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Food Science

Periodic Graphics

Periodic Graphics: Hunger, digestion, and feeling full

Chemical educator and Compound Interest blogger Andy Brunning explains what’s going on in our guts before, during, and after a hearty meal

by Andy Brunning, special to C&EN
November 19, 2023 | A version of this story appeared in Volume 101, Issue 38


Infographic on hunger, digestion, and feeling full. Ghrelin, produced by an empty stomach, causes feelings of hunger. Stomach rumbling is caused by the migrating motor complex, contractions that sweep indigestible substances from the stomach and through the small intestine. Smelling food signals the brain's primary salivary centers in the medulla oblongata. Neurotransmitters then signal glands to produce saliva.
Mechanical and chemical digestion break down the food we eat. Different enzymes break up different biological molecules. The salivary glands produce amylase, a carbohydrase that breaks carbohydrates into simple sugars. The stomach produces proteases, which breaks proteins into amino acids. The pancreas produces amylase, proteases, and lipases. Lipase breaks lipids into fatty acids and glycerol.
Eating makes fat cells release the hormone leptin, making us feel full. Sleepiness after eating is caused by an insulin response: after carbohydrates raise blood sugar, insulin reduces blood sugar, causing feelings of tiredness.
Credit: Andy Brunning

To download a pdf of this article, visit

References used to create this graphic:

Klok, M. D., S Jakobsdottir, and M. L. Drent. “The Role of Leptin and Ghrelin in the Regulation of Food Intake and Body Weight in Humans: A Review.” Obes. Rev.( 2007). DOI: 10.1111/j.1467-789X.2006.00270.x.

Reactions. “Why Does Food Make Your Mouth Water?” March 15, 2016. YouTube video, 3:10.

Wood-Black, Frankie. “The Great Thanksgiving Chemistry Debates.” inChemistry, Nov. 17, 2017.

A collaboration between C&EN and Andy Brunning, author of the popular graphics blogCompound Interest

To see more of Brunning’s work, go to To see all of C&EN’s Periodic Graphics, visit



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