I'm wandering if mammals that can eat many different kinds of food (omnivores) vary their preference for food not only based on the availability, but also based on dietary needs?
I'm looking at this site Food nutritional content and see that "not all foods are created equal" - the vitamin/mineral and amino acid contents can vary dramatically. Is there some part of an omnivore brain/digestive system that "monitors" the micronutrient density of digested food and adjusts food preferences towards foods that make the diet more complete?
For what concerns amino acids, mice rapidly reject meals that are not balanced in essential amino acids and continue to look for other kind of foods. This behavior is called aversion response and it is an adaptive phenomena that can be observed already 20 minutes after exposure to the unbalanced food. The mechanism involves brain sensing of uncharged tRNAs. The significance of the aversion response is to minimize depletion of essential amino acids that can not be directly synthethized but are required for protein synthesis. You can find good reviews in PubMed, for instance Gietzen et al. 2007, Annual Review of Nutrition, and Gietzen and Aya 2012, Molecular Neurobiology.