Jia et al. BMC Cancer 2010, 10:446 http://www.biomedcentral.com/1471-2407/10/446 significant risk factor for NPC in many studies [13-15,18,19]. Several nitrosamines have been detected in samples of Chinese salted fish, most of which are capable of inducing nasal cavity tumours in experimen- tal animals [20-22]. In studies of Chinese populations, the relative risk of NPC associated with weekly con- sumption of salted fish ranged from 1.4 to 3.2 when compared with no or rare consumption; for daily con- sumption, the relative risk ranged from 1.8 to 7.5 [23]. We have not found any apparent difference in the mag- nitude of the association between the present study and the previous studies. In addition, previous studies observed that childhood exposure seems more strongly related to NPC risk than adulthood exposure [12,13,15,18,24-26]. We obtained a consistent result in this study, whereby OR was 2.45 (2.03-2.94) in child- hood vs. 1.58 (1.20-2.09) in adulthood at consumption frequency of weekly or more. Besides salted fish, exposure to other preserved foods, such as salted vegetables and preserved meat, was also found to be associated with an increased risk of NPC. This is fairly consistent with previous publications [13,15,16,18,23]. It has been reported that mouldy bean curd, salted shrimp paste, salted eggs and various pre- served vegetables were associated with an increased risk of NPC. Intake of preserved foods has also been shown to be associated with NPC risk among Arabs in Maghre- bian countries [27,28]. Similar to Chinese salted fish, these NPC-associated preserved foods contain carcino- genic nitrosamines and other genotoxic substances [29]. Noteworthy, we have not found convinced evidence that fermented pastes (bean, fish or shrimp paste) increase the risk of NPC. Our data demonstrate that NPC patients had ingested significantly fewer fresh fruits than control subjects, which was consistent with previous several reports, especially for childhood exposures [12,13,15,16,18,25]. Fresh fruit contains high levels of vitamin C, which not only blocks nitrosamine formation in vivo but also inhibits mutagenesis and carcinogenesis in vitro and inhibits tumour cell growth and carcinogen-induced DNA damage [30]. These findings offer a biological rationale for the observed protective effect of fresh fruit in NPC development as reported by previous studies [31]. Interestingly, intake of herbal tea and herbal slow- cooked soups are found to be associated with decreased risk of NPC. Aboriginal habitants of the Guangdong fre- quently consume Canton-style herbal tea, guided by the belief that it can heal or prevent common ailments like the flu and sore throats. The tea contains traditional Chinese medicinal herbs such as mulberry (Morus alba) leaf, yellow or white chrysanthemum (Chrysanthemum) flowers, prunella (Prunella vulgaris) spike, roughhaired Page 5 of 7 holly (Radix Ilicis Asprellae) root, ural licorice (Radix Glycyrrhizae) root and others. The type of herbs varies among several brands of herbal teas or homemade her- bal teas depending on their medicinal purposes. Unlike herbal tea, herbal slow-cooked soup contains meat and is normally simmered for several hours. The herbs in a typical slow-cooked soup include Coix Seed (Semen Coicis), Lily Bulb (Lilium Lancifolium), Gordon Euryale Seed (Semen Euryales), Dioscoreae Rhizome (Rhizoma Dioscoreae Oppositae), Fragrant Solomonseal Rhizome (Rhizoma Polygonati Odorati), lotus seed (Semen Nelumbinis), Chinese Date (Fructus Jujubae) and others. Studies conducted on Filipino populations contradict our findings; these studies suggest that herbal medicines are independently associated with an increased risk of NPC[32] and that herbal medicines interact with EBV through a direct proliferative effect on EBV-transformed cells[33]. One study conducted in Taiwan also observed an association between herbal medicines and increased risk of NPC [34]. However, studies conducted by Yu et al. in mainland China failed to detect an independent effect of herbal medicine use on NPC risk [13,35]. Spe- cific herbal medicines were examined, but no single plant family appeared to be responsible for the associa- tion observed between herbal medicine use and NPC. The possible explanation of the disparity in the study results may be that the term “herbal medicine” is too broad; the involved herbal plants could be heteroge- neous between studies and/or within a study. On the other hand, the herbal tea and herbal slow-cooked soup investigated in the present study are Canton-style diet- ary staples that involve a limited number of herbal plants without remarkable variance throughout the Guangdong province. Thus it is a more homogeneous variable than traditional herbal medicines treated as a whole. Several studies have shown that some Chinese herbal plants can play their anticancer role by inducing apoptosis and differentiation [36,37], by enhancing the immune system, inhibiting angiogenesis or reversing multi-drug resistance (MDR)[34]. It is noteworthy that Prunella vulgaris extracts were shown to have anti-viral, anti-proliferative and immunostimulatory effects [38,39]. Coix seed has inhibitory effects on Epstein-Barr virus activation and has anti-tumour promoting activities. It is possible that the herbal tea and herbal slow- cooked soups consumed in Guangdong province contain herbal plants that have similar anticancer capabilities. However, it is also possible that this association came from the fact that part of our controls were recruited from individuals taking health examinations, who are more concerned about their health and hence tend to consume more herbal tea or soup. Therefore, the rela- tionship between dietary habits including Chinese