Nutrients and toxic elements in lettuce (Lactuca sativa): studies of bioaccessibility, bioavailability, biofortification and speciation

Lettuce can accumulate large amounts of metals and nonmetallic elements in the leaves when grown in contaminated soil. However, it is not easy to assess whether the consumption of lettuce can bring health benefits to contain nutrients or also can offer risks for being susceptible to contamination by toxic elements. Thus, besides the total quantification of the elements present in lettuce, it is also important the evaluation of the amount that is released from the food during the gastrointestinal digestion (bioaccessibility) and the amount that is absorbed by the body for the use of physiological functions (bioavailability). Thus, as a first step of the study, different types of lettuce were evaluated with respect to the total content and the bioaccessibility of Al, Cd, Co, Cu, Fe, Mo, Ni, Pb and Zn. The samples evaluated were "crespa" (loose-leaved leaves), "lisa" (cup-shaped leaves), "americana" (iceberg), and "mimosa" (oak leaf). Furthermore, since some other components may substantially influence the release of certain elements during the gastrointestinal digestion, the total polyphenol content was also determined and through theoretical calculations the evaluation of a potential correlation between the polyphenols content and the bioaccessibility was made. The results for the total concentration of the elements in lettuce samples showed the average of (in g per 100 g fresh sample): 623 for Al; 0.3 for Cd; 30 for Cu; 730 for Fe; 3.5 for Mo and 220 for Zn. The total concentration for Co, Ni and Pb were below the LOQ, as well as the bioaccessible concentration for Mo. Moreover, it was observed that the values depend on the type of lettuce, but an increasing order for bioaccessibility can be observed (considering the average for all samples): Cd> Cu> Zn> Fe> Al. The results also showed that the polyphenols might interact with the elements and carry them to the soluble fraction during the digestion. However, the cellulose contribution for the bioaccessibility values must be more pronounced, since the concentration of this substance is much higher than polyphenols. In the second part of the work, the Se biofortification was carried out using a red type of letuce (cv. "Veneza roxa"). The evaluation of the selenate or selenite application was done, as well as the biofortification effect on macro and micronutrients. Additionally, the bioaccessibility and bioavailability for Se and other nutrients was estimated, as well as the Se speciation during the gastrointestinal digestion in order to evaluate the best Se specie for being used during the biofortification program. The results showed that both Se species did not lead to a mass reduction of the shoot and the selenate promoted a greater Se accumulation by the plant, while there was an increase of at least 207 mg kg-1 at different concentrations of selenate, for selenite the maximum variation was only of 16 mg kg-1. However, the Se bioaccessibility was quite similar for all plants obtained in the different assays. On the other hand, the speciation analysis indicated that for selenite-enriched plants the main specie found was SeMet or SeMetO, while for selenate-enriched plants, the most abundant specie is still Se(VI). Thus, the lettuce plant is able to biotransform selenite more efficiently in organic forms than transforms selenate. It was also showed that the presence of different Se forms promotes changes in the nutritional balance of the lettuce, considering Cu, Fe, Mn, Mo, Ca, K, Mg, Na, P, and S. While selenate promoted the absorption of Mo and S and the reduction of the K, Mn and P concentration in the shoot, selenite increased Mn and reduced the Mo accumulation. Since the absorption of Cu and Fe was always negatively affected, the concentration of Ca and Mg showed a slight increase, and Na and Zn were not affected on the presence of any Se specie. The bioaccessibility for all elements was not substantially affected on the presence of different Se forms. Finally, the results showed that only Mo, Se and Zn were absorbed in significant quantities by the Caco-2 cells, and only for Se the absorption was strongly influenced by the Se speciation (AU)