This study aimed to determine the contributions of basal excess glycaemia (BEG) and prandial excess glycaemia (PEG) to overall excess glycaemia in type 2 diabetes (T2D) patients treated with metformin alone.

Outpatients with T2D treated with metformin alone (n=46) who underwent continuous glucose monitoring (CGM) were divided into tertiles according to glycated haemoglobin (HbA_1c) levels. For each CGM trace, the glucose area under the curve (AUC)>5.5mmol/L was expressed as the AUC_overall, representing overall excess glycaemia. The sum of glucose AUCs above the premeal glucose level at 4h after breakfast, lunch and dinner was expressed as the AUC_peg, representing PEG. The contribution of PEG to overall excess glycaemia was calculated as (AUC_peg/AUC_overall)×100%. The contribution of BEG was calculated as [(AUC_overall−AUC_peg)/AUC_overall]×100%. Factors related to PEG contribution were also analysed.

BEG constituted more than half the overall excess glycaemia in all HbA_1c tertiles. The contribution of PEG was negatively correlated with HbA_1c and mean glucose values before each meal. Prebreakfast and predinner glucose values were the dominant factors affecting PEG contribution and was independent of HbA_1c.

In patients treated with metformin alone, BEG was the major contributor to excess glycaemia at HbA_1c levels ≥7.7%, while PEG and BEG contributions were similar and stable below this level. For HbA_1c levels ≥7.7%, add-on therapy to metformin should preferentially target control of BEG, whereas targeting both BEG and PEG could be of equivalent importance with lower HbA_1c levels.