"Bottled water may contain trace chemicals, micro-plastics, and less vitamins and minerals, which you would naturally get in tap water," Dr. Nesheiwat said. One of those minerals is fluoride, a substance released from rocks into the soil, water, and air. The Centres for Disease Control and Prevention states that, in addition to naturally-occurring fluoride in the environment, extra amounts are often added to tap water to help protect against tooth decay. "Whenever given the option, I always go for tap water, it's simply more natural," she said.