To evaluate the representativeness of our validation study sample, we compared distributions of selected baseline characteristics for the 195 substudy participants to the remainder of the CTS cohort (age ≤85 years) (; see also ref 15
). Substudy participants were similar to the entire cohort in terms of age, body mass index, physical activity, and broad dietary composition (i.e., average daily caloric intake and the contribution of fat and fruits and vegetables to the diet). Menopausal status at baseline was also comparable for substudy participants compared to the remainder of the cohort (50% vs. 53% being postmenopausal; p=0.61) [15
]. However, substudy participants were more likely than the remainder of the cohort to be Latina (8% vs. 4%) and less likely to be African American (1% vs. 3%) or white (84% vs. 88%), reflecting the racial/ethnic composition of the substudy area compared to the state as a whole.
Selected baseline characteristics of participants in the validation substudy and the California Teachers Study cohort.
presents the variation in average daily intake of total calories, macronutrients, fiber, selected micronutrients, and alcohol as measured by the pre- and post-study FFQs and the average of the four 24-hour recalls. Usual intakes during the previous year, as measured by FFQ at the beginning and at the end of the 10-month study period, were generally constant, and correlations reflecting the FFQ’s reproducibility for these nutrients were generally high (ranging from rSF=0.60 for oleic acid to rSF=0.87 for alcohol). The average of all reproducibility correlations was 0.71. Correlations quantifying the validity of the FFQ assessment compared to the average of the four 24-hour recalls were low for caloric intake, linoleic acid, and oleic acid (rP=0.14 to 0.44) but moderate to high for all other nutrients (ranging from 0.57 for protein and vitamin A to 0.85 for β-carotene). The average of all validity correlations was 0.63; excluding caloric and oleic and linoleic acid intake, the average correlation was 0.71. Correlations between the recalls and both the pre-study and post-study FFQs were generally similar for all nutrients.
Reproducibility, validity and precision of the food frequency questionnaire (FFQ) dietary assessment used in the California Teachers Study.
Multiple 24-hour dietary recalls can be used as a gold standard of dietary assessment, under the assumption that each gives an independent and unbiased estimate of true nutrient exposure. For individual nutrients, the number of recalls needed to achieve the same degree of measurement precision represents a useful metric for evaluating a FFQ; the higher the number of recalls a FFQ represents, the greater its precision for estimating the intake of a given nutrient. For most nutrients, estimates obtained from our FFQ corresponded to approximately two to six recalls (). Dietary fiber, saturated fat, vitamin A and vitamin C were measured particularly well by our FFQ as they were comparable to more than four recalls. As reflected by the low validation coefficients, our FFQ captured caloric and linoleic and oleic acid intake less well. For each, the FFQ assessment was worth less than one recall.
To understand whether the performance of the FFQ differed by selected characteristics of the participants (i.e., race/ethnicity, age, BMI, physical activity, and HT use), we examined the slope of the calibration equation for each dietary factor (). There was a significant relationship between age and the slope for percent of calories from fat, however, the trend was not monotonic. For white women with a BMI<25 who did not exercise or use HT, the slopes for ages <50, 50–64, and ≥65 years were 0.97 (= 0.78 + 0.19, i.e., as shown in , the baseline slope estimate + the estimate for women age <50), 0.36, and 0.78, respectively, evidence that the FFQ captured intake less well for women age 50–64 compared to either younger or older women. Similarly, the slope for β-carotene differed by participant age; BMI modified performance of the FFQ for fiber, vitamins C and D, and alcohol; physical activity influenced the estimates for saturated fat (data not shown), vitamin D, and folate; and the slopes for fiber and alcohol differed by HT use.
Expanded calibration model for dietary components by participant demographic and lifestyle factors; the estimates represent the amount of the change in baseline slope (and the standard error).
presents the uncorrected and corrected relative risks for the association between dietary intake and postmenopausal breast cancer risk in the study cohort. For all nutrients except linoleic acid, the corrected risk estimates were within 13% of the uncorrected estimates. The only dietary factor significantly associated with breast cancer risk was alcohol (uncorrected RR=1.25 per 20g/1000kcal/d, 95% CI: 1.10–1.42). Correcting for measurement error provided a relative risk of 1.36 per 20g/1000kcal/d (95% CI: 1.03–1.51) for this association. When simultaneous adjustment for measurement error in caloric intake, alcohol consumption, and each nutrient of interest was made using the methods described by Rosner et al.[19
] and Spiegelman et al.[6
] corrected relative risks of similar magnitude were observed (data not shown).
Uncorrected and correcteda relative risks for the relationship between specific nutrientsb and postmenopausal breast cancer risk in the California Teachers Study cohort.