A Cochrane review 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.
Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment and lack of reporting on blinding of outcome assessors).