Peptides affect the intake of specific nutrients and the sympathetic nervous system.

Food intake can be increased or decreased after either central or peripheral administration of peptides. Galanin, neuropeptide Y, opioid peptides, growth-hormone-releasing hormone, and desacetyl-melanocyte stimulating hormone increase food intake whereas insulin, glucagon, cholecystokinin, anorectin, corticotropin-releasing hormone, neurotensin, bombesin, cyclo-his-pro, and thyrotropin-releasing hormone reduce food intake. Many of these peptides have reciprocal effects on food intake and sympathetic activity with those peptides that stimulate food intake reducing sympathetic activity and vice versa. In addition, neuropeptide Y specifically increases carbohydrate intake. Galanin and opioid peptides on the other hand increase fat intake whereas enterostatin reduces fat intake. Glucagon decreases protein intake. The effect of peptides on specific nutrients suggests that peptides may work in part by modulating basic feeding mechanisms to lead to the selection of specific nutrients from the diet. This hypothesis might be called a nutrient-specific model of peptide-induced food intake.