TABLE 4.

Challenges and recommendations in dietary assessment of (poly)phenol intakes¹

Challenges	Recommendations/resources needed
Dietary assessment tool not designed to capture (poly)phenol diet sources and variabilities	1) Choose a tool that covers the food sources of target compounds, and has foods with different (poly)phenol profiles differentiated2) Consider the frequency and timing of measurement to make sure the target time period is represented
	3) Use multiple measurements of dietary records rather than FFQs if possible
Dietary assessment methods not validated/insufficiently validated to measure (poly)phenol intakes	1) Validate the tool specifically for measuring the intake of target (poly)phenols2) Use other well-established dietary assessments and established biomarkers as reference methods3) Conduct multiple statistical analysis to reflect validation status: correlation coefficients, cross-classification (Cohen's κ), Bland-Altman
	4) Provide evidence of validity and reproducibility
Limited data on (poly)phenol content in foods	1) Choose a database that covers the content data of all food sources of the target compound; combine different sources of data to make up the limitations of single databases
	2) Choose databases of high quality: with reliable analytical methods and data source, and consistent data between multiple sources; use data from comparable analytical methods if need to summarize the total
	3) Choose the data that can match up with the food item in the measured diet, in terms of food origin and species; apply food-processing yield factors if applicable
	4) Check the updates of the database and search for newly published data if possible
	5) Use standard recipes that can reflect the diet in target population
Insufficient reporting on methods	1) Follow STROBE-nut framework (21)
	2) Describe the dietary assessment methods used in detail: food groups and number of items measured, whether similar foods are distinguished in items; how the assessment was conducted, time range measured, and validation of the methods
	3) Report clearly whether the dietary assessment method is validated for targeted (poly)phenols; if it is validated, describe the reference method used including sample size and characteristics of the population, how the reference method was conducted, statistical analysis methods used and validity/reproducibility results; if biomarkers are used to validate the dietary assessment, report details of the biomarkers and analytical methods applied
	4) Report the name of the database used or cite the reference paper; describe the analytical method used to get the food content data and whether compounds were measured individually or in aglycones; report the retention factors used
	5) Report how food items were matched, how missing items and missing compound values were analyzed, and the adjustment made on the intake amount

¹FFQ, food frequency questionnaire; STROBE-nut, Strengthening the Reporting of Observational Studies in Epidemiology—Nutritional Epidemiology.