Dietary carbohydrate intake and mortality: reflections and reactions – Authors' reply
Authors: Sara B Seidelmann, Aaron R Folsom, Eric B Rimm, Walter C Willett, Scott D Solomon
Journal: The Lancet Public Health (2018)

We appreciate the interest in our work. We found that in long-term observational studies, low carbohydrate dietary intake (<40% of total energy from carbohydrates) was associated with higher mortality when animal-based fat and protein were substituted for carbohydrate.1Seidelmann SB Claggett B Cheng S et al.Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis.Lancet Public Health. 2018; 3: e419-e428Summary Full Text Full Text PDF PubMed Scopus (370) Google Scholar However, when plant-based sources of fat and protein such as whole grains, legumes, and nuts are incorporated into low carbohydrate diets, they were associated with lower mortality. We fully acknowledge the limitations of observational studies, while recognising the difficulties associated with long-term randomised trials in the area. For example, the large Women's Health Initiative trial was largely uninformative because of poor adherence to the assigned diets as indicated by lack of difference in biomarkers sensitive to dietary fat.2Willett WC The WHI joins MRFIT: a revealing look beneath the covers.Am J Clin Nutr. 2010; 91: 829-830Crossref PubMed Scopus (24) Google Scholar We agree that our findings should be considered within the context of previous work, including both observational data3Micha R Penalvo JL Cudhea F Imamura F Rehm CD Mozaffarian D Association between dietary factors and mortality from heart disease, stroke, and type 2 diabetes in the United States.JAMA. 2017; 317: 912-924Crossref PubMed Scopus (580) Google Scholar, 4Satija A Bhupathiraju SN Spiegelman D et al.Healthful and unhealthful plant-based diets and the risk of coronary heart disease in U.S. Adults.J Am Coll Cardiol. 2017; 70: 411-422Crossref PubMed Scopus (326) Google Scholar and randomised trials,5Li SS Blanco Mejia S Lytvyn L et al.Effect of plant protein on blood lipids: a systematic review and meta-analysis of randomized controlled trials.J Am Heart Assoc. 2017; 6: e006659Crossref PubMed Scopus (58) Google Scholar and these strongly support better blood lipid patterns and lower risks of cardiometabolic disease and premature death with consumption of plant-based foods compared with animal-based foods. It is also essential to recognise that our study assessed long-term dietary patterns and outcomes in a predominantly healthy community population; we did not study prescribed diets to treat obesity or specific diseases. We also did not evaluate very-low carbohydrate diets (5–10% of energy) because so few participants in a general population consume such diets: the available evidence on these diets is limited to short-term studies, many of them without a control group. We provided additional details on regression and quintile results in our Article's appendix.1Seidelmann SB Claggett B Cheng S et al.Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis.Lancet Public Health. 2018; 3: e419-e428Summary Full Text Full Text PDF PubMed Scopus (370) Google Scholar As we noted, the food frequency questionnaire used in the Atherosclerosis Risk in Communities study was designed to capture eating patterns over the course of a year, and a similar questionnaire has been formally validated6Willett WC Sampson L Stampfer MJ et al.Reproducibility and validity of a semiquantitative food frequency questionnaire.Am J Epidemiol. 1985; 122: 51-65Crossref PubMed Scopus (3524) Google Scholar by comparison with weighed and measured food intake. Although that questionnaire underestimated total energy intake, adjustment of specific nutrients for it provides values with good validity and that have predicted biomarkers of diet and disease risks in hundreds of published studies. Recall of diet inevitably includes some error; however, bias with respect to the outcome was avoided by the prospective design. We observed no association between carbohydrate intake and change in bodyweight at 3-year and 6-year intervals, as noted in table 1 of our Article,1Seidelmann SB Claggett B Cheng S et al.Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis.Lancet Public Health. 2018; 3: e419-e428Summary Full Text Full Text PDF PubMed Scopus (370) Google Scholar which is similar to the overall results of randomised trials. We adjusted for waist-to-hip ratio with no meaningful difference in the overall association, as did many of the studies included in our Article's meta-analysis. We did not exclude participants with heart disease, diabetes, and stroke from the primary analysis, but this exclusion was performed in the sensitivity analysis provided in the appendix. Finally, we acknowledge the problems of residual confounding, as with any observational data. In conclusion, we reiterate that it is not enough to focus on carbohydrates alone, but to consider the types of food replacing them. This area is important for further research. We declare no competing interests. Dietary carbohydrate intake and mortality: reflections and reactionsI read Sara Seidelmann and colleagues study1 with interest, and there are several shortcomings that deserve attention. Although, the authors appropriately acknowledge the limitations of data collection by questionnaire, in their study, participants were expected to recall their food intake over 25 years, in detail, over two sessions. Full-Text PDF Open AccessDietary carbohydrate intake and mortality: a prospective cohort study and meta-analysisBoth high and low percentages of carbohydrate diets were associated with increased mortality, with minimal risk observed at 50–55% carbohydrate intake. Low carbohydrate dietary patterns favouring animal-derived protein and fat sources, from sources such as lamb, beef, pork, and chicken, were associated with higher mortality, whereas those that favoured plant-derived protein and fat intake, from sources such as vegetables, nuts, peanut butter, and whole-grain breads, were associated with lower mortality, suggesting that the source of food notably modifies the association between carbohydrate intake and mortality. Full-Text PDF Open AccessDietary carbohydrate intake and mortality: reflections and reactionsEven if the Food Frequency Questionnaire had been robust and accurately reflective of what people had eaten during the whole 25-year study, a mean calorie intake of 1560–1660 kcal per day had been explained, and people had been allocated correctly to groups that reflected their actual carbohydrate consumption after a health diagnosis. Full-Text PDF Open AccessDietary carbohydrate intake and mortality: reflections and reactionsThe Article by Sara Seidelmann and colleagues1 presents the analysis from the Atherosclerosis Risk in Communities study of adults aged 45–64 years over a median duration of 25 years, at six timepoints. The mean caloric intake by each adult was 1558–1655 kcal per day. I have several issues with this study. First, this analysis seems to ignore the fact that, after 25 years, participants are aged 70–89 years, which is older than the 2016 estimate of life expectancy by the US Centers for Disease Control and Prevention: 78·6 years. Full-Text PDF Open AccessDietary carbohydrate intake and mortality: reflections and reactionsThere are several problematic issues in the Article by Seidelmann and colleagues.1 First, the quality of the dietary data is weak, since they are based only on two samples. The food frequency questionnaire that they used appears not to have been independently validated, and it contained only 66 questions and excluded several popular items, such as pizza.1 Food was clearly underreported, as evidenced by a reported average energy intake of roughly 1500 kcal per day. Full-Text PDF Open AccessDietary carbohydrate intake and mortality: reflections and reactionsIn their Article in The Lancet Public Health (September 2018), Sara Seidelmann and colleagues1 use data from a long-term prospective cohort study and do a meta-analysis of other related cohorts to assess the association between carbohydrate intake and mortality. Our main problem with this Article1 was regarding the confounders that are related to an individual's consumption of carbohydrates and to that individual's associated mortality. Some of these confounders were included as control variables, but since it is impossible to account for all possible confounders, some confounders necessarily went unobserved. Full-Text PDF Open Access