Carbohydrate quality and human health: A series of systematic reviews and meta-analyses

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This review, commissioned by WHO, aimed to quantify the predictive potential of markers of carbohydrate (CHO) quality in order to determine which markers are the most useful and to establish an evidence base for quantitative recommendations for intakes of dietary fibre. Previous systematic reviews and meta-analyses explaining the relationship between CHO quality and health have often examined a single marker and a limited number of clinical outcomes.

 

This review involved systematic review and meta-analyses of prospective studies and clinical trials reporting on the relationship between the most widely studied indicators of CHO quality (dietary fibre, whole grains, dietary glycaemic index, or glycaemic load) and mortality and incidence of a wide-range of non-communicable diseases and their risk factors. These included coronary heart disease (CHD), stroke, type 2 diabetes and colorectal cancer, plus the risk factors associated with these (body weight, HbA1c, total cholesterol, blood pressure). By including data from both prospective studies and clinical trials, the authors were able to explore the extent to which changes in disease risk factors associated with altering intake of dietary carbohydrate aligned with the effect of carbohydrate quality on disease risk observed in prospective studies. Dose-response curves were generated and the benefits from different amounts of total dietary fibre were calculated. GRADE criteria was used to assess the quality of evidence and the importance of the observed associations.

 

Just under 135 million person-years of data from 185 prospective studies and 58 clinical trials with 4635 adult participants were included in the analyses. Only one eligible trial of children was identified and data for this study was not included with adult data. Prospective studies and trials reporting on participants with chronic disease, weight loss trials or trials involving supplements were excluded.

 

Observational data showed that higher intakes of total dietary fibre are associated with a 15-31% reduction in the risk of specified clinical outcomes. Data from prospective studies was used to assess the dose-response relationship for total fibre and total mortality, incidence of CHD, Type 2 diabetes and colorectal cancer. The relationship was found to be linear in most cases, with no sign of plateau. When comparing the lowest fibre intakes with pre-specified ranges, the greatest benefits were observed for individuals consuming 25-29g per day. In randomised trials, higher intakes of dietary fibre reduced bodyweight and lowered cholesterol and systolic blood pressure. These findings were supported by data from cohort studies which demonstrated a reduced risk of CHD incidence, CHD mortality and type 2 diabetes incidence. The consistency between the results from trials and prospective studies, together with the dose-response relationships, indicates that the effect of dietary fibre consumption on cardiometabolic diseases is likely to causal and not a consequence of confounding factors.

 

Studies specifically examining whole grain consumption found that higher levels of consumption are associated with a 13-33% reduction in the risk of all critical outcomes. The randomised controlled trials involving an increase in consumption of whole grains showed reduction in bodyweight and cholesterol. The similar protective effects of higher intakes of whole grain foods suggest their beneficial effect may be due to their high dietary fibre content.

 

An 11% risk reduction of type 2 diabetes incidence was observed for individuals consuming low glycaemic index diets. However, this was reduced to 5% after sensitivity analysis due to high heterogeneity. Stroke mortality was lower among subjects consuming lower glycaemic index diets. The findings from prospective studies of glycaemic load were inconsistent and the results from trials showed no consistent benefits on clinical outcomes when changing the glycaemic index of a diet. Based on current evidence, dietary glycaemic index or glycaemic load might be less useful as overall measures of carbohydrate quality compared to dietary fibre and whole grain content.

 

The authors concluded that there is convincing evidence to make nutrition recommendations that replace refined grains with whole grains and increase dietary fibre to at least 25-29g per day, with additional benefits likely to accrue with greater intakes, Given that most people worldwide currently consume less than 20g of dietary fibre per day, significant reinforcement and facilitation of nutrition policy regarding fibre will be required to achieve the potential reduction in disease risk. Dietary fibre is naturally occurring in foods but can be extracted or synthesised and added to manufactured foods. The large body of evidence reported in this review relates specifically to fibre-rich foods (as most of the studies were conducted before extracted or synthetic fibre were widely used). It therefore seems appropriate that dietary advice should emphasise the benefits of naturally occurring dietary fibre in whole grains, vegetables and fruits that have been minimally processed.

 

 

Reynolds A, Mann J, Cummings J et al. Lancet 2019; 393: 434-45

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