Picture this: deep in your gastrointestinal tract, there is a bustling metropolis of microscopic life. Bacteria, archaea, and fungi, all going about their business. We have known for a while that these tiny tenants help with digestion and support our immune system. They also make neurotransmitters, like serotonin. But scientists have discovered something truly mind-blowing - these microbes are miniature vitamin factories. If we nourish them, could they produce more vitamins and replace supplements?