Salted food preferences in rats (Rattus norvegicus): responses to liquid milk products

Sprague-Dawley rats do not prefer salted (NaCl) solid food when it is presented with its unsalted counterpart but prefer isotonic saline when it is presented with plain water. Preferences in liquid foods were examined by giving rats one-hour choices between various salted liquid milk products and their unsalted counterparts. When heavy cream and half and half were tested, rats consumed either equal amounts of the salted and unsalted food, or more of the unsalted food. The absence of salt preference in heavy cream and half and half was evident using other testing procedures and, with heavy cream, additional salt concentrations. In contrast, rats consumed more skin milk containing 0.150 M NaCl than plain skim milk. The role of fat content in modulating the expression of these salt preferences was examined in tests using whole milk and low-fat milk, both of which have water, protein and carbohydrate contents similar to skim milk, but a greater fat content. Unlike skim milk, preferences for 0.150 M NaCl in whole milk and low-fat milk were approximately 50%. The role of solidity in salted food preferences was also investigated by adding gelatin to salted (0.150 M NaCl) and unsalted skim milk. Whereas the rats ate equal quantities of salted and unsalted gelled skim milk, they preferred salted skim milk when it was a liquid. Results from these experiments suggest that several cues contained within food influence the expression of rats' salt preferences.