However, as 'natto' and 'natto jiru' (miso soup with natto) are the season words for winter for haiku (Japanese seventeen-syllable poem) and also as there is a proverb saying that 'in the natto season, there is no need for a doctor,' winter had been the season for natto.
しかし「納豆」「納豆汁」が冬の季語である事や、「納豆時に医者要らず」という諺があったように、元々納豆の時期は冬とされている。