Glycemic Index and Glycemic Load: What does the research tell us about disease risk?

The glycemic index and glycemic load are numbers assigned to foods to indicate the expected response of blood glucose when eaten. They are among the many characteristics used in nutrition research and dietary advice to designate carbohydrate quality and quantity.

Over the ensuing two decades, numerous population studies examined these two measures in relation to important health outcomes such as type 2 diabetes and coronary heart disease.  Yet, the nutrition community remains in debate over the relevance of the glycemic index and glycemic load of foods as strategies for the prevention and treatment of chronic diseases. As such, many health care providers remain cautious in making recommendations for their patients to make food choices based on these indices.

Two new independent studies featured in the October 2019 issue of The American Journal of Clinical Nutrition conducted meta-analyses to summarize the relationship between the glycemic index and glycemic load on health-related outcomes. The study by Parvane Saneei (Isfahan University of Medical Sciences), Ahmad Esmaillzadeh (Tehran University of Medical Sciences) and Farnaz Shahdadian (Isfahan University of Medical Sciences) examined the role of dietary glycemic index and glycemic load in the incidence of deaths from all-causes and cardiovascular disease mortality in healthy and unhealthy adults. Lu-Lu Chen (Huazhong University of Science and Technology) and colleagues reviewed the effects of dietary glycemic index on established cardiometabolic risk factors such as fasting blood glucose, body weight, and blood lipids in patients with glucose intolerance. A corresponding editorial by Walter Willett, and Simin Liu added insights to the complexities of these research findings in greater detail.

The meta-analysis by Shahdadian, et al of prospective cohort studies found no association between glycemic index or glycemic load with mortality from all-causes or deaths due to cardiovascular disease in men, but did observe a positive association of glycemic index with all-cause mortality in women. Chen and colleagues reported that among participants with impaired glucose tolerance, low glycemic index diets were effective at reducing glycated hemoglobin, fasting glucose, body mass index, total cholesterol, and low-density lipoproteins, but had no effect on fasting insulin, insulin resistance, high-density lipoproteins, triglycerides or insulin requirements.

Therefore, while a population level impact of glycemic index or glycemic load on mortality was not observed across several observational cohort studies, these aspects of carbohydrate quality may still benefit those with diabetes, which is characterized by a poor ability to control blood sugars that elevate after a meal, particularly when the diet is high in carbohydrates.

“While further research on the relation of glycemic index and glycemic load to health outcomes is desirable, available evidence strongly suggests considering them as important components of carbohydrate quality,”

Dr. Walter Willett, nutrition researcher at the Harvard T.H. Chan School of Public Health.

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