High glycemic index associated with chronic disease outcomes
A meta-analysis of very large cohorts showed that foods with a high glycemic index were associated with T2D, CVD, diabetes-related cancer, and all-cause mortality. Diets high in fiber or whole grains showed reductions similar to those seen for low-GI foods.
This summary is based on the publication of Jenkins DJA, Willett WC, Yusuf S, et al. - Association of glycaemic index and glycaemic load with type 2 diabetes, cardiovascular disease, cancer, and all-cause mortality: a meta-analysis of mega cohorts of more than 100 000 participants. Lancet Diabetes Endocrinol. 2024 Feb;12(2):107-118. doi: 10.1016/S2213-8587(23)00344-3
Introduction and methods
Background
According to the WHO, the glycemic index (GI)—i.e., the degree to which a fixed amount of carbohydrate raises the postprandial blood glucose concentration, expressed as a percentage of consuming a standard amount of glucose—is not a relevant dietary factor in the prevention of chronic diseases [1,2]. However, a 2022 updated review of meta-analysis and observational studies suggested that a high GI, as well as a high glycemic load (GL), is associated with an increased risk of chronic diseases, such as T2D and CVD [3]. The GL is the total impact of food consumed, calculated by multiplying the GI by the total amount of carbohydrate in the food or diet.
Aim of the study
The study aim was to assess the associations between both GI and GL and the incidence of chronic disease outcomes and whether these GI and GL associations were similar to those observed with diets high in fiber or whole grains.
Methods
For this meta-analysis, the authors performed a systematic search in the Cochrane Library, MEDLINE, PubMed, Embase, Web of Science, and Scopus to identify large cohorts (≥100,000 participants each) from the Richard Doll Consortium in which the associations between either GI or GL and T2D, CVD, diabetes-related cancer, and all-cause mortality were prospectively examined. From 10 prospective cohorts (6 from the USA, 1 from Europe, 2 from Asia, and 1 international), a total of 48 GI or GL studies (34 (71%) on various types of cancer, 9 (19%) on CVD, 5 (10%) on T2D, and 3 (6%) on all-cause mortality) were selected. Associations between high-fiber and/or high–whole-grain diets and the 4 chronic disease outcomes were also examined. Data on fiber and whole-grain consumption were only included if the data on GI or GL had been measured in the same cohort, resulting in 35 fiber or whole-grain studies. Comparisons between the lowest and highest quantiles were assessed using the most adjusted model, in which in which the study authors had attempted to control their findings for all available relevant covariates at their disposal.
Outcomes
Primary endpoints were incident T2D, total CVD (i.e., CVD, CHD and MI, stroke, mortality from CVD, mortality from CHD and MI, and mortality from stroke), diabetes-related cancer (i.e., bladder, breast, colorectal, endometrial, hepatic, and pancreatic cancer, and non-Hodgkin lymphoma), and all-cause mortality.
Main results
Associations between GI or GL and chronic disease outcomes
- Consumption of high-GI foods was associated with an increased incidence of T2D (relative risk (RR): 1.27; 95%CI: 1.21–1.34; P<0.0001; heterogeneity: I²=71%), total CVD (RR: 1.15; 95%CI: 1.11–1.19; P<0.0001; I²=35%), diabetes-related cancer (RR: 1.05; 95%CI: 1.02–1.08; P=0.001; I²=23%), and all-cause mortality (RR: 1.08; 95%CI: 1.05–1.12; P<0.0001; I²=90%) compared with low-GI foods.
- High GL was also associated with T2D (RR: 1.15; 95%CI: 1.09–1.21; P<0.0001; I²=60%) and total CVD (RR: 1.15; 95%CI: 1.10–1.20; P<0.0001; I²=38%) but not diabetes-related cancer, for which nonsignificant or significant inverse associations were found, possibly due to unmeasured or uncontrolled confounding. There was no significant association between high GL and all-cause mortality.
Associations between fiber or whole-grain consumption and chronic disease outcomes
- To allow for direct comparison with the fiber and whole-grain data, GI associations were reversed and explored as lowest versus highest GI quantile for the chronic disease outcomes. Low-GI foods and a high-fiber diet showed similar associations for the outcomes individually, as well as for the 4 outcomes combined (RR for GI: 0.90; 95%CI: 0.88–0.92; RR for fiber: 0.88; 95%CI: 0.86–0.91; P=0.22).
- Comparable results were found for the direct comparison of low-GI foods versus a high–whole-grain diet (RR for GI: 0.83; 95%CI: 0.80–0.85; RR for whole grain: 0.84; 95%CI: 0.82–0.86; P=0.58).
Conclusion
This meta-analysis of 10 large cohorts (≥100,000 participants each) showed that the intake of high-GI foods was associated with the incidence of T2D, CVD, diabetes-related cancer, and all-cause mortality, whereas high GL was only associated with incident T2D and CVD. Diets high in fiber or whole grains showed reductions in these 4 chronic disease outcomes similar to associations seen for low-GI foods. According to the authors, “[t]hese findings justify the combination of GI with fiber and whole grains in dietary recommendations to reduce the risk of diabetes and related chronic diseases.”
References
1. Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet 2019; 393: 434–45.
2. WHO. Carbohydrate intake for adults and children: WHO guideline. Geneva: World Health Organization, 2023.
3. Dwivedi AK, Dubey P, Reddy SY, Clegg DJ. Associations of glycemic index and glycemic load with cardiovascular disease: updated evidence from meta-analysis and cohort Studies. Curr Cardiol Rep 2022; 24: 141–61.