Alteration of tissue levels of 13C in pigs by natural abundance 13C labeling of diets.

A study was conducted to determine the feasibility of using differences in the natural abundance of dietary 13C as a means of in vivo labeling. Four female pigs were fed a C3 plant diet (barley, soybean meal and alfalfa) and four were fed a C4 plant diet (corn and corn gluten meal). The pigs were continually fed the diets from 8 wks of age to maturity and through pregnancy until they had themselves produced offspring. The resulting piglets were sacrificed at various time intervals (0 to 165 days) after switching either the piglets or their mothers' to the opposite diet. Changes in the delta 13C levels in samples of milk and tissues following the diet change were monitored by isotope ratio mass spectrometry. The differences in the delta 13C content of the two diets was 12.9 parts per mil (/1000). After piglets born to a sow fed the C3 diet were switched at one day of age to a surrogate sow fed the C4 diet, tissue delta 13C levels of the piglets were nearly identical to the diet of the surrogate sow by 11 days while those switched from C4 to C3 grew more slowly and differed from the diet by 2-5/1000. When the mothers diet was switched, 15-20 days later the delta 13C in the milk had changed by only 2-3/1000, indicating that the body rather than the diet was the primary source of carbon in the milk. At 28 days after the piglets diet was switched at weaning, the tissue delta 13C had changed by 8.4/1000 (C4 to C3) and 2.9/1000 (C3 to C4). The delta 13C level in the liver changed more rapidly than in the muscle or fat and the rate of change was also influenced by the size and growth rate of the pig.