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LETTERS TO THE EDITOR
doi: 10.3945/ajcn.116.147470.
Reply to G-C Chen et al. Dear Editor: We appreciate Chen et al.’s comments on our meta-analysis (1) with regard to protein intakes and the risk of diabetes. We agree that some cases from the Dutch and Swedish cohorts of the European Prospective Investigation into Cancer and Nutrition (EPIC) study overlapped with those from the EPIC-InterAct case-cohort study (2–4). However, the total number of diabetes cases from the Dutch and Swedish cohort studies was larger than reported for these 2 countries in the EPICInterAct study, likely because EPIC-InterAct excluded participants without suitable blood samples (2–5). Given the disparities in diabetes cases, we included all 3 studies in the meta-analysis. In addition, we conducted a sensitivity analysis to determine pooled estimates for the remaining studies after the Swedish and Netherlands EPIC cohorts were removed, and the pooled RR was 1.08 (95% CI: 1.05, 1.11), which was similar to the overall result (RR: 1.09; 95% CI: 1.06, 1.13). We also agree that expressing protein intake as a continuous variable in the Women’s Health Initiative increased the weight of this study in the meta-analysis relative to other studies that analyzed protein intake as quantiles. As per several previous meta-analyses that included studies with exposure expressed as both continuous and categorical variables (6, 7), we chose to include the Women’s Health Initiative in our meta-analysis. Nevertheless, as shown in our Supplemental Table 4, the pooled RR for total protein intake and incident diabetes for the remaining studies after removing the Women’s Health Initiative was 1.12 (95% CI: 1.05, 1.18), which is consistent with the primary results (RR: 1.09; 95% CI: 1.06, 1.13) reported in our study. Chen et al. found a nonlinear relation between animal protein intake and incident diabetes, and this relation was independent of BMI, although additional adjustment for BMI attenuated this association. Animal protein intake (energy-adjusted) was expressed in grams per day in 2 studies included in their meta-analysis, and they converted this value to percentage of energy intake from animal protein so as to perform meta-analysis of the dose-response relation between each 5% of energy intake from animal protein and incident diabetes (4, 8). However, this conversion is questionable, because protein intake was calculated as a relative energyadjusted value where energy is kept constant. Another finding of great interest was that below an animal protein intake of 12% of total energy there was no significant association between animal protein intake and
diabetes. This may reflect the fact that individuals with lower animal protein intakes also consumed more plant protein because we observed a high inverse correlation between animal and plant protein intake in the Melbourne Collaborative Cohort Study (9). Of the studies included in the meta-analysis, plant protein intake was not adjusted for in 2 studies when analyzing the association between animal protein intake and incident diabetes (4, 8). Although this might have biased the association between animal protein intake and incident diabetes, it is reasonable to conclude that higher animal protein intake was associated with higher risk of diabetes. In brief, Chen et al. added some interesting new analyses to our analysis, and the overall conclusions are consistent. The authors did not report any potential conflicts of interest relevant to this letter.
Xianwen Shang David Scott Allison M Hodge Graham G Giles Peter R Ebeling Kerrie M Sanders From the Faculty of Medicine, Dentistry and Health Sciences, Melbourne Medical School-Western Campus (XS, e-mail:
[email protected]), Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health, The University of Melbourne, Melbourne, Australia (AMH, GGG); Department of Medicine, School of Clinical Sciences at Monash Health, Monash University, Melbourne, Australia (DS, PRE); and Institute for Health and Ageing, Australian Catholic University, Melbourne, Australia (KMS).
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