As loneliness deepens in one of the world's fastest-ageing nations, a network of women delivering probiotic milk drinks has become a vital source of routine, connection and care.

A woman in a neat navy suit and powder-blue shirt cycles purposefully down a quiet residential street in Tokyo. It's 08:30 but already balmy, and she's grateful for the matching visor that shields her eyes from the summer sun.

She arrives at her first stop, parks her bike and knocks on the door of a small wooden house with potted plants flanking the entrance. Inside, an elderly woman waits. Her face breaks into a broad smile as she opens the door – she has been expecting this visit.

Japan is the world's most rapidly ageing major economy. Nearly 30% of its population is now over 65, and the number of elderly people living alone continues to rise. As families shrink and traditional multi-generational households decline, isolation has become one of the country's most pressing social challenges.

The suited woman is a Yakult Lady – one of tens of thousands across Japan who deliver the eponymous probiotic drinks directly to people's homes. On paper they're delivery workers, but in practice they're part of the country's informal social safety net. In a country grappling with a rapidly ageing population and a deepening loneliness crisis, Yakult Ladies have become an unlikely source of community, helping to reduce the problem of isolation one drop-off at a time.

With their distinctive squat plastic bottles and shiny red caps, Yakult pioneered a genre. The probiotic drink was launched in Japan 90 years ago – long before "microbiome" became common parlance. But today, the women who deliver them are as important to the brand's identity as the product itself.