High protein diets suppress appetite and facilitate weight loss, but are difficult to adhere to. Understanding the mechanisms by which protein suppresses appetite may establish targets for more acceptable interventions to treat or prevent obesity. Of particular interest is the concept of functional foods or novel products, which aim to potentiate satiety.
Receptor systems that respond to amino acids have been identified. However, the specific mechanisms regulating protein-induced satiety are unknown. Previous work has investigated the effect of specific amino acids which act as ligands for the following G-protein coupled receptors: CaR, T1R1/T1R3 and GPRC6A on food intake in rodents.
L-cysteine activates the CaR, the T1R1/T1R3 and the GPRC6A. A diet that includes high levels of whey protein, which contains high levels of L-cysteine, has been reported to be more satiating, and to suppress circulating levels of the orexigenic hormone ghrelin to a greater extent, than other types of protein in humans. Pilot studies suggested that ligands for the GPRC6A receptor can reduce food intake in rodents, and that this effect is at least partly mediated by a reduction in circulating ghrelin levels.
The role of L-cysteine in food intake in humans was thus investigated. Following an overnight fast, healthy volunteers were given either oral L-cysteine (0.04 or 0.07 g/kg), glycine (0.04 or 0.07 g/kg) or vehicle control in a double-blinded randomised manner. Following administration of the amino acid or control, visual analogue scales were completed and gut hormone analysis carried out every 15 min over a 2.5-h period. Results showed that oral administration of L-cysteine significantly reduced feelings of hunger (P<0.05) and reduced circulating ghrelin levels (P<0.05) compared to vehicle or glycine controls. These data suggest that L-cysteine may reduce appetite, and this effect may be mediated by a reduction in ghrelin.