Objective: beta-carotene (beta-C) is a major and safe vitamin A source for a vast population in China. But there is little understanding on beta-C-vitamin A equivalence in Chinese population. This study was designed to evaluate intestinal and whole-body conversion of beta-C to vitamin A in some Chinese adults by using a stable isotope dilution technique.
Methods: Fifteen healthy adult farmer volunteers (9 males and 6 females) aged 50-60 years from a few villages of China's eastern Shandong Province were recruited in a 55-days experiment. A Physiological dose of 2H8 beta-C (11011 nmole or 6 mg) in oil was given with a liquid diet (25% energy from fat) to the volunteers in the first day of the study. Three days after the 2H8 beta-C dose, the volunteers each took a reference dose of 2H8 RAC (8,915 nmole or 3 mg) in oil with the same liquid diet. Serum samples were collected at 0, 3, 5, 7, 9, 11, 13 hours of the first and the fourth days of study, and fasting serum samples were also collected daily in first 10 days and then weekly at 14th, 21st, 28th, 35th, 42nd, 49th, and 56th day after a 12-hour overnight fast, beta-C and retinol were extracted from serum and isolated by high performance liquid chromatography. The serum retinol and beta-C enrichments were respectively determined by using a gas chromatography/electron capture negative chemical ionization mass spectrometry (GC/ECNCI-MS) and liquid chromatograph/mass spectrometry with an atmospheric pressure chemical ionization interface (LC/APCI-MS).
Results: All the subjects responded well to the 2H8 retinyl acetate reference dose. However, the responses to the 2H8 beta-C dose were unexpectedly Poor in 4 of the 15 subjects. The repeated GC-MS analysis on the backup serum samples confirmed that these 4 subjects responded less efficiently to ingested 2H8 beta-C dose. These 4 subjects referred as to poor converters will be re-examined to confirm the observation. Therefore, the data of beta-C metabolism of the 4 poor converters were not analyzed together with that of the 11 normal converters. In the normal converters (7 males and 4 females), the calculated conversion factor of beta-C to retinol in1, 6, 13, 20, 27, 34, 41, and 52 days are 7.1 +/- 3.4, 5.6 +/- 2.7, 5.4 +/- 2.9, 5.1 +/- 2.8, 5.0 +/- 3.0, 5.0 +/- 2.9, 4.9 +/- 2.9, 4.8 +/- 2.8 to 1 on a molar basis respectively, and the corresponding post intestinal absorption conversion is estimated as (19.7 +/- 17.8)%, (22.7 +/- 20.5)%, (26.3 +/- 21.2)%, (27.8 +/- 22.7)%, (28.6 +/- 22.7)%, (29.5 +/- 22.8)% and (30.1 +/- 22.1)% of the total converted retinol.
Conclusion: The different time conversion of beta-C to vitamin A in the tested Chinese adults had been quantitatively determined by using a stable isotope reference method, and the long term as 52 days conversion factor value was 9.1 to 1 on a weight basis, and about 30% of total converted retinol came from absorbed intact beta-C.