The Standard Format of a Food Frequency Questionnaire
At its core, a food frequency questionnaire (FFQ) is a structured, retrospective dietary assessment tool. Whether on paper or a digital form, its layout is designed to be easy for a participant to complete, focusing on three main components: a list of food items, a scale for frequency of consumption, and sometimes, a reference for portion sizes. The exact appearance can vary significantly depending on the study's specific goals and target population, with some FFQs having as few as 20 items and others including over 200.
The Food List
The food list is the central element of any FFQ. This list is not an exhaustive record of every possible food, but rather a curated selection of items chosen for their relevance to the research question and their contribution to the nutrient intake of the target population. For example, a questionnaire for a study on calcium intake might include a detailed list of dairy products, leafy greens, and fortified juices, while another focused on general health might cover broader food groups. Foods are often grouped into categories to help with navigation, such as:
- Dairy Products: Milk, yogurt, cheese (e.g., cheddar, mozzarella, cottage cheese).
- Meat, Poultry, and Fish: Beef, chicken, salmon, shellfish.
- Fruits and Vegetables: Specific fruits like apples, oranges, berries, and vegetables like broccoli, carrots, and potatoes.
- Grains: Breads (white, whole-wheat), rice, pasta, cereals.
- Fats and Oils: Butter, margarine, olive oil.
- Snacks and Sweets: Chips, candy, cookies, cakes.
- Beverages: Coffee, tea, soda, juice, alcoholic drinks.
The Frequency Columns
Adjacent to the food list are columns representing different frequencies of consumption. A respondent marks a checkbox to indicate how often they consume a particular food or beverage over a specified period, such as the past month or year. The frequency options typically range from 'never or rarely' to multiple times per day, allowing for a nuanced picture of eating habits. An FFQ might include categories like:
- Never or rarely
- 1-3 times per month
- Once per week
- 2-4 times per week
- 5-7 times per week (or once a day)
- 2-3 times per day
- 4+ times per day
The Portion Size Component
Some FFQs, known as semi-quantitative FFQs, also include information on portion sizes to enable a more detailed calculation of nutrient intake. This is often presented in one of two ways:
- Standardized Portion: The questionnaire provides a specified portion size (e.g., '1 slice of bread' or '1/2 cup of rice'), and the respondent reports how many of those standard portions they eat per frequency.
- Relative Portion: The questionnaire asks the respondent to estimate their portion size relative to a standard one (e.g., smaller than, similar to, or larger than a medium portion). Some advanced questionnaires may even include pictorial aids to help with portion size estimation.
Semi-Quantitative vs. Non-Quantitative FFQs
Different FFQs collect varying levels of detail, primarily categorized by how they handle portion size. This comparison table highlights the differences between the main types.
| Feature | Non-Quantitative FFQ | Semi-Quantitative FFQ |
|---|---|---|
| Purpose | Rank individuals by consumption frequency. | Estimate nutrient and food quantity intake. |
| Portion Size Data | Not collected. | Collected via standardized portions or relative size options. |
| Complexity | Simpler and faster to complete. | More complex, higher participant burden due to portion estimation. |
| Level of Detail | Provides a broad picture of food group frequency. | Offers more precise data for nutrient calculations. |
| Best For | Large-scale epidemiological studies focusing on dietary patterns. | Research requiring specific nutrient intake estimates. |
Completing the Questionnaire: A Step-by-Step Guide
Filling out an FFQ requires careful reflection on your long-term eating patterns. Unlike a food diary that tracks recent consumption, an FFQ relies on your memory of your usual diet over a period, typically the last six months to a year. Here are some tips on how to approach it:
- Read the Instructions Thoroughly: Pay close attention to the time frame and any specific instructions on how to mark your responses.
- Focus on Your Usual Habits: Avoid reporting on short-term diets or recent unusual circumstances. The goal is to reflect your long-term, habitual intake.
- Use the Frequency Scale as a Guide: If you eat a food sporadically (e.g., only during the holidays), average it out over the year. For example, eating a holiday item once a year would fall under the 'never or rarely' category on a 1-year FFQ.
- Consider Portion Sizes Carefully: If the FFQ asks for portion size, do your best to estimate accurately. Aids like photo booklets or household measures can help.
- Answer Every Question: Unless otherwise instructed, try to provide a response for each food item. If you truly never or rarely eat something, mark that option.
Interpretation and Use of FFQ Data
After a participant completes an FFQ, the data is coded and analyzed by researchers. Responses about frequency and portion size (if applicable) are multiplied by the nutritional content of the food items to estimate nutrient intakes. This is done using specific food composition tables and computer software. The primary use of this data is for population-level studies to investigate links between dietary patterns and health outcomes, such as disease risk. For example, researchers might use FFQ data from a large cohort to see if higher reported consumption of processed meats is associated with an increased risk of a particular disease. It is important to note that FFQs are not ideal for assessing the absolute intake of a single individual but are powerful tools for ranking dietary intake within a study group.
Conclusion
In summary, a food frequency questionnaire is a straightforward, checklist-style document comprising a specific food list, frequency checkboxes, and, for semi-quantitative versions, portion size references. It is a retrospective tool used to efficiently assess long-term, habitual dietary patterns in large populations, though it relies on memory and is prone to some reporting biases. By understanding the components of an FFQ and how to properly complete one, participants can provide valuable data for important nutritional research and public health studies. While not perfect for individual dietary precision, its simplicity and low cost make it an indispensable resource in nutritional epidemiology. For more on dietary assessment tools, see the National Cancer Institute's guide.
