The Science of Hunger & Medications to Combat Obesity | Dr. Zachary Knight

A fast brainstem system controls meal size over 10–20 minutes using gut-derived signals such as gastric stretch and intestinal hormones (e.g., CCK). A slower hypothalamic system tracks body fat stores over weeks to years via hormones like leptin. The hypothalamus modulates brainstem meal-size circuits so that short-term eating behavior matches long-term energy needs.

Leptin is secreted by fat tissue in proportion to total body fat and acts mainly in the brain to suppress hunger and increase energy expenditure. In rare monogenic obesity (e.g., OB/DB mouse analogs, some humans), leptin or its receptor is mutated and leptin therapy can be powerful. However, most people with obesity already have high leptin and are leptin resistant, so simply adding more leptin does little for weight loss, though it may be useful after weight loss to help maintain lower body weight.

AgRP neurons in the hypothalamus drive the appetitive phase of feeding—searching and wanting food. Knight’s lab showed that in hungry mice, AgRP activity shuts down within seconds of seeing/smelling accessible, palatable food, well before ingestion. The size of this rapid drop predicts how many calories the mouse will eat over the next ~30 minutes, indicating these neurons perform a learned, multi-variable prediction about future intake rather than simply reporting current energy deficit.

Twin and genetic studies suggest ~80% of variation in body weight is heritable, with hundreds to thousands of genes (mostly expressed in the brain) influencing appetite and expenditure. The obesity epidemic since the 1970s is driven by environmental change—cheap, abundant, ultra-processed, highly palatable foods, and other lifestyle shifts—that “pull the trigger” on genetic susceptibilities. Genetics sets where you tend to fall on the leanness–obesity spectrum; environment shifts the entire distribution upward.

Natural GLP-1 from the gut has a ~2-minute half-life and physiologic roles in enhancing glucose-stimulated insulin secretion, not in robustly regulating body weight. DPP-4 inhibitors that raise endogenous GLP-1 threefold improve diabetes but do not cause meaningful weight loss. Engineered GLP-1 agonists (e.g., liraglutide, semaglutide) extend half-life to hours or days, raising GLP-1 receptor activation 1,000–10,000-fold and continuously stimulating brainstem sites (NTS, area postrema) to reduce appetite and drive ~15–20% body-weight loss over a year.