Critical review of the safety assessment of titanium dioxide additives in food

Nanomaterial engineering provides an important technological advance that offers substantial benefits for applications not only in the production and processing, but also in the packaging and storage of food. An expanding commercialization of nanomaterials as part of the modern diet will substantially increase their oral intake worldwide. While the risk of particle inhalation received much attention, gaps of knowledge exist regarding possible adverse health effects due to gastrointestinal exposure. This problem is highlighted by pigment-grade titanium dioxide (TiO2), which confers a white color and increased opacity with an optimal particle diameter of 200-300 nm. However, size distribution analyses showed that batches of food-grade TiO2 always comprise a nano-sized fraction as inevitable byproduct of the manufacturing processes. Submicron-sized TiO2 particles, in Europe listed as E 171, are widely used as a food additive although the relevant risk assessment has never been satisfactorily completed. For example, it is not possible to derive a safe daily intake of TiO2 from the available long-term feeding studies in rodents. Also, the use of TiO2 particles in the food sector leads to highest exposures in children, but only few studies address the vulnerability of this particular age group. Extrapolation of animal studies to humans is also problematic due to knowledge gaps as to local gastrointestinal effects of TiO2 particles, primarily on the mucosa and the gut-associated lymphoid system. Tissue distributions after oral administration of TiO2 differ from other exposure routes, thus limiting the relevance of data obtained from inhalation or parenteral injections. Such difficulties and uncertainties emerging in the retrospective assessment of TiO2 particles exemplify the need for a fit-to-purpose data requirement for the future evaluation of novel nano-sized or submicron-sized particles added deliberately to food.