A Cochrane review [Abstract] 1 included 11 studies (lasting 1 to 12 months) with a total of 402 subjects. 10 studies compared a low glycaemic index (low-GI) diet with a higher glycaemic index diet and 1 study compared the low-GI diet to a diet using measured carbohydrate exchanges. 3 studies had participants with type 1 diabetes, 7 with type 2 diabetes and 1 study had participants with either type 1 or type 2 diabetes. 2 studies involved children, all of whom had type 1 diabetes.
Pooled data from studies reporting glycated haemoglobin A1c (HbA1c) with participants whose glycated haemoglobin was not yet optimised showed that low-GI diets significantly decreased glycated haemoglobin A1c (HbA1c) levels, indicating improved glycaemic control (WMD -0.5 % HbA1c, 95% CI -0.8 to -0.2; 6 studies). 2 studies reported on hypo- or hyperglycaemic events. Hypoglycaemic episodes significantly decreased with low-GI diet compared to high glycaemic index diet (difference of -0.8 episodes per patient per month, P < 0.01; 1 study). In the second study, the control diet was a measured carbohydrate exchange diet in children with type 1 diabetes and there was no difference reported in hypoglycaemic episodes. Proportion of participants reporting more than 15 hyperglycaemic episodes per month was lower for low-GI diet compared to measured carbohydrate exchange diet (35% versus 66%, P = 0.006; 1 study). No study reported on mortality, morbidity or costs.
A meta-analysis 2 included 6 RCTs with type 2 diabetes. In 2 studies, the improvement in HbA1c was higher with low-GI diet (difference -0.5%, 95% CI -0.61% to -0.39%) than with high-cereal fibre diet (difference -0.18%, 95% CI -0.29% to -0.07%); or with low-GI legume diet (difference -0.5%, 95% CI -0.6% to -0.4%) vs high-wheat fibre diet (difference -0.3%, 95% Cl -0.4 to -0.2%). Results in HbA1c favoured low-GI-diet compared to control (mean difference -0.22, 95%CI -0.31 to -0.13, 5 trials, n=574).Comparint the effect of the low-GI diet vs higher-GI diet or control on fasting blood glucose, the meta-analysis favoured the low-GI diet (mean difference -6.59, 95% CI -12.12 to -1.05, 3 trials, n=241). However, the differences were not significant in the sensitivity test (MD -5.42, -12.79to 1.95, p = 0.15).
Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment and lack of reporting on blinding of outcome assessors).
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