Hidarugami

You are walking a mountain pass, healthy, well-fed, in good condition.

And it hits you mid-stride.

Hunger — not ordinary hunger, but the total kind. Weakness in the legs. Numbness spreading through the limbs. Fatigue that arrives all at once, out of a clear sky. And within moments you cannot advance a single step, and you are sitting down on the path whether you decided to or not.

Untreated, people died of it. Starved — with a full stomach, on a mountain road, in an hour.

Who they are

The hidarugami are the spirits of people who starved to death on those roads and were never buried and never given rites.

Not travellers who fell ill. The ones who ran out — who miscalculated the distance, or hit weather, or simply had nothing, and sat down on a pass and did not get up. Nobody found them, or nobody who found them did anything about it.

And what they do to you is the only thing they have to give. They share it.

The rule, which is the best rule in Japanese folklore

Here's the countermeasure, and it's genuinely superb:

Never go into the mountains without food. And never eat all of it.

A single mouthful of rice, or a bite of mochi, breaks the possession instantly. The legs come back. You get up and walk on.

Which means the practical instruction — recorded from the Kii peninsula and Shikoku, and repeated as living advice on Japanese mountain paths today — is: carry riceballs, and always leave a few grains behind in case of emergency.

Never finish your food on the mountain. Keep a reserve you don't intend to eat, because the moment you need it you will not be capable of getting it out of your own pack unless it's already there.

That is not folklore decoration. That is a hiking rule, and it is correct.

And if you've got nothing

There's a fallback for the traveller with an empty pack, and it may be the most portable counter-spell in the tradition.

Write the character for rice — 米 — on your palm, and lick it.

The word standing in for the grain. When you have no food at all, you eat the idea of food, and the tradition says that will do.

I find that unreasonably moving. It's the last thing available to a person alone on a pass with nothing, and it costs nothing, and it gives them something to do with their hands at the moment their legs stop working.

What is actually happening

The modern readings are unglamorous and they make the folklore look better rather than worse.

One is hypoglycemia — the sudden blood-sugar crash that endurance athletes call bonking. It arrives fast, it produces exactly these symptoms, and it does not care how well you ate at breakfast if you have been climbing for four hours.

The other, proposed for particular valleys, is carbon dioxide pooling in still air on a forested climb.

So: the bonk, deified.

But look at what the folk cure actually prescribes. Immediate sugar, in a small dose, from a reserve you were specifically instructed never to finish.

That is precisely correct treatment for hypoglycemia, arrived at without any concept of blood sugar, and encoded in a form that a fourteenth-century porter would remember on a mountain at dusk.

And there are named places. The Otogi Pass on the Shiga–Mie border, where the thing was said to appear as a bloated hungry ghost. The Kumotori peaks in Wakayama, which have a Gaki-ana — a "hungry ghost cave" — that reliably produced encounters.

Which is to say: certain specific climbs, in certain specific places, were known to do this to people, consistently, for centuries. The mountain kept its record. Somebody just wrote it in the only notation available.