In Japan, somewhere between 8 and 10 million people eating this weird food every day.

It costs about ¥50 — roughly 35 cents. It comes in a small polystyrene container, wrapped in thin paper, tucked between the onigiri and the yogurt in every convenience store in the country. It has a powerful smell. It pulls into long, glistening strings when you lift it with chopsticks. And depending on where you were born in Japan, it is either the most comforting food imaginable — or something you'd never put near your mouth.

It's called natto (納豆). And the research on what it does to your cardiovascular system is something I've wanted to write about for a long time.

A thousand years of accidental discovery

The origin of natto has been disputed for centuries — at least four competing legends claim to have invented it.

The most enduring story points to around 1086 AD, during a military campaign led by the warrior Minamoto no Yoshiie in northeastern Japan. His troops were boiling soybeans when enemy forces attacked. In their rush to retreat, soldiers packed the half-cooked beans hastily into straw bags and fled on horseback. Several days later, when the bags were finally opened, the beans had fermented — naturally inoculated by Bacillus subtilis bacteria living in the rice straw. Hungry soldiers ate them anyway. They didn't die. Some apparently found they liked the taste.

Whether or not this story is accurate, the geography of that campaign matters. The region Yoshiie was fighting through is what we now call Ibaraki Prefecture — specifically the area around Mito City. And today, Mito is still considered the natto capital of Japan. The annual Natto Festival draws tens of thousands of visitors. Local natto brands are gifted the way Kyoto gifts matcha. The connection between northeastern Japan and natto is one of the most persistent in the country's food history.

Other records suggest Buddhist monks may have been fermenting soybeans in straw as far back as the Nara period (710–794 AD), predating Yoshiie entirely. The monk Kukai — founder of Shingon Buddhism, based in Koyasan — is sometimes credited with bringing early fermentation techniques from Tang-dynasty China. There is no single definitive origin. What is clear is that for at least a thousand years, natto has been a part of the Japanese diet — first as a preserved ration for soldiers and monks, then as a daily staple for the population at large.

East Japan loves it. West Japan is not convinced.

Here is something foreigners rarely hear: even within Japan, natto is polarising.

In eastern Japan — Tokyo, Ibaraki, Tohoku, Hokkaido — natto is simply breakfast. It has been for generations. People grow up with it, they eat it without thinking, and they find the smell comforting rather than alarming. Many eastern Japanese can't imagine a morning without it.

In western Japan — Osaka, Kyoto, Hyogo — the relationship is complicated. Kansai residents have a well-documented aversion to natto that borders on cultural identity. The smell is considered excessive. The texture is wrong. It is, in the words of many Osaka people I've spoken to, "not food." Surveys consistently show natto consumption in Kansai running at less than half the rate of the Kanto and Tohoku regions.

I grew up in Kagawa, on the island of Shikoku — technically western Japan, though Shikoku occupies an interesting middle ground. Natto was present but not dominant in my childhood. My own relationship with it developed gradually as I got older and started paying attention to what the oldest, healthiest people around me actually ate every day.

What I found was that natto consumption correlated strongly with the people in my community who seemed to be ageing slowest. That observation is obviously not clinical evidence. But it was the beginning of my interest in what was actually inside those small containers.

What natto actually is

Natto is fermented soybeans — but the fermentation is unlike anything in Western food culture. Soybeans are steamed, then inoculated with a specific bacteria called Bacillus subtilis var. natto. They ferment at around 40°C for 18 to 24 hours, developing the characteristic sticky strings that stretch between beans like a spider's web when you lift them with chopsticks.

The traditional method uses rice straw — wara natto — where beans are packed directly into dried straw bundles, which carry the bacteria naturally. This method is still used by artisan producers in Ibaraki and gives a more complex, earthier flavour than modern factory production. If you ever see wara natto at a Japanese market, it's worth trying — it's closer to what a thousand years of natto eaters actually tasted.

Why it divides people so sharply

The smell is the first thing. Real natto has a fermented protein smell — ammonia-forward, funky, with an undertone that some people describe as blue cheese and others describe as something they'd rather not name. This is not irrational to find off-putting. The brain doesn't easily distinguish "safe fermentation" from "potential spoilage" without cultural context, and for anyone who didn't grow up eating natto, there's no context.

Then there's the texture. The sticky strings are genuinely unlike anything else. When you lift natto with chopsticks, it pulls and stretches in a way that has no equivalent in Western food. The beans themselves are soft but intact. The surface is slightly wet and glistening. For people encountering it for the first time, the whole sensory package is a lot.

But here's the thing that happens to almost everyone who keeps trying: it flips. Somewhere between the third and seventh exposure, the smell stops triggering aversion and starts triggering appetite. The strings become satisfying rather than alarming. People who once couldn't be in the same room as natto start seeking it out. There's something addictive in the flavour — that deep, umami-saturated fermented richness — that takes a few encounters to unlock.

I've watched this happen with foreign visitors to Japan many times. The first reaction is almost always a polite "no thank you." By the end of a week of nudging and gentle insistence, a surprising number of them are requesting it at breakfast.

Three things it does that nothing else does

How to actually eat it — a beginner's guide

If you've never tried natto, I want to be honest: the first attempt will probably be uncomfortable. This is normal. It doesn't mean it's not for you. Here are five ways to eat it, starting from the gentlest possible introduction and working up to the full traditional experience.

One consistent tip across all preparations: eat it immediately after stirring, while the rice is hot. Cold natto from the fridge, eaten alone without any preparation, is the hardest possible introduction. Don't do that to yourself on the first attempt.

Where to find it outside Japan

Natto is widely available at Japanese grocery stores and many Asian supermarkets globally — look in the refrigerated section, often near the tofu. It comes frozen for export. Thaw overnight in the fridge before eating. Brands worth looking for: Okame, Natto King, Takaokaya. In the US, most Japanese grocery chains (Mitsuwa, Marukai, H Mart) carry it. Online ordering through Japanese grocery retailers is also reliable.

A note on frozen export natto: it's milder in flavour than fresh Japanese natto, which makes it a reasonable starting point. Once you've adjusted, seek out fresh natto if you have a Japanese grocery nearby — the probiotic activity and flavour are both meaningfully better.

What I'm writing about next

The next gem comes from the beauty pillar — a skincare ingredient that Japanese women have used for 300 years, which Western brands are only now beginning to study. It's been hiding in plain sight in rice fields across the country. I'll explain what it is, what the research shows, and where to find it.