Low Glycemic Diet Improves A1c, Other Risk Factors in Diabetes

A diet rich in vegetables and low in carbs — a so-called low glycemic index (GI) diet ― is associated with clinically significant benefits beyond those provided by existing medications for people with type 1 and type 2 diabetes, compared with a higher glycemic diet, findings from a new meta-analysis show.

"Although the effects were small, which is not surprising in clinical trials in nutrition, they were clinically meaningful improvements for which our certainty in the effects were moderate to high," first author Laura Chiavaroli, PhD, of the Department of Nutritional Sciences, Temerty Faculty of Medicine, University of Toronto, Ontario, Canada, told Medscape Medical News.

The GI rates foods on the basis of how quickly they affect blood glucose levels.

Fruits, vegetables, and whole grains have a low GI. They also help to regulate blood sugar levels. Such foods are linked to a reduced risk for heart disease among people with diabetes.

But guidelines on this ― such as those from the European Association for the Study of Diabetes ― reflect research published more than 15 years ago, before several key trials were published.

Chiavaroli and her colleagues identified 27 randomized controlled trials ― the most recent of which was published in May 2021 ― that involved a total of 1617 adult participants with type 1 or 2 diabetes. For the patients in these trials, diabetes was moderately controlled with glucose-lowering drugs or insulin. All of the included trials examined the effects of a low GI diet or a low glycemic load (GL) diet for people with diabetes over a period 3 or more weeks. The majority of patients in the studies were overweight or had obesity, and they were largely middle-aged.

The meta-analysis, which included new data, was published in The BMJ. The study "expands the number of relevant intermediate cardiometabolic outcomes, and assesses the certainty of the evidence using GRADE [grading of recommendations assessment, development, and evaluation]," Chiavaroli and colleagues note.

"The available evidence provides a good indication of the likely benefit in this population and supports existing recommendations for the use of low GI dietary patterns in the management of diabetes," they emphasize.

Improvements in A1c, Fasting Glucose, Cholesterol, and Triglycerides

Overall, compared with people who consumed diets with higher GI/GL ratings, for those who consumed lower glycemic diets, glycemic control was significantly improved, as reflected in A1c level, which was the primary outcome of the study (mean difference, -0.31%; P < .001).

This "would meet the threshold of ≥0.3% reduction in HbA1c proposed by the European Medicines Agency as clinically relevant for risk reduction of diabetic complications," the authors note.

Those who consumed low glycemic diets also showed improvements in secondary outcomes, including fasting glucose level, which was reduced by 0.36 mmol/L (-6.5 mg/dL), a 6% reduction in low-density cholesterol (LDL-C) (-0.17 mmol/L), and a fall in triglyceride levels (-0.09 mmol/L).

They also lost marginally more body weight, at -0.66 kg (-1.5 lb). Body mass index (BMI) was lower by -0.38, and inflammation was reduced (C-reactive protein, -.41 mg/L; all P < .05).

No significant differences were observed between the groups in blood insulin level, high-density lipoprotein cholesterol level, waist circumference, or blood pressure.

Three of the studies showed that participants developed a preference for the low GI diet. "In recent years, there has been a growing interest in whole-food plant-based diets, and there are more options, for example, for pulse-based products," Chiavaroli commented.

This meta-analysis should support the recommendation of the low-glycemic diet, particularly among people with diabetes, she reiterated.

Will Larger Randomized Trial Show Effect on Outcomes?

The authors note, however, that to determine whether these small improvements in intermediate cardiometabolic risk factors observed with low GI diets translate to reductions in cardiovascular disease, nephropathy, and retinopathy among people with diabetes, larger randomized trials are needed.

One such trial, the Low Glycemic Index Diet for Type 2 Diabetics trial, includes 169 high-risk patients with type 2 diabetes and subclinical atherosclerosis. The investigators are evaluating the effect of a low GI diet on the progression of atherosclerosis, as assessed by vascular MRI over 3 years.

"We await the results," they say.

The study received funding from the the Diabetes and Nutrition Study Group of the European Association for the Study of Diabetes (EASD) as part of the development of the EASD Clinical Practice Guidelines for Nutrition Therapy. The study was also supported by the Canadian Institutes of Health Research through the Canada-wide Human Nutrition Trialists' Network. The Diet, Digestive Tract, and Disease (3D) Center, which is funded through the Canada Foundation for Innovation and the Ministry of Research and Innovation’s Ontario Research Fund, provided the infrastructure for the study.

BMJ. Published August 5, 2021. Full text

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