Evaluating Dietary Assessment Methods in Adults with Cystic Fibrosis in the Modulator Era: A Comparison of Food Frequency Questionnaires and Three-Day Food Diaries

Background/Objectives: With advances in treatment, nutritional priorities in Cystic Fibrosis (CF) have shifted towards long-term metabolic health. However, uncertainty remains regarding whether commonly used dietary assessment methods provide comparable estimates of dietary intake during this transition. This study aims to compare and evaluate agreement between dietary intakes derived from food frequency questionnaires (FFQs) and three-day food diaries in adults with CF. Methods: Cross-sectional study of Irish adults with CF. Semi-quantitative FFQs and three-day food diaries gathered dietary data. Results: Forty-four participants were included (variant-specific therapies: 81.8%). No significant differences were observed between methods for energy, carbohydrate, protein, or total fat intake (p > 0.05). However, significant differences were identified for several nutrients and food groups. Compared with the three-day food diary, the FFQ reported higher intakes of total sugar, percentage energy from saturated fat, monounsaturated fat, polyunsaturated fat, sodium, zinc, and vitamin E, but lower intakes of fibre and vitamin D (p < 0.05). Associations between methods ranged from weak to strong, with the strongest observed for fibre (r = 0.701). There were wide limits of agreement for many dietary variables (e.g., energy: −1335.3 to 1242.3 kcal/day), despite comparable group-level estimates. Both methods identified poor adherence to dietary recommendations. Conclusions: The FFQ and three-day food diary yielded broadly similar estimates in adults with CF. However, substantial individual-level variability and poor agreement for several nutrients and food groups suggest these methods are not interchangeable for individual dietary assessment. While three-day food diaries may be preferable when detailed assessment of current intake is required, FFQs remain useful for characterising habitual dietary intake in epidemiological and population-based research.