Dietary carbohydrate intake and mortality: reflections and reactions
Authors: Marc F Bellemare, Amelia B Finaret
Journal: The Lancet Public Health (2018)

In their Article in The Lancet Public Health (September 2018), Sara Seidelmann and colleagues1Seidelmann 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 (354) Google Scholar 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 Article1Seidelmann 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 (354) Google Scholar 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. This problem is illustrated in the figure. In any observational study, unobserved confounders prevent the identification of a causal relationship between carbohydrate intake and the risk of associated mortality. In this context, any statistical association between these two variables cannot be argued to be causal, no matter the level of statistical significance. What could be an unobserved confounder? One example could be whether individuals tended to have highly variable bodyweight due to extreme diets (ie, diets that are too low or too high in carbohydrate intake), which was not observed in this study. Such variability in bodyweight is associated with an increased risk of diabetes and increased cardiometabolic risk factors.3Delahanty LM Pa Q Jablonski KA et al.Effects of weight loss, weight cycling, and weight loss maintenance on diabetes incidence and change in cardiometabolic traits in the Diabetes Prevention Program.Diabetes Care. 2014; 37: 2738-2745Crossref PubMed Scopus (75) Google Scholar, 4Dulloo AG Montani JP Pathways from dieting to weight regain, to obesity and to the metabolic syndrome: an overview.Obes Rev. 2015; 16: 1-6Crossref PubMed Scopus (86) Google Scholar Without a variable that measured whether a respondent dieted in this way (and there is no indication that Seidelmann and colleagues controlled for this confounder), the estimated association between carbohydrate consumption and related mortality could be biased. Hence, instead of concluding that diets that are either too low or too high in carbohydrates cause a higher risk of associated mortality, in fact, bodyweight variability due to extreme diets would be responsible. Some of our concerns could be easily assuaged with the inclusion of more information about the analysis, including the regression results for the Atherosclerosis Risk in Communities study itself instead of merely presenting only the adjusted odds ratios of interest and one figure. Additionally, robustness checks on the results could be presented, including falsification tests.5Prasad V Jena AB Prespecified falsification end points: can they validate true observational associations?.JAMA. 2013; 309: 241-242Crossref PubMed Scopus (174) Google Scholar With the increase in computing power, decreasing costs of data storage, and increased use of big data in the health sciences,6Murdoch TB Detsky AS The inevitable application of big data to health care.JAMA. 2013; 309: 1351-1352Crossref PubMed Scopus (1080) Google Scholar we would have expected additional analyses to support the findings. MFB reports personal fees from PepsiCo. ABF declares 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 AccessHigh ambitions for health equity in the AmericasOn Oct 1, 2019, the Pan American Health Organization (PAHO) published Just Societies: Health Equity and Dignified Lives, the report of the Commission on Equity and Health Inequalities in the Americas—a Commission established by PAHO , chaired by Sir Michael Marmot, and tasked to analyse the effect of drivers influencing health, while proposing actions to address health inequalities in the region. 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 reactions – Authors' replyWe 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.1 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. Full-Text PDF Open Access