Analyzing the δ2H values in individual amino acids of proteins extracted from vertebrates, we unexpectedly found insome samples, notably bone collagen from seals, more than twice as much deuterium in proline and hydroxyproline residues than inseawater. This corresponds to at least 4 times higher δ2H than in any previously reported biogenic sample. We ruled out diet as aplausible mechanism for such anomalous enrichment. This finding puts into question the old adage that “you are what you eat”.