I haven't the answer, but wondered if it would be worthwhile to ask the members of ResearchGate who have been involved with this:

Hugerth, L. W., Andreasson, A., Talley, N. J., Forsberg, A. M., Kjellström, L., Schmidt, P. T., ... & Engstrand, L. (2019). No distinct microbiome signature of irritable bowel syndrome found in a Swedish random population. Gut, gutjnl-2019.

Article No distinct microbiome signature of irritable bowel syndrome...

They used questionnaires

A FFQ includes questions on a set of food and beverage items. For each food or beverage item, there may be questions asking about the following:

• Frequency of consumption (with a "never" option as well as options ranging from very infrequent to several times a day).
• Portion size information, which may be framed in terms of portion size for each round of consumption or total portion size. Respondents may be asked to translate usual consumption amount to number of specified units, such as a cup of rice. Some questionnaires include portion size images (such as photographs) in an attempt to enhance reporting accuracy.
• Other information, such as frequency of consumption at particular times of the year, or the subtype or brand that people consume.

In addition to foods and beverages, FFQs often ask about the frequency of intake and dosages of commonly consumed dietary supplements.

Dear Felicitas

Thank you.