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Geographic variations in the carbon isotope composition of the human diet and human hair were investigated. The carbon isotopic composition of common foodstuffs purchased in Chicago, USA, Tokyo, Japan and Munich, FRG, were determined by combustion and differential isotope ratio mass spectrometry. The dietary protein carbon for the United States (-18.1%) was calculated to be enriched in 13C relative to the Japanese (-21.2%) and the German (-23.6%) diets. To a large degree, the differences reflected the consumption of corn-fed animal products in the United States and Japan, as well as seafood in Japan. The carbon isotopic composition of hair (-16.4, -18.0 and -20.4%) for the three respective populations correlated with the calculated values of the dietary protein, but were 2-3% enriched in 13C. Changes in the isotopic composition of beard hair were shown to record the changes in dietary composition in travelers visiting the respective regions.