Objective: Increasing recognition of the potential importance of phytochemicals in the aetiology of cancer and heart diseases has highlighted the need for methods to measure individual phytochemical consumption that are sufficiently simple to be used in large epidemiological studies and whose reproducibility and accuracy have been quantified. d-Limonene is a natural component of a variety of foods and beverages and is found mainly in citrus fruits. However, d-limonene is not assessed by any nationally available analysis database.
Design: We designed our study to assess the d-limonene content of different citrus juices and beverages and to develop a dietary assessment instrument to measure consumption of citrus foods (fruit, juice and peel) and d-limonene intake and test it for reliability.
Subjects and methods: A total of 120 citrus juice samples were analysed and used to develop the preliminary d-limonene database. A self-administered citrus food-frequency questionnaire was developed and administered twice to participants, separated by a 2-month interval. The questionnaire was tested for reproducibility of estimates of citrus food consumption and d-limonene intake among 120 participants.
Results: Correlation coefficients between the two administrations of the questionnaire ranged from 0.50 for citrus peel use to 0.82 for orange juice. Mean intakes (range) of d-limonene from citrus juices among consumers were 13.0 (0.24-141.9) mg day-1 and 13.2 (0.07-83.9) mg day-1 (r = 0.60, P < 0.001).
Conclusion: The citrus frequency questionnaire developed in this study provided highly reproducible estimates of citrus foods, citrus peel and d-limonene intakes. This instrument may be a useful tool in studies of the associations between citrus peel use, d-limonene intake and risk for chronic disease.