Why are you always hungry?
Your hunger and your appetite are regulated by your endocrine system, but how it manages this has always been something of a mystery. We originally suspected that a rapid rise in insulin, followed by a fall in blood sugar, stimulated appetite—and overeating (for an excellent description of this outdated theory, see reference 1, pages 65-66).
Upon reviewing the available evidence, scientists specializing in appetite began to have their doubts about this mechanism’s ability to induce overeating[2-3], but realistic alternatives to this theory were lacking. This changed in 1999 with the discovery of the hormone ghrelin[4-5].
A product of the gut[6-17], ghrelin was the first hormone discovered to directly stimulate hunger in humans[18]. Ghrelin’s potential to regulate body weight has since led to rigorous investigation into its properties. Here’s a summary of the most important of these:
- Stimulates growth hormone release in humans[4, 19-29], and is possibly the most potent stimulator of growth hormone release in the body[30]. Few conflicting results exist[30-31].
- Higher concentrations directly increase hunger[18, 32-39].
- Levels fall after meal ingestion[33, 34, 37, 38, 40-46]
- Directly related to body mass[20, 47-61]; the more fat mass, the lower the levels of ghrelin.
- Higher levels are found in women[40, 49, 62].
- Possible role in male sex hormone production[63-65].
These properties of ghrelin position it high on the list of body weight regulators[18, 66-68], especially since it signals overall fat stores and nutritional status of the body[39, 48, 69-72]—i.e., the more fat you possess and the more you eat, the lower your levels of ghrelin.
Ghrelin appears as a direct link between the gut and the brain[73-76], and there’s even evidence that it causes a timing effect for meal ingestion during the day[77]. It may also even trigger a deeper state of sleep in humans[78]. Even the success of gastric bypass surgery to reduce weight seems to be related to ghrelin secretion—or a lack thereof[51, 58, 79, 80].
Long story short, this is a hormone requiring particular consideration in any type of diet, whether you’re a serious athlete or not[81]—and it’s definitely a hormone we’re going to target for manipulation.