The glycaemic index of potatoes: the effect of variety, cooking method and maturity

Objective: The aim of this study was to determine the impact of variety, cooking method and maturity on the GI of potatoes, it was hypothesised that new potatoes may have a relatively lower GI. Design and subjects: Ten healthy volunteers were recruited as subjects through advertising on the campus of the University of Sydney. Equal (50 g) carbohydrate portions of eight potato meals (three varieties, four cooking methods, two states of maturity) and two reference while bread meals were fed in random order to each of the subjects over a period of 10 weeks. Capillary blood samples were taken in the fasting state and then at 15, 30, 45, 60, 90 and 120 min from the start of each meal. Samples were analysed for plasma glucose concentrations and incremental areas under plasma glucose curves were calculated. The GI of the potato was calculated as the AUC of the potato expressed as a percentage of the individual's average AUC of the white bread. This was then multiplied by 0.7 to index the GI to glucose as the reference food. Results: GI values (mean +/- s.e.m.) ranged from 65 +/- 9 (canned new potatoes) to 101 +/- 15 (boiled Desiree potatoes), glucose = 100. No significant difference was found among the three varieties of potato tested (P = 0.38) or among the four different cooking methods (P = 0.55). The GI values of the canned new potato and boiled Desiree potato were significantly different (P = 0.047). The average size of the tuber was found to correlate with the GI (r = 0.83, P < 0.05). Conclusions: Potatoes, regardless of variety, cooking method and maturity, have exceptionally high GI values. New potatoes have relatively lower GI values which is attributed to differences in starch structure.