Interests and limitations
Interests and limitations of TDS in comparison to other surveillance tools
In order to cover the whole diet with limited costs, TDS uses food sample pooling in order to limit the number of foods or food groups to around 200 core foods. In dietary surveys, more than 1000 different foods are usually identified, and in some countries more than 5000 (EFSA, 2009). It is not possible to analyse separately all these foods because costs are prohibitive. “Pooling” means that different foods from the same food group are mixed to make a unique sample. The quantities of the different foods mixed together are related to their intake. This pooling method means that acute exposure assessment cannot be performed on TDS data without auxiliary information. The reason is that food pooling reduces the food contamination variability and that maximum contamination values are not estimated by TDS. There is no record of the real food contamination variability because of this sample pooling. Acute exposure needs to take account of food contamination variability because the highest acute exposures are directly explained by the highest food contamination values. Hence, the main aim of TDS is chronic exposure assessment and not acute exposure assessment.
The utility of TDS is very high because there are far more relevant chronic exposure assessments to be done than acute exposure assessments. The reason why there are many chronic exposure assessment to be performed is that a lot of substances may be toxic at low dose in the long term. The second issue is related to the representation of the variability of food exposure. Several intake profiles need to be built to capture subpopulation diets. The core foods are chosen depending of the different subpopulation to be studied so that TDS exposure can be considered as representative. Another main interest of TDS is the standardisation of the methods and their representativeness. TDS methodology allows assessing time and country trends because of this standardization (Rose M et al, 2010). However, there are still differences in TDS methodology between countries and this could be a limitation not only in the essential comparison of exposure to contaminants between countries for public health purposes but also for research.
In a TDS, foods are analysed after being prepared as usual for consumption. Thus, they might contain some chemicals, such as nitrosamine, which are formed during food processing. On the other hand, they might not contain certain chemicals originally present in the raw foods, which are destroyed during heating or removed during washing and peeling. Thus, the chemicals in the foods analysed in a total diet study are more closely representative of what is actually ingested by the consumer rather than what is produced (raw agricultural commodities) or purchased at the retail level (Moy, 2011).
Unlike most surveillance samples, total diet samples are usually analysed for many different chemicals to save sampling costs. This has the additional benefit of facilitating risk-benefit analysis for different chemicals, such as polychlorinated biphenyls, mercury and omega-3 fatty acids in fish.