Model for determination of 13C substrate oxidation rates in humans in the fed state.

: The effect of a test diet of conventional North American foods on breath 13CO2 abundance was determined in 4 healthy males. Subjects consumed a diet containing 41.9% energy as fat and a polyunsaturated:saturated fatty acid ratio of 0.217 for 5 days at a level equal to estimated requirements for total energy. One subject underwent four 5-day feeding periods. Over the feeding period mean change in subjects' body weight was -0.165 +/- 0.64% (means +/- SEM) of initial body weight. On day 5 breath samples were collected hourly from 0745 to 1645 h and analyzed for 13CO2 enrichment relative to the pre-breakfast fasted state (level at 0745 h). Breakfast and lunch were consumed at 0820 and 1300 h respectively. Mean enrichment peaked at 1445 h at 0.00311 atom % excess above fasting level for all subjects and 0.00243 atom % excess for the four repeated trials on one subject. Between subject variation (SEM) in 13CO2 enrichment (0.000618 atom %) was significantly greater than within subject variation (0.000308 atom %). These results indicate that ingestion of normal meals during the breath test period increases breath 13CO2 abundance due to a shift in substrate oxidation. The small within subject variation in repeated 13CO2 enrichment profiles indicates the reliability of 13C breath tests using controlled diets. It is concluded that for tests conducted under identical conditions, an initial background 13CO2 abundance profile determined once for each subject, can be subtracted from the subsequent enrichment profile generated by a labeled test substrate.