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Variation in food quality may threaten the validity of epidemiological studies in China.

Food safety has become one of the biggest concerns in China, and this may not only put the health of the population at risk but also threaten the validity of epidemiological studies investigating the possible relationship between diet and risk of disease in the country. This month in China there was a new warning on the quality of milk powder. In 2003, it was reported that at least 13 babies died from malnutrition in China because they were fed fake formula milk, which contained no protein. In the same year, about 20% of 2000 types of food products, which had been officially inspected, failed to meet government standards. The food products that fail to meet the official requirement usually do not contain the basic amount of nutrients that are required, contain additives that are banned in food products or contain an overdose of pesticide residual. There is great variation in food quality in China. It would not be appropriate to consider the same types of food to be similar if they were produced by different manufacturers or were from different areas, and they may not contain the standard amount of nutrients. Epidemiological studies on the relationship between dietary intake and risk of disease conducted in China usually obtained have information on dietary intake through food frequency questionnaires. 4 These studies were conducted on the basis of the assumption that the quality of food consumed by different subjects was similar over a long period. Furthermore, studies that focused on the effect of certain types of nutrients needed to further assume that the amount of nutrients in food consumed by subjects over the observation period was similar to that expected by the study. Large variation in food quality would certainly affect the validity of these studies. Future studies that involve measurement of dietary intake may need to consider the potential bias that could be caused by the variation in food quality in the country.