URINARY-EXCRETION OF NITROSAMINO ACIDS AND NITRATE BY INHABITANTS OF HIGH-RISK AND LOW-RISK AREAS FOR NASOPHARYNGEAL CARCINOMA IN SOUTHERN CHINA

The hypothesis that endogenous synthesis of nitrosamines from dietary precursors is a risk factor for nasopharyngeal carcinoma (NPC) in Chin a was tested by applying the nitrosoproline (NPRO) test to subjects li ving in high- and low-risk districts for NPC in Zangwu county, Guangxi region, in southern China. Samples of 12-h urine were collected from 77 subjects: (a) before any treatment; (b) after ingestion of proline; and (c) after ingestion of proline together with vitamin C. NPRO, oth er nitrosamino acids, and nitrate were measured as indices of exposure to preformed and endogenously formed nitrosamines or their precursors . The NPRO level after proline intake was significantly increased in s ubjects from the high-risk area (P = 0.012) and markedly reduced after ingestion of ascorbic acid (P = 0.007), but such an effect was not se en in subjects from the low-risk area. Levels of N-nitrosothiazolidine -4-carboxylic acid and the sum of nitrosamino acids in subjects in the high-risk area were significantly reduced by ascorbic acid (P < 0.01) but were not reduced in subjects from the low-risk area. The urinary nitrate level was about twice as high in subjects from the high-risk a rea. In subjects from high- and low-risk areas combined, NPRO levels i n any of the three dose groups were highly correlated with nitrate lev els (P = 0.0001). These results demonstrate a higher potential for end ogenous nitrosation in subjects living in the high-risk area of NPC an d suggest the occurrence of nitrosation inhibitors in the diet consume d in the low-risk area. Thus, in addition to infection by Epstein-Barr virus and genetic predisposing factors, dietary habits that may entai l higher nitrosamine exposure appear to play a role in NPC etiology.