The isotopic ratio of atoms from the water and beverages you drink is locked into your hair and could be used as a chemical tracking system.

The isotope ratio of the hydrogen and oxygen in the water used to produce bottled water, soda pop, and local beers leaves a natural chemical imprint of the geographic origin. The ratio is different depending on elevation, location, and proximity to the coast. Given that the body uses water’s hydrogen and oxygen atoms in protein synthesis, including the easily analyzed proteins in hair, imbibing local drinks leaves a chemical tracer in your hair of your travels over time. Isotopic ratio can be readily obtained from a hair sample using mass spectrometry for epidemiological or forensic studies.

• Links between Purchase Location and Stable Isotope Ratios of Bottled Water, Soda, and Beer in the United States
  L. A. Chesson, L. O. Valenzuela, S. P. O’Grady, T. E. Cerling, J. R. Ehleringer,
  J. Agric. Food Chem. 2010, 58 (12), 7311–7316.
  DOI: 10.1021/jf1003539