Low Protein Diets and Their Effects on Brown Adipose Tissue

Abstract: Obesity is a rising health issue and has been linked with a relative decrease of dietary protein, which promotes higher food intake. Brown adipose tissue is a heat-producing, energy-wasting organ that has been shown to be enhanced by a low protein diet. However, the mechanism of how this occurs is largely unknown.Protein and amino acid concentration are detected biologically by several mechanisms: taste receptors in the tongue and inside the gut secrete many endocrine factors upon feeding, such as CCK, GLP-1 and PYY. GCN2 binds to unbound tRNA instead of the usual binding partner for tRNA: amino acids. Upon low protein feeding, GCN2 alters behaviour in mice and rats and induces many downstream signalling pathways, including FGF21. mTORC1 indirectly detects protein concentration and downregulates protein synthesis and growth when amino acid supply is low.Animals respond to low protein feeding using many endocrine factors, including CCK, GLP-1, PYY and FGF21, affecting brown fat in various ways. CCK induces hyperthermia, where brown fat can be used as an effector. GLP-1 promotes brown fat via the brain but does not pass the blood-brain barrier. GLP-1 in the periphery may down-regulate brown fat. PYY has been directly linked to promoting lowered food intake when mice are fed a high protein diet. In humans, PYY decreases food intake by causing nausea. PYY has been poorly studied in regards to brown fat. FGF21 is released from the liver when mice are fed a low protein diet and promotes brown fat via the brain. FGF21 could act directly on brown adipose tissue, but a direct effect has only been shown when pharmacological doses are used.In conclusion; the response to low protein feeding in humans and other animals is complex and uses many varying mechanisms and mediators, including brown adipose tissue.