Schooling et al. BMC Public Health 2011, 11:345 http://www.biomedcentral.com/1471-2458/11/345 seated height (both continuous)) and lifestyle habits (smoking, alcohol use and physical activity). These were selected for inclusion in the final model on a change in estimate criteria for white blood cell count, on which basis longest held occupation, leg length, seated height, alcohol use and physical activity were dropped. The final model (model 1) included age, childhood socio-economic position (from parental possessions), education and smoking status. Model 2 additionally adjusted for body mass index and waist-hip ratio. Results Of the 10,088 participants, 9,867 (99%) had complete data on inflammatory markers, as well as age, body mass index and waist-hip ratio and were included. There were more women (7,369) than men (2,498), and the women were younger (mean age 59.2 (standard deviation 7.6)) than the men (mean age 63.2 (standard deviation 7.6)). Age ranged from 50 to 96 years, but few participants were older than 75 years. Table 1 shows that recalled childhood meat eating was positively associated with height and socio-economic position. Table 2 shows that the associations of child- hood meat eating with total white blood cell count or its differentials did not differ with age, but did differ by sex for total white blood cell count and lymphocyte count. Table 3 shows that among men, childhood meat eat- ing was positively associated with total white blood cell count and lymphocyte count in model 1. The associa- tions were attenuated slightly by additional adjustment for body mass index and waist-hip ratio in model 2. In contrast, among women childhood meat eating was not clearly associated with total white blood cell count, lym- phocyte count or granulocyte count. Discussion In a large study from an understudied non-western devel- oping population, we found that a marker of early life conditions, i.e., recalled childhood meat eating, had sex- specific associations with some inflammatory markers. More frequent childhood meat eating, adjusted for child- hood socio-economic position, was positively associated with white blood cell count and lymphocyte count among men but not women. Despite using a large study, there are caveats. First, we used self-report of childhood meat eating as a proxy for childhood nutrition, which is undoubtedly subject to measurement error. Random misclassification most likely makes the results conservative, for which our large sam- ple size compensates. Recall bias is also possible, although the participants were unlikely to have been aware of their inflammatory status, nor is it obvious why such status should affect their recall of childhood events. Page 3 of 6 As would be expected, recalled childhood meat eating was strongly positively associated with childhood socio- economic position, education and height. Second, our participants were not a randomly selected, population representative sample; however that should not affect internal associations, unless we missed people with speci- fic combinations of childhood meat eating and inflamma- tory markers. Third, survivor bias is possible, in which case we would have expected differences in associations by age, of which there was no evidence. Fourth, a single measurement of total white blood cell count and its dif- ferentials might not reflect long-term immune function. However, white blood cell count is used as a marker of immune status in clinical settings. Within the normal range, white blood cell count is associated with cardio- vascular mortality [3]. Fifth, white blood cell count or its differentials could be affected by acute infection or trauma, however people in such a condition would have been unlikely to participate in our study. Finally, we did not adjust for potential confounding by diet, because of the difficulty of obtaining reliable and accurate dietary data in large-scale studies of free-living participants, par- ticularly amongst the Chinese who share several dishes during a meal making individual intake difficult to gauge. Childhood conditions, in long-term developed coun- tries, are usually consistently negatively associated with markers of a pro-inflammatory state [4-8]. However, a study from a similarly middle income country [9] also found that childhood living conditions had sex-specific associations with an inflammatory marker that were less favourable among men. In the United States, early life socio-economic position has been observed to have a negative association with infectious pathogens among women but not men [34]. Our findings, of childhood meat eating positively associated with some potential markers of a pro-inflammatory state among men but mot women, has some commonality with these later two studies [9,34]. Our study went further by specifically examining a measure of childhood nutrition, recalled childhood meat eating, adjusted for childhood socio- economic position. There are several possible explanations for our obser- vations. Childhood meat eating could be a marker of a generally unhealthy childhood diet, including fast food, trans fats and a low fruit and vegetable intake, with cor- responding consequences. However, our participants spent their lives in China. Their childhood in China spanned the mid-20th century when living standards were low, and diet was very limited with no fast food and little trans fat [35,36]. More frequent meat eating in our population almost certainly represents a less limited diet, which is consistent with the taller height and ear- lier age of menarche amongst those with more frequent recalled childhood meat eating [27,28].