Table 1.

Advantages and disadvantage of dietary assessment methods

Dietary assessment tool	Strengths	Weaknesses
Retrospective
Dietary recalls—short-term method, where foods and drinks consumed are recalled.	- Self-reported manually or electronically. - Can be interview led, face-to-face, by phone, or online. - Typically, recall past 24 h but can be employed to recall longer durations/instances. - Multiple 24-h recalls can estimate habitual intakes. - Facilitates collection of extra information (meal timing, frequency, and location). - Flexibility of collected data is applicable to diverse research questions and analytical methods. - Limited literacy skills and cultural differences can be overcome using an interviewer. - Moderate participant burden and high compliance rates. - A skilled interviewer using multi-pass methods can prompt information, increasing accuracy.	- Habitual intake can change during the recall period. Overcome if participants are not forewarned. - Limited accuracy when recalling distant periods. - Unsuitable for subjects with memory disorders or elderly. - Items often omitted and incorrect items can be recalled. - One 24-h recall has limited accuracy, typically underestimating intake and overlooks day-to-day variation. - Moderate-to-high burden when analysing, requiring standardised protocols. - Expensive to employ face-to-face interviews in studies with large samples. - Reliance on subjects’ ability to remember portion size.
Food frequency questionnaire—retrospective methods recording frequencies of common foods over a period of time (weeks, months, years). Can be qualitative (frequency only), semi-qualitative (estimated portion size), or quantitative (portion size queried).	- Self-reported manually or electronically or interviewer led. - Useful for estimating long-term intakes retrospectively. - Low cost and participant burden, higher completion rate, applicable to large population studies. - Comprehensive questionnaires can estimate total nutrient intake if the portion size is prompted. - Can utilise short questionnaires specific to foods or nutrients pertinent to the research question. - Analysis is typically less burdensome on researchers.	- Arduous for participants if > 100 food items are queried. - Limits comparisons across cultures/countries unless comparable diets. - Typically, shorter questionnaires have less reliability and accuracy of intake. - Relies on participants’ memory, literacy, and numeracy skills. Longer periods of time reduces the accuracy of intakes. - Requires a proxy for accurate reporting in children. - Prone to misreporting, particularly with longer questionnaires. - Finite list of items included in the questionnaire. - Expensive software required to convert frequencies to nutrients.
Prospective
Food diaries—prospective methods where details of everything consumed is logged over several days. Portions can be either estimated by the subject or via photographical evidence or weighed by the subject or research assistant at the time of consumption.	- Provides detailed depiction of foods and drinks consumed, including portions. - Generates good estimates of short-term dietary intake, if conducted thoroughly. - Facilitates collection of contextual data (meal timing, location, satiety levels). - Not influenced by subjects’ memory if recorded prospectively. - Weighed provides more accurate quantitative intake, can also include ingredients and food waste. - Can be conducted via digitally or manually. - Prompts can promote the inclusion of specific foods, nutrients or occasions, pertinent to the research question and limit misreporting. - Reasonably cost-effective. - Accuracy increases with standardised protocols and analysis.	- Not applicable to retrospective studies. - High participant burden, particularly over longer durations, adding to the high participant burden of microbiome research. - Costly in time and resources for coding and analysis. - Compliance rate reduces as the duration of recording increases. - Requires sufficient literacy and numeracy skills of subject/proxy. - Heavy reliance on subjects’ perception of portions (can be improved with photographs). - Relies on trust that the diary is complete at the time of consumption and not as a recall.
Dietary checklist—prospective short-term method where specified foodstuffs are ticked from a checklist over a number of days. Can include frequencies or portion sizes. Typically used as a screening tool. Shares many strengths and weaknesses of FFQs.	- Useful for estimating dietary patterns over short periods. - Low cost. - Low participant and researcher burden. - Relatively simple coding.	- Generally, very short, cannot determine total intakes. This is of concern for microbial research as determination of effects are limited. - Cross-cultural/cross-country comparisons are limited unless diets are comparable. - Restricted to items that are listed in the instrument.
Retrospective and prospective
Diet histories—combination of multiple methods, typically 24-h recalls, food frequency questionnaires, and food diaries. More applicable in clinical settings by experienced dieticians to generate an in-depth analysis at an individual level.	- Long periods > 1 month can determine habitual intake. - Combinations of methods is ideally suited to capture accurate dietary intake during a period of interest surrounding faecal matter collection. - Facilitates assessment of meal patterns and food preparation. - Typically uses automated tools that have been adapted for self-administration.	- No standardised protocols available. - Meal based approaches is not suitable for individuals with irregular eating patterns. - High participant and researcher burden. - Requires complex analytical methods. - Expensive, as requires experienced interviewer and researcher to code data.
Novel technologies—collect and process dietary data using wearable hardware (such as sensors) and software (such as web-based programmes and mobile apps based on traditional dietary assessment tools). Many have close agreement to traditional methods, yet noticeable differences persist when comparing against the gold standard, doubly labelled water techniques [92].	- Facilitates real-time data entry and results irrespective of location. - Enhanced portion size quantitation and food waste estimating using digitally captured photos. - Reduces participant burden and increases motivation (dependent on participants’ technological ability). - Facilitates easier prompting to reduce mis-recording. - Automation of web-based recording reduces the burden on researchers and interviewers.	- Due to novelty, no validation performed to determine the quality of the technology. - Prone to similar measurement errors as traditional assessment tools. - Potential security risk using a web-based computer or mobile-technologies. - Requires participant education/training if the tool is not intuitive. - Potential high initial costs of specialist equipment and software.

This table is adapted from Nutritools [89]