained 232 mg/L and 5 mg/L of cafestol, respectively, suggesting that espresso brews may have an intermediate contribution to the intake of cafestol and kahweol compared with other types of coffee. To this can be added variations in the size of coffee cups. Italians drink small cups of espresso, and one cup is defined as a 30 mL serving.21 There was no standardised cup size in the questionnaire for Tromsø7, and, therefore, up to each subject to define. Norwegians are used to large cups of filtered coffee, and this habit could lead to large cups of espresso as well. If one cup of ‘Norwegian’ espresso is four times the size of a cup of Italian espresso, more diterpenes will be ingested per cup of coffee.
Several additional factors influence diterpene contents, which affect all coffee brews. First, the variability of cafestol and kahweol contents in commercially roasted and ground coffee is high; they are actually blends of the two species Coffea arabica (arabica) and Coffea canephora (robusta).25 Moeenfard et al performed a literary review where arabica coffee was found to have kahweol contents between 182 mg/100 g and 1265 mg/100 g and cafestol contents between 182 mg/100 g and 1308 mg/100 g. Robusta coffee concentrations, on the other hand, were 151–363 mg/100 g and 0–20 mg/100 g, respectively.26 This shows that arabica coffee contains the highest concentrations of cholesterol-raising diterpenes. Second, an inverse relationship was found between roasting degree and the cafestol concentration in brews prepared without using a paper filter.27 Third, an inverse relationship between particle size of ground coffee beans and diterpene concentrations in espresso has been demonstrated.23 Taken together, these findings may explain why studies yield differing results.
Results regarding filtered coffee’s association with serum cholesterol vary in the literature, although it is consistently found to be less S-TC-raising than boiled/plunger coffee.6 28 29 Strandhagen and Thelle found in a controlled study that filtered coffee did raise serum cholesterol and warranted a study on the paper filter quality and physical properties of the filters.29 Two recent studies tested various types of commercially available filters, to assess whether the filter’s diterpene-retaining function varied.30 31 Cafestol and kahweol concentrations in the brews varied from 1.62 mg/L to 2.98 mg/L and 0.73 to 1.95 mg/L, respectively, and the highest concentrations were obtained using filters with micro perforations. The filters showed high fat permeability. The porosity of the paper filter and the particle size of the ground roasted coffee were determinant factors in obtaining filter coffee brews with lower diterpene contents. The variation in diterpene concentrations passing through the paper filter may be what leads to the variance in results.
The association between espresso and S-TC was stronger for men than for women. Non-physiological explanations for this could hypothetically be (1) a smaller number of women drinking larger quantities of coffee yielding non-significant results, (2) smaller cups of espresso for women than for men, (3) other brewing methods of espresso dominating (capsule, espresso machine or mocha pot) in women compared with men and (4) sex differences regarding subjective views on coffee intake may lead to different reporting in the questionnaire. Furthermore, Weggermans et al found a discrepancy between men and women in serum cholesterol response when given concentrates of cafestol and kahweol.32 The adjusted response of both S-TC and S-LDL cholesterol to cafestol was 0.22 mmol/L higher in men than in women, after adjusting for potential confounders. Lack of compliance, the menstrual cycle, contraceptives and a smaller total intake of energy could not explain the sex discrepancy, suggesting that there could be other unknown physiological sex differences explaining why men’s cholesterol metabolism responds more strongly to diterpene intake.
Previous studies regarding instant coffee found no association with increased S-TC.28 This makes sense, as instant coffee contains negligible amounts of diterpenes.33 Although there was a significant association between instant coffee and S-TC in our study, there was no linear trend within the group drinking instant coffee, and no dose–response curve.
Results regarding boiled/plunger coffee coincide with previous studies and with the same magnitude of effect,5 28 34 with a positive association and dose–response relationship between consumption and S-TC concentrations.
Interestingly, coffee contains more than a thousand diverse phytochemicals.35 The intake of each compound also depends on the variety of coffee species, roasting degree, type of brewing method and serving size. Experimental studies show that cafestol and kahweol, in addition to raising S-TC, exert multiple potential pharmacological actions such as anti-inflammatory, hepatoprotective, anti-cancerogenic, anti-diabetic and anti-osteoclastogenesic activities.36 This demonstrates how coffee contains compounds that may lead to multiple mechanisms operating simultaneously.
Strengths and limitations
The present study has several strengths, including that all inhabitants ≥40 years of age in the municipality were invited, the high attendance proportion and the broad diversity of health variables allowing to adjust for potential confounding factors. Furthermore, it captures the heterogeneous coffee habits in Northern Norway, making it possible to compare different habits in the same population. The questionnaire, clinical examinations and blood samples are executed with standardised methods and with experienced personnel. Well-known potential confounders are registered and controlled for in the statistical analysis.
However, there are study limitations that require further comments. First, some variables were self-reported, which could lead to overestimation or underestimation of the prevalence of risk factors. Second, although we adjusted for a broad variety of well-known potential confounders related to socioeconomic status and lifestyle, there could be confounding factors like diet, adding milk or sugar to the coffee, or unknown socioeconomic factors not accounted for in the analysis. Third, because of participants with missing values (n=7194), most regarding coffee consumption (n=6287), we used missing indicator methods when analysing data. Comparing the group with complete data sets and the missing group showed that the latter were older, had higher S-TC, smoked less, had fewer years of education and had a larger proportion of women. Furthermore, for women, the association between espresso consumption and S-TC was slightly stronger in complete case analysis than in the missing indicator analysis. Fourth, the cross-sectional design limits causal inference. Last, the external validity refers to Caucasian middle-aged and elderly adults and is not necessarily generalisable to other groups.
Implications
The main finding in the present study was that espresso coffee was associated with increased S-TC. Further research regarding espresso would be beneficial to review these new findings. The preferable study design would be by conducting an RCT, including standardised brewing methods, coffee beans, roasting degree, coffee particle and cup sizes. The goal should be to explore whether there is a reliable dose–response curve to diterpene intake and raise in S-TC, and whether there are sex differences.
Our findings regarding boiled/plunger coffee are the same as in the 1980s,3 4 6 pointing toward results being generalisable. This supports previous health recommendations15 37 to reduce intake of boiled/plunger coffee because of its capabilities to increase S-TC.