Relative validity and reproducibility of a food frequency dietary questionnaire for use in the Italian EPIC centres

A pilot questionnaire was developed for the EPIC centres of northern Italy, and validated in the feasibility part of the cohort project. The questionnaire was self-administered and of the food frequency type with portion size estimated by means of pictures. It was structured by courses within a meal characteristic of Italian dietary habits. Dietary intake estimated by the questionnaire was compared to the corresponding estimates obtained from 8-14 24-hour recall interviews administered over a 1-year period. The reference method was validated by means of urinary nitrogen in 4-6 repeated 24-hour urine collections. One hundred and ninety-seven volunteers (47 men and 150 women) were enrolled and completed the study lasting 1 year. They filled out two questionnaires at the beginning and at the end of the study, and had a 24-hour recall interview once a month. Twenty-four-hour urine samples were collected at regular intervals. Usual intake of energy, the major nutrients and some vitamins were estimated for the questionnaires and the reference method by means of food composition tables compiled for this study. The agreement between the questionnaire and the 24-hour recalls was only good for alcohol consumption: Pearson's correlation was 0.73 and 0.77 in men and women respectively. Otherwise the relative validity of the questionnaire ranged between 0.28 for fat to 0.52 for carbohydrates in men and 0.25 and 0.50 in women for the same nutrients. The validity of the two interview methods in estimating protein intake, compared to mean urinary nitrogen was 0.24 (M) and 0.18 (W) for the questionnaire and 0.63 (M) and 0.48 (W) for 24-hour recalls. The main causes of low performance of the questionnaire were identified to be the estimated intake of dressing and cooking fats, vegetables and meat. Remedies were devised and introduced in the final version of the questionnaire currently in use in the EPIC project.