Background Associations of coffee consumption with multiple health outcomes have been researched extensively. Coffee consumption, usually reported in cups a day, is a heterogeneous measure due to numerous preparation methods and cup sizes, leading to misclassification. This paper develops a new ‘unit’ measure of coffee and uses coffee consumption data from a representative sample of the UK population to assess misclassification when cup volume and preparation type are not taken into account.
Methods A coffee unit measure was created using published estimates of caffeine and chlorogenic acid concentrations, and applied across volumes and preparation types. Four-day food diary data in adults from the UK National Diet and Nutrition Survey (NDNS; 2012–2016) were used to quantify coffee intake. Participant self-reported cups a day were compared with cups a day standardised by (a) 227 mL volume and (b) 227 mL instant coffee equivalents (unit measure), and the degree of misclassification was derived. Sensitivity analyses were conducted to model coffee drinking preferences of different populations and caffeine:chlorogenic acid weighting assumptions of the unit measure.
Results The NDNS sample consisted of 2832 adult participants. Coffee was consumed by 62% of participants. Types varied, with 75% of caffeinated coffee cups being instant, 17% filter, 3% latte, 2% cappuccino, 2% espresso and <1% other types. Comparing reported cups to volume-standardised cups, 84% of participants had correct classification, and 73% when using the coffee unit measure, 22% underestimated and 5% overestimated, largely by one cup. Misclassification varied by gender, age and income. Sensitivity analysis highlighted the benefits of using the unit measure over volume alone to cater for different populations, and stability of the unit composition assumption.
Conclusion Cup volume and preparation type should be taken into account, through the application of a standardised coffee unit measure, when coffee consumption is classified in future research studies.
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Contributors RP conceptualised and created the coffee unit measure, extracted coffee drinking data, performed the analysis and wrote the first draft of the manuscript; SE advised on aspects of the analysis and revised the manuscript; JP conceptualised the study and revised the manuscript; JAF provided comments and revised the manuscript; and PR conceptualised the study, revised the manuscript and is the guarantor.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests JP reports personal fees from Siemens Healthineers, outside the submitted work; JAF reports personal fees and other from Novartis, personal fees from Merck Sharp & Dohme, grants from GlaxoSmithKline, grants from Intercept Pharmaceuticals, personal fees from Galecto Biotech and personal fees from Gilde Healthcare, outside the submitted work.
Patient consent for publication Not required.
Data availability statement National Diet and Nutrition Survey data used in this study are available via the UK Data Service.
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