The impact of liquid preloads varying in macronutrient content on postprandial kinetics of amino acids relative to appetite in healthy adults

The underlying mechanisms for the effect of proteins on appetite regulation, especially in presence of variable macronutrient composition, are not fully elucidated. The present study investigated the absorption kinetics of proteins after co-ingestion with the other macronutrients and examined the impact of circulating amino acids on appetite and satiety-related gut hormones. A randomized, within-subjects, 2-level full factorial design was implemented, where thirty six healthy subjects consumed seven preloads with similar energy density (3.1 kJ/g) and volume (670 mL) but with varying macronutrient content. The energy from protein (%) and the CHO:fat ratio were the two factors combined in three levels of 9, 24, 40 and 0.4, 2, 3.6 respectively. Blood and appetite parameters were evaluated until the serving of the ad libitum lunch after 210 min and the amino acid concentrations were measured in a subgroup of seven male subjects. The amino acid concentrations peaked at 90 min after all preloads and returned to the baseline values until 210 min. Protein intake affected amino acid profiles (P < 0.05), while no differences (P > 0.05) were detected between the two high protein preloads despite the different CHO:fat ratio (40%/0.4 CHO:fat and 40%/3.6 CHO:fat), indicating that neither carbohydrate nor fat influenced the profiles. Most of the amino acids were not related to appetite sensations or gut hormones (P > 0.05), while glutamate was positively associated with prospective consumption and inversely related to ghrelin (P < 0.05). Valine, leucine, isoleucine, lysine and α-aminobutyric acid were inversely associated with energy intake (P < 0.05). Overall, postprandial amino acid profiles were solely affected by protein content and were not consistently related to appetite regulation. Further investigation of glutamate's effect on appetite is needed.