Diurnal postprandial responses to low and high glycaemic index mixed meals

Glycaemic index testing is conducted in the morning, however postprandial glycaemia has a diurnal rhythm. The study aimed to evaluate the effect of glycaemic index on glucose tolerance at different times during the day.

A randomised controlled crossover study was conducted in ten healthy participants after a standardised premeal and eight hour fast. Low (37) and high glycaemic index (73) meals, matched for energy, available carbohydrate, protein and fat, were consumed at 08:00 h and 20:00 h. Blood samples were taken for 2 h postprandially.

Postprandial glucose area under curve showed effect with time of day after both meals (Low p < 0.001, High p = 0.003), and a trend (p = 0.06) to higher glycaemic responses in the evening for low glycaemic index meal. No differences were observed in insulin responses. Despite the calculated difference in meal glycaemic index little difference was observed in morning responses, but differences were seen in the evening when insulin insensitivity is increasing, the glycaemic response increase was proportionally greater for low glycaemic index meals.

Low glycaemic index foods are of less value in glycaemic control in the evening than the morning. Consuming food late in the day has a detrimental metabolic impact irrespective of glycaemic index.