What is the validity of the food frequency questionnaire?
The validity of a food frequency questionnaire (FFQ) refers to its ability to measure what it is intended to measure: a person's typical or habitual dietary intake over a specific period, such as the past year. While FFQs are popular for their low cost and low respondent burden, their validity is not absolute and depends heavily on numerous factors, including the target population, the questionnaire's design, and the reference method used for validation.
The role of reference methods in validation
To determine an FFQ's validity, researchers must compare its results against a more accurate, or 'gold standard,' method. While weighed food records are often considered the most precise, they are also very burdensome for participants and costly. Therefore, multiple 24-hour dietary recalls (24HRs) are frequently used as the reference method in validation studies.
- Multiple 24-Hour Recalls (24HRs): This method involves trained interviewers asking participants to recall everything they ate and drank in the previous 24-hour period. Repeating these recalls on non-consecutive days helps account for day-to-day variations and improves accuracy. Validation studies comparing FFQs to 24HRs often reveal key differences. For instance, some research shows FFQs tend to overestimate intake of certain food groups, such as fruits and vegetables, while underestimating others.
- Biomarkers: In some studies, objective biomarkers are used to validate nutrient intake. For example, 24-hour urinary urea nitrogen can be measured to validate protein intake. This method provides an objective measure of intake, circumventing the recall bias inherent in self-reported dietary assessments.
Factors that influence FFQ validity
Several factors can affect the accuracy and validity of an FFQ:
- Cultural and Ethnic Differences: An FFQ developed and validated for one population cannot be reliably used in another with different eating habits. Localized food items, cooking methods, and dietary patterns must be considered. Studies conducted in countries like Lebanon or Peru emphasize the need for culturally specific FFQs.
- Portion Size Estimation: Many FFQs are semi-quantitative, requiring participants to estimate portion sizes (e.g., small, medium, or large). The accuracy of these estimates is often limited, and the concept of portion size can vary widely among individuals. Visual aids and household measuring units can help, but discrepancies remain a source of measurement error.
- Recall and Social Desirability Bias: An FFQ requires participants to recall their diet over a long period, typically the past year, which is prone to memory limitations. Participants may also overreport 'healthy' foods and underreport 'unhealthy' foods, a phenomenon known as social desirability bias.
- Food List Specificity: A shorter food list can miss important details, but a list that is too long can cause respondent fatigue and reduce accuracy. The selection of foods should prioritize major contributors to nutrients of interest and foods that explain variations between individuals.
Comparison of dietary assessment methods
| Feature | Food Frequency Questionnaire (FFQ) | 24-Hour Dietary Recall (24HR) | Weighed Food Records (WFR) |
|---|---|---|---|
| Timeframe | Habitual intake (long-term: months to a year) | Recent intake (short-term: past 24 hours) | Actual intake (specific period: days) |
| Cost & Burden | Low cost, low respondent burden | Medium cost, medium respondent burden | High cost, high respondent burden |
| Key Strength | Captures long-term dietary patterns; ideal for large epidemiological studies | Captures detailed, specific food and nutrient data; less reliant on long-term memory | Provides highly accurate, quantitative data on intake |
| Primary Weakness | Heavily reliant on memory; potential for recall and social desirability bias; challenges with portion size estimation | Multiple days required to capture usual intake; relies on memory for 24-hour period; resource-intensive | High participant burden can alter eating habits; requires high literacy and motivation |
Can validity be improved?
Despite its limitations, the FFQ's validity can be strengthened through several methodological approaches:
- Population-Specific Validation: FFQs must be developed and validated for the specific population they intend to study. This involves tailoring the food list and portion sizes to local dietary habits, as shown in studies conducted in the Peruvian Amazon and Fujian Province, China.
- Combining Methods: Using a combination of dietary assessment methods, such as FFQs with a subset of 24HRs or biomarkers, can provide a more comprehensive picture of diet while mitigating individual method weaknesses. This can help calibrate results and correct for measurement error.
- Interviewer-Administered Surveys: For populations with lower literacy rates or older adults with cognitive decline, having a trained interviewer administer the FFQ can improve data quality and reduce reporting bias.
- Optimized Design: Improvements in the questionnaire's design, such as using visual aids for portion sizes or incorporating technology like optical scanners, can enhance the accuracy of collected data.
Conclusion
The validity of the food frequency questionnaire is not a universal constant but a relative measure determined by the specific context of its use. While FFQs offer a practical and cost-effective way to assess long-term dietary patterns in large studies, they are less accurate for measuring absolute intake due to inherent biases. Their primary strength lies in their ability to rank individuals relative to one another, which is essential for investigating diet-disease relationships in nutritional epidemiology. By carefully developing and validating FFQs for specific populations and understanding their limitations, researchers can make meaningful contributions to public health research and interventions.
