Reproducibility measures and their effect on diet-cancer associations in the Boyd Orr Cohort

OBJECTIVES: To quantify measurement error in the estimation of family diet intakes using 7-day household food inventories and to investigate the effect of measurement-error adjustment on diet-disease associations. DESIGN AND SETTING: Historical cohort study in 16 districts in England and Scotland, between 1937 and 1939. SUBJECTS: 4999 children from 1352 families in the Carnegie Survey of Diet and Health. 86.6% of these children were traced as adults and form the Boyd Orr cohort. The reproducibility analysis was based on 195 families with two assessments of family diet recorded 3-15 months apart. METHODS: Intraclass correlation coefficients (ICCs) were calculated for a variety of nutrients and food groups. Diet-cancer associations reported previously in the Boyd Orr cohort were reassessed using two methods: (a) the ICC and (b) the regression calibration. MAIN RESULTS: The ICCs for the dietary intakes ranged from 0.44 (beta carotene) to 0.85 (milk and milk products). The crude fully adjusted hazard ratio (HR) for cancer mortality per 1 MJ/day increase in energy intake was 1.15 (95% CI 1.06 to 1.24). After adjustment using the ICC for energy (0.80) the HR (95% CI) increased to 1.19 (1.08 to 1.31), and the estimate from regression calibration was 1.14 (0.98 to 1.32). The crude fully adjusted odds ratio (OR) for cancer incidence per 40 g/day increase in fruit intake was 0.84 (95% CI 0.73 to 0.97). After adjustment using the fruit ICC (0.78) it became 0.81 (0.67 to 0.96) and the OR derived from regression calibration was 0.81 (0.59 to 1.10). CONCLUSIONS: The diet-disease relationships for the dietary intakes with low measurement error were robust to adjustment for measurement error.