Plain language summary
Previous trials have shown that low carbohydrate diets are beneficial for short-term weight loss. However, the long-term impact of carbohydrate restriction on mortality is less clear, with research producing conflicting results. Additionally, previous studies have not addressed the source or quality of proteins and fats consumed in low-carbohydrate diets. This study aimed to find out whether there is an association between carbohydrate consumption and mortality. It also looked at whether animal-based or plant-based foods had any impact on the association. The researchers began by studying over 15,000 adults in the US, enrolled between 1987 and 1989. At the start of the study and again six years later, participants completed food frequency questionnaires. These were used to estimate the percentage of calories they derived from carbohydrate, fat and protein. The results showed a U-shape association between overall carbohydrate intake and life expectancy, with low (less than 40% of calories from carbohydrates) and high (more than 70%) intake of carbohydrates associated with a higher risk of mortality compared with moderate intake (50-55% of calories). The researchers estimated that the average life expectancy was 4 years shorter for those with low carbohydrate consumption, and 1 year shorter for those with high carbohydrate consumption, compared to those with a moderate carbohydrate intake. However, the authors point out that since diets were only recorded at the start of the trial and six years later, participants’ diets could have changed during the 25-year follow-up period. Next, the authors performed a meta-analysis of data from eight previous studies. This revealed similar trends, with participants whose overall diets were high and low in carbohydrates having a shorter life expectancy than those with moderate consumption. In further analyses examining the source of proteins and fats, animal-derived protein and fat sources, such as lamb, beef, pork and chicken, were associated with higher mortality, whereas plant-derived protein and fat intake, from sources such as vegetables, nuts, peanut butter and whole-grains, were associated with lower mortality. The authors suggest that, when restricting carbohydrate intake, replacement of carbohydrates with predominantly plant-based fats and proteins could be considered as a long-term approach to promote healthy ageing.
Abstract
BACKGROUND Low carbohydrate diets, which restrict carbohydrate in favour of increased protein or fat intake, or both, are a popular weight-loss strategy. However, the long-term effect of carbohydrate restriction on mortality is controversial and could depend on whether dietary carbohydrate is replaced by plant-based or animal-based fat and protein. We aimed to investigate the association between carbohydrate intake and mortality. METHODS We studied 15 428 adults aged 45-64 years, in four US communities, who completed a dietary questionnaire at enrolment in the Atherosclerosis Risk in Communities (ARIC) study (between 1987 and 1989), and who did not report extreme caloric intake (<600 kcal or >4200 kcal per day for men and <500 kcal or >3600 kcal per day for women). The primary outcome was all-cause mortality. We investigated the association between the percentage of energy from carbohydrate intake and all-cause mortality, accounting for possible non-linear relationships in this cohort. We further examined this association, combining ARIC data with data for carbohydrate intake reported from seven multinational prospective studies in a meta-analysis. Finally, we assessed whether the substitution of animal or plant sources of fat and protein for carbohydrate affected mortality. FINDINGS During a median follow-up of 25 years there were 6283 deaths in the ARIC cohort, and there were 40 181 deaths across all cohort studies. In the ARIC cohort, after multivariable adjustment, there was a U-shaped association between the percentage of energy consumed from carbohydrate (mean 48·9%, SD 9·4) and mortality: a percentage of 50-55% energy from carbohydrate was associated with the lowest risk of mortality. In the meta-analysis of all cohorts (432 179 participants), both low carbohydrate consumption (<40%) and high carbohydrate consumption (>70%) conferred greater mortality risk than did moderate intake, which was consistent with a U-shaped association (pooled hazard ratio 1·20, 95% CI 1·09-1·32 for low carbohydrate consumption; 1·23, 1·11-1·36 for high carbohydrate consumption). However, results varied by the source of macronutrients: mortality increased when carbohydrates were exchanged for animal-derived fat or protein (1·18, 1·08-1·29) and mortality decreased when the substitutions were plant-based (0·82, 0·78-0·87). INTERPRETATION Both 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. FUNDING National Institutes of Health.
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