The human gut microbiome contains up to one thousand species of bacteria that, among many functions, produce neurotransmitters and hormones—including an estimated 90 percent of the body’s serotonin, the “feel-good” hormone that shapes your overall sense of well-being. It’s why scientists often refer to the gut as a “second brain,” influencing everything from digestion to metabolism, anxiety, and allergies, and explains the constant spate of probiotic supplements on the market today, whether kombucha, apple cider vinegar tonics, or trendy juice cleanses.