a1 Discipline of Exercise and Sport Science, Faculty of Health Sciences, University of Sydney, Sydney, NSW, Australia
a2 School of Molecular Bioscience, Faculty of Science, University of Sydney, Sydney, NSW, Australia
a3 Boden Institute of Obesity, Nutrition, and Exercise, University of Sydney, Sydney, NSW, Australia
The purpose of the present study was to determine the influence of the glycaemic index (GI) and glycaemic load (GL) on the ratio of tryptophan (TRP) relative to other large neutral amino acids (LNAA). Ten healthy men (age 22·9 (sd 3·4) years; BMI 23·5 (sd 1·6) kg/m2) underwent standard GI testing, and later consumed each of a mixed-macronutrient (1915 kJ; 66·5 % carbohydrate (CHO), 17 % protein and 16·5 % fat) high-GI (MHGI), an isoenergetic, mixed-macronutrient low-GI (MLGI) and a CHO-only (3212 kJ; 90 % CHO, 8 % protein, 2 % fat) high-GI (CHGI) meal on separate days. The GI, GL and insulin index values (e.g. area under the curve) were largest after the CHGI meal (117, 200, 158), followed by the MHGI (79, 59, 82) and MLGI (51, 38, 56) meals, respectively (all values were significantly different, P < 0·05). After the MHGI and MLGI meals but not after the CHGI meal, TRP was elevated at 120 and 180 min (P < 0·05). After the CHGI, LNAA was lower compared with the MLGI (P < 0·05); also the rate of decline in LNAA was higher after CHGI compared with MHGI and MLGI (both comparisons P < 0·05). The percentage increase from baseline in TRP:LNAA after CHGI (23 %) was only marginally higher than after the MHGI meal (17 %; P = 0·38), but it was threefold and nearly significantly greater than MLGI (8 %; P = 0·05). The present study demonstrates that the postprandial rise in TRP:LNAA was increased by additional CHO ingestion and higher GI. Therefore, the meal GL appears to be an important factor influencing the postprandial TRP:LNAA concentration.
(Received July 12 2010)
(Revised October 29 2010)
(Accepted November 12 2010)
(Online publication February 24 2011)
p1 Present address: Research and Education Centre, ASPETAR – Qatar Orthopaedic and Sports Medicine Hospital, PO Box 29 222, Doha, Qatar.
Abbreviations: CHGI, carbohydrate-only high glycaemic index; CHO, carbohydrate; GI, glycaemic index; GL, glycaemic load; LNAA, large neutral amino acids; MHGI, mixed-macronutrient high glycaemic index; MLGI, mixed-macronutrient low glycaemic index; TRP, tryptophan