Ghrelin: Why You Get Hungry


Here’s a look at some of my original research notes. This should be of interest to everyone, and it’s all definitely still relevant, so I’ll be presenting these excerpts from time to time. Enjoy.

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in insulin, followed by a fall in blood sugar, stimulated appetite—and overeating (for an excellent description of this outdated theory, see reference 1, pages 65-66).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. You’ll have to wait for part two to learn how to do so.

References (click to expand)
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John Kiefer


Physicist turned nutrition and performance scientist. Currently considered one of the industry’s leading experts on human metabolism.

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Ghrelin: Why You Get Hungry

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