Essentials: How to Control Hunger, Eating & Satiety

This Essentials episode looks at how hormones affect feeding, hunger and satiety, which it describes as the feeling that you don't want to eat or that you've eaten enough. Early on, the episode says hormones don't act alone here. They work "in cooperation with the nervous system," so the discussion starts with the brain before it gets to blood-borne signals.

The first region is the ventromedial hypothalamus, which has long interested researchers because damaging it produces opposite results. In some animals or people, lesioning or disrupting its neurons led to hyperphagia, a drive to eat "like crazy." Other lesions led to anorexia, where food became aversive. The episode explains the paradox by saying the region holds several neuron populations: some promote feeding and others promote not eating. That makes it an important control station, but on its own it doesn't explain what is happening at a deeper level.

The second region is the insular cortex. It processes interoception, meaning information about what's going on inside the body, and it gets input from touch receptors in the mouth. The episode says this region has "powerful control" over whether you enjoy a food, want to avoid it, have had enough, or want to keep eating. It ties that control to the tactile side of eating, such as chewing and food consistency. The episode notes that people usually think about taste receptors and often overlook "the touch or tactile essence of food."

To explain why the hypothalamus can push feeding either way, the episode describes a classic experiment in which two rats were parabiosed. Surgery linked their blood supply, while their brains and mouths stayed separate. When researchers lesioned the ventromedial hypothalamus in one rat, that rat became "very, very fat." The connected partner became very thin and actually lost weight.

The episode reasons that because only the blood supply was shared, something carried in the blood must have passed between the animals. That points to hormonal, or endocrine, signals involved in the desire to eat, hunger and appetite. This experiment is the bridge to the rest of the episode, which promises entry points for timing meal frequency and predicting when you'll be hungry.

The episode calls the discovery of the arcuate nucleus one of the major developments in the science of feeding and appetite over the past 20 years. It describes this area as containing neurons that release molecules into the blood that act "as accelerators on feeding and appetite or brakes."

POMC (proopiomelanocortin) neurons make alpha-melanocyte-stimulating hormone (alpha MSH), which the episode calls a powerful molecule that reduces appetite. MSH release goes up after eating. AgRP neurons stimulate eating. Their activity goes "way up" when animals or people haven't eaten for a while.