Reply to G Livesey

Dear Sir: We thank Livesey for his comments on and interest in our articles. He raised concerns about the validity of our dietary data and potential discrepancies between the recent article (1) and our earlier one (2). To address the first concern, we would like to note that our methods using cumulative averages of repeated dietary measurements over time not only reduce measurement errors due to intraindividual variations over time but also improve the precision of our estimates (3). In addition, our validation study showed that using repeated measures from food-frequency questionnaires (FFQs) in prospective studies can reduce measurement errors and improve the validity of dietary measures when compared with diet records (4). Repeated measures are also more likely to represent long-term dietary intake. More precisely, in terms of carbohydrate intake, the deattenuated correlation between the 1986 FFQ and the average of six 1-wk diet records collected in 1980 and 1986 was 0.73 (5), and the deattenuated correlation between the average of the 1980, 1984, and 1986 FFQs and the average diet records was 0.78 (4). Although we did not assess validation coefficients for subgroups (eg, different alcohol intake groups), the use of repeated measures is likely to have substantially enhanced the validity of our dietary measurements, especially with a longer duration of follow-up. In addition, corrected correlations for individual carbohydrate-rich food items were found to be high (0.84 for sugar-sweetened and diet soft drinks, 0.56 for other carbonated soft drinks, 0.56 for fruit punch, 0.71 for white bread, 0.77 for dark bread, 0.66 for potatoes, and 0.79 for cold breakfast cereals) (6). Moreover, the ability of the FFQ to assess dietary glycemic index and glycemic load was documented further in a study that evaluated the relations of these 2 variables to plasma concentrations of HDL and triacylglycerol in postmenopausal women (7). On the basis of these studies, we believe that the semiquantitative FFQ methods can reasonably measure long-term dietary intakes, including dietary glycemic index and load. There is no evidence that the validity of FFQs has decayed over time. Instead, the availability of multiple dietary measurements during the course of 2–3 decades enables us to reduce measurement errors and investigate long-term dietary exposures in relation to disease outcomes. We are in the process of conducting a new validation study of the FFQ used in the Nurses’ Health Study, and the results will be available in the near future. As for the discrepancies in estimates between the current article (1) and our earlier article by Halton et al (2), the earlier article compared extreme deciles of glycemic load, whereas the present article compared extreme quintiles. For the decile comparison, the age-adjusted RR (95% CI) for glycemic load and type 2 diabetes would be 1.34 (1.21, 1.49) in the present study, which is close to 1.37 (1.18, 1.60) found in the earlier analysis. Our multivariate RR would be 1.65 (1.38, 1.97) for extreme deciles, which is somewhat lower than 2.47 (1.75, 3.47) as reported by Halton et al, but the 95% CIs broadly overlap. It should be noted that these estimates cannot be exactly the same because of different follow-up periods and age structures as well as differences in adjusted covariates. In addition, the ranges for glycemic load quintiles presented in the more recent article were derived from the 1990 FFQ, whereas the data in the Halton et al article were derived from the baseline data in 1980 (the variation in glycemic load across quintiles was ∼94 in 1990 compared with 84 in 1980). The greater contrast in glycemic load in later years was due to the expanded carbohydrate-containing foods in the FFQs since 1984 as well as the increasing carbohydrate intakes over time. Similarly, the difference in estimated intakes of total energy intake between the 2 articles was also due to changes in FFQs between 1980 and subsequent years. The age-standardized intakes of total energy, protein, and total fat were 1743 kcal, 18.9% of energy, and 31.4% of energy, respectively, in 1990, whereas the corresponding intakes were 1562 kcal, 19.1% of energy, and 38.9% of energy in 1980. The lower estimate of total energy intake in 1980 is due to the smaller number of food items in the original FFQ. It should be noted that the changes in the FFQs since 1984 have been relatively small, and thus the estimated intakes of total energy and other nutrients have remained relatively stable over time, with consideration of the aging of the cohort participants and secular trends of dietary intakes. In summary, there is sufficient evidence that our FFQs have reasonably high validity in measuring dietary intakes including glycemic index and load. The use of repeated measures over time not only reduces measurement errors but also shows long-term dietary habits. In addition, there is consistent and robust epidemiologic and clinical evidence to implicate higher dietary glycemic load in the development of type 2 diabetes, especially in populations with elevated insulin resistance.