December 4, 2025
Add Us On GoogleAdd SciAm
Why Leftover Pizza Might Actually Be Healthier
Researchers have discovered that cooling starchy foods—from pizza to rice—creates “resistant starch,” a carb that behaves like fiber and alters your blood sugar response
This video is part of “Innovations In: Type 1 Diabetes,” an editorially independent special report that was produced with financial support from Vertex.
Tom Lum: On the surface, this may look like your garden-variety Internet fun fact, the kind that you half remember and try to retell at a party, like …
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
[Lum pretends to be a guest at a party. He is holding a slice of pizza and talking over music.]
Lum: Did you know I read somewhere that leftover pizza is actually better for—uh—so how do you know Lauren?
But the secret is that this is just the surface of the fact, and the deeper we go, the more fun and weirder the science gets.
Because your first thought on hearing this is probably “Why?” Why is leftover pizza healthier for me? And the answer has to do with what happens when you cool the delicious crust. When you cool a pizza to below 40 degrees Fahrenheit, some of the starches in the dough will start to mingle together to form these long chains called resistant starches.
They resist digestion, and another word for a carbohydrate that resists digestion is fiber! And even if you reheat the pizza, the chains stay intact, so your body doesn’t break them down to sugar. They mostly pass through.
This could help reduce blood sugar spikes for people with diabetes or people who just need more fiber for a healthier gut. And this seems to work for a lot of starches, like rice, pasta, potatoes—even beans and lentils. Heating then cooling the starch changes its properties. It’s like tempering chocolate or forging a stronger steel.
But we can go even deeper into this fun fact because another question you might have is “How?” How did scientists study, analyze, and figure this out? And for that, we need to go to the actual papers.
And this is where you’ll find electron microscope photographs of old rice, showing these long starchy fibers forming and then sticking around through “simulated digestion.” And you’ll also find studies on humans to try to measure these health changes, like this one where brave participants had to be at the lab at 6 A.M. to eat old rice for science, which they had to do so that nothing else they ate that day interfered with their measurements.
This study also measured how long participants were chewing the rice, which may seem like overkill until they point out digestion starts in the mouth. And it’s this clever attention to detail that is the most important part because that’s how you get the fun fact.
Like, humans have been eating food the entire time humans have existed, but the way it interacts with our body is so complex that we’ve only just learned that apparently our fridge is a forge for fiber. And I think that and the details of the study are so much more interesting than the fun fact. It just might not be the best at parties.
[Lum pretends to be a guest at a party again.]
Lum: Hi, it’s Tom. Did you know digestion starts at the mouth?
It’s Time to Stand Up for Science
If you enjoyed this article, I’d like to ask for your support. Scientific American has served as an advocate for science and industry for 180 years, and right now may be the most critical moment in that two-century history.
I’ve been a Scientific American subscriber since I was 12 years old, and it helped shape the way I look at the world. SciAm always educates and delights me, and inspires a sense of awe for our vast, beautiful universe. I hope it does that for you, too.
If you subscribe to Scientific American, you help ensure that our coverage is centered on meaningful research and discovery; that we have the resources to report on the decisions that threaten labs across the U.S.; and that we support both budding and working scientists at a time when the value of science itself too often goes unrecognized.
In return, you get essential news, captivating podcasts, brilliant infographics, can’t-miss newsletters, must-watch videos, challenging games, and the science world’s best writing and reporting. You can even gift someone a subscription.
There has never been a more important time for us to stand up and show why science matters. I hope you’ll support us in that mission.
