Protein Appetite at the Interface between Nutrient Sensing and Physiological Homeostasis

Abstract

Feeding behavior is guided by multiple competing physiological needs, as animals must sense their internal nutritional state and then identify and consume foods that meet nutritional needs. Dietary protein intake is necessary to provide essential amino acids and represents a specific, distinct nutritional need. Consistent with this importance, there is a relatively strong body of literature indicating that protein intake is defended, such that animals sense the restriction of protein and adaptively alter feeding behavior to increase protein intake. Here, we argue that this matching of food consumption with physiological need requires at least two concurrent mechanisms: the first being the detection of internal nutritional need (a protein need state) and the second being the discrimination between foods with differing nutritional compositions. In this review, we outline various mechanisms that could mediate the sensing of need state and the discrimination between protein-rich and protein-poor foods. Finally, we briefly describe how the interaction of these mechanisms might allow an animal to self-select between a complex array of foods to meet nutritional needs and adaptively respond to changes in either the external environment or internal physiological state.

Keywords: amino acids; feeding behavior; homeostasis; macronutrient; protein.

Figure 1

Conceptual mechanism of food choice as an interaction between short-term meal-related cues and long-term signals of the nutritional state. The ability to alter food choice in response to internal nutritional state requires at least two interdependent mechanisms. Within the meal, the animal must sense the nutrient composition of food using sensory cues (visual, gustatory, and/or olfactory inputs), with this information interacting with prior experiences (food memories) to produce a central representation of the nutrient content of a particular food. Separately, the chronic restriction of protein/amino acid availability is signaled via endocrine (possibly FGF21) and/or neural mechanisms, which produce a central representation of interoceptive state. Then, these two central representations interact, such that need state signals shift the value of nutrient-specific cues and/or memories, thereby coordinating adaptive, unconscious shifts in the food preferences that increase the intake of the needed nutrient, in this case protein or essential amino acids.