[Effect of natural isotope contents of nutrients on the background in 13C breath tests].

The long-term variation of the mean delta 13C-value of breath-CO2 of Europeans (base value -26%) was +/- 1% over a period of several months. However, the ingestion of products from corn, a C4-plant, caused remarkably higher shifts. In a fasted state and during rest a stability of the base value of +/- 0.3% could be attained for several hours. Longer fasting periods imply a shift of the CO2-delta-value due to the metabolism of body products. - By a change of the main energy source to corn products the delta 13C-base line could be shifted to -18% within one day. After this shift even the metabolism of products from C3-plants could be investigated. The degradation curve and the integral degradation of fats were largely dependent on the method of application, and the base value largely interfered with the results. The period of maximum disintegration for a naturally and uniformly labelled fat was different from that of an artificially labelled trilinoleate. The results shall be used for the adjustment and the correction of the base line in 13C-breath tests, especially with patients under parenteral nutrition.