What It Means To Be Full

a story of three hormones

The Cast

Main characters:

Cholecystokinin, a spunky hormone that responds to amino acids in the small intestine. Friends call it CCK.

Leptin, a pretty serious hormone that runs fat patrol and sends out signals as adipocytes fill up. Can’t stand overeating.

Insulin, a pretty famous hormone that keeps blood from becoming syrup and is responsible for orchestrating storage in the body. A bit of a gym rat¹.

Side characters:

Ghrelin, kind friend to CCK and Leptin, this hormone notices when adipocytes start to empty and signals the brain to acquire more food. Extremely punctual. A bit of a hypochondriac.

Glucagon, BFF with Insulin, this hormone is responsible for taking things out of storage and distributing them throughout the body. Sometimes overzealous².

Act 1

Our story begins in the middle because the beginning is so hard to find. By this point, you’ve found something tasty to eat and chewed it up thoroughly. Characters not in this current story, such as Amylase and Lipase, have been hard at work stoking your digestive fires and performing complex chemical dances; breaking down large molecules from your food into smaller constituent components which can be ferried through your body and put to good use in keeping you not only alive but thriving.

Scene 1

We are inside your stomach. Hydrochloric Acid — stomach acid — has activated Protease, an enzyme which disassembles protein. Your stomach has been stretching to accommodate food as you eat. The time has come. The gastric emptying process begins and your small intestine starts to receive the chyme³ for further digestion. This is the alarm bell which wakes up CCK: amino acids from the protein you've eaten (along with some prodding from your stomach expanding) activate CCK and get it moving to tell your brain that you're getting nutrients. At the same time, new enzymes are awoken by the addition of sodium bicarbonate into your small intestine and begin breaking apart the carbohydrate and fat in your meal. The introduction of glucose into your bloodstream slaps Insulin awake and it calls your fat cells to task storing newly created lipids. Fat cells filling up can only mean one thing. Turn on the Leptin signal! Leptin joins CCK in telling your brain that you're full: it's time to stop eating.

And that’s it. Now that you’ve stopped eating, after some time Glucagon gets to work pulling food out of storage to distribute around your body and Ghrelin notices the fat cells depleting so it shouts to you to eat. You’re hungry again!

Of course, a lot more goes on in your body as these hormones speed around sending signals and unlocking cellular doors, but that’s the basic structure. Only one act, only one scene. It happens all at once.

In fact, it never stops happening. Your body acts not based on whether or not a hormone is present but the concentration of the hormone in relation to other, “antagonistic” hormones. For instance, Ghrelin and Leptin are always around but you’ll be hungry or full based on the ratio of the two that are around and how sensitive your cells are. Insensitivity — called “resistance” — is what happens a lot with Leptin and Insulin, and is a primary cause of modern obesity: there is too much of a hormone in your blood and your cells have stopped listening. Your body has cried wolf to itself.

That’s All?

Of course not! Your body is insanely complex. In fact, it’s so complex that even the scientific knowledge we have about it is tenuous at best. Some of what I’ve described here is old hat but some of it is based on fairly new journal papers. Some of it might turn out to be wrong or incomplete in the future.

But what about fat? What about Calories?

Sorry, bub. Your body doesn’t care about those when it comes to being full.

The only things that trigger the feeling of fullness are your stomach expanding, your fat (read: storage) cells filling up, and protein. The exception is that if you aren’t getting enough fat then your body can begin to fail to process nutrients and then you will remain hungry. The caveat here is that your body isn’t looking for the fat per se but it needs the nutrients which are not being absorbed (fat also happens to be the most efficient way to produce ATP — the currency of cellular exchange).⁴

I’ll be writing more (maybe even making a video) about Calories later. The short of it, though, is that they are not the correct metric to be using so you might as well get a head start of it and ignore them from here on out.

1. Insulin is responsible for feeding your muscles, too!

2. If there’s not enough to go around, Glucagon can start to request material from all sorts of places; muscle, bone marrow, and organs.

3. Chyme is what we call the mass of partially digested foodstuff that exits your stomach.

4. As usual, there is a lot more to this story. Variety of chemical composition of the foods you eat as well as their novelty seem to have an effect.