Glycemic index a valuable determinant of composite meal glycemic response

Glucose responses were not related to fat or protein content of breakfasts.

Issue: August 2006

Used in real-life settings, glycemic index is a significant determinant of the glycemic effect of mixed meals; with available carbohydrate content, it can explain approximately 90% of the variation in glycemic response, according to researchers from the University of Toronto and the University of Sydney.

Glycemic responses were not related to the fat or protein content of the test meals used, but insulin response was related to the glycemic response.

Prior studies have been conducted in this area, but the studies used fewer mixed meals and had examined the validity of glycemic load by using only individual foods. The researchers therefore conducted new experiments to ascertain whether the glycemic index and carbohydrate content of individual foods were determinants of the glycemic effect of normal breakfasts that varied in energy, fat, protein and carbohydrate content.

“Because of large between- and within-person variation of glycemic responses, the carbohydrate source and amount can not be used to predict the absolute glycemic response of a person on a single occasion,” the researchers wrote in the American Journal of Clinical Nutrition. “However, they can provide a guide to the relative glycemic effect or the rank order of the average glycemic responses elicited by different mixed meals eaten on multiple occasions over a period of time. Indeed, the present results suggest that, when properly applied, the [glycemic index] and the carbohydrate content are remarkably good predictors of the average glycemic effect of mixed meals with a range of nutrient contents.”

What’s for breakfast?

Sixteen participants in Sydney (eight men, eight women) tested six breakfast meals, and 10 patients (7 men, 3 women) in Toronto tested eight meals. Participants in Sydney had a mean age of 23; in Toronto, 26. Participants in Sydney had a mean BMI of 21.3; in Toronto, 22.4.

Participants were studied after a 10- to 14-hour overnight fast. Fasting blood samples were taken. Participants then consumed a test meal and additional blood samples were taken in 15-minute increments for two hours after they started eating.

Insulin was measured in Sydney but not in Toronto due to differences in blood measuring capacity. The Toronto group had also already demonstrated a very close correlation between plasma glucose and insulin responses, brought about from five mixed meals that varied in energy, carbohydrate, fat protein and glycemic index (P=.0002).

Thirteen of the test meals were normal breakfast meals and one was 50 g oral glucose. The 14 meals varied in energy (223 kcal to 451 kcal), protein (0 g to 17.5 g; 0% to 31% of energy), fat (0 g to 18.2 g; 0% to 52% of energy), available carbohydrate (15.5 g to 79.4 g; 28% to 100% of energy) and glycemic index (35 to 100).

Mean glucose explained

Six test meals from Sydney showed a similarity in the mean glucose (P<.001) and insulin (P<.001) predicted glycemic effect. Heterogeneity also was demonstrated in the predicted glycemic response for the eight test meals from Toronto (P<.0001).

The fat and protein contents of the test meals did not correlate with glycemic response; the amount of available carbohydrate (P=.0018) and glycemic index (P=.022) were related. Available carbohydrate and glycemic index explained 88% of the variation of mean glucose response (P<.0001).

Mean insulin glycemic response after six test meals in Sydney was correlated with mean glucose response (P=.005), glycemic load (P=.005) and available carbohydrate content (P=.013). The correlation between mean insulin glycemic response and fat, protein and glycemic index were not significant.

“The utility of glycemic index has been criticized on the grounds that the glycemic responses of foods are not closely related to their insulinemic effects,” the researchers wrote. “Our previous and current results do not support this hypothesis because the insulin response rates of [Sydney] test meals were closely correlated with their glycemic response.”

For more information:

Wolever TMS, Yang M, Zeng XY, et al. Food glycemic index, as given in glycemic index tables, is a significant determinant of glycemic responses elicited by composite breakfast meals. Am J Clin Nutr. 2006;83:1306-1312.