IJzer- en zinkabsorptie na het consumeren van ontbijtgranen afkomstig van behandelde tarwe.

Wheat is a widespread staple crop and a dominant source of energy and minerals for humans. Particularly the aleurone layer of the miller's bran is rich in iron (Fe) and zinc (Zn) and accounts for most of their content in whole wheat (20 - 35 mg/kg). For the human body to fully benefit from the minerals, they need to be bioavailable. Mineral bioavailability refers to the fraction of minerals that is absorbed in the intestinal lumen and enters the systemic circulation to exert bioactivity in the human body. The key minimum requirement for bioavailability is bioaccessibility, which is the extent to which minerals are released from their matrix in the gastrointestinal tract. Unfortunately, the bioaccessibility of wheat Fe and Zn is only 3 - 5%. This is mainly due to their chelation with phytic acid. As these chelates (i.e. phytates) are insoluble at intestinal pH, and because intestinal phytate-degrading enzymes (i.e. phytases) only have limited activity, these minerals in wheat are not bioaccessible. Unit operations that target increased phytase activity in wheat can be effective at increasing mineral bioaccessibility. During steeping and sprouting of wheat, phytase is activated and de novo synthesized. As a result, phytate is broken down, and previously bound minerals are released. Another relevant unit operation is hydrothermal processing, in which wheat grains are incubated under low pH and high temperature conditions that allow endogenous phytase activity. In addition, complexes formed during wheat processing (e.g. with acetate, lactate, or citrate) may further enhance mineral bioaccessibility. Prior to this work, it was unclear whether these unit operations, which increase Fe and Zn bioaccessibility in wheat, also enhance Fe and Zn bioavailability. To address this knowledge gap, the overall objective of this work was to investigate the effect of sprouting or hydrothermal processing of wheat on Fe and Zn bioavailability using both in vitro and in vivo approaches. In the first part, the effect of steeping and sprouting of wheat on its phytate content and Fe and Zn bioaccessibility was investigated. Strong positive correlations were observed between sprouting time, phytate reduction, and Fe and Zn bioaccessibility. Steeping (36 h, 15 °C) and sprouting (48 - 120 h, 26 °C) of wheat caused significant phytate breakdown (25 - 40%), accompanied by a 1.5 to 2.7-fold increase in Fe bioaccessibility and a 1.6 to 2.3-fold increase in Zn bioaccessibility, expressed relative to the total mineral content in wheat. However, long sprouting times (at least 48 h) and substantial phytate reduction were required to significantly increase Fe and Zn bioaccessibility. Furthermore, a multifactorial experiment in which grains were subjected to various combinations of incubation time, temperature, pH, and medium revealed that hydrothermal processing of wheat at 60 °C and pH 4.0 led to maximal phytate breakdown. The incubation medium significantly affected the extent of phytate reduction: processing in acetate or lactate buffer reduced the phytate content maximally by 54% and 61%, respectively, while using a citrate buffer achieved a maximal reduction of only 42%. Surprisingly, despite substantial phytate reduction in wheat processed in acetate or lactate buffer, Fe and Zn bioaccessibility did not differ markedly from those of untreated wheat. In contrast, hydrothermal processing (60 °C, pH 4.0) of wheat using citrate buffer increased bioaccessibility by factors 9.8 (Fe) and 8.8 (Zn) times, probably due to the formation of soluble mineral-citrate chelates. In the second part, the effects of sprouting or hydrothermal processing of wheat on Fe and Zn bioavailability were estimated using Caco-2 cells, a model system for intestinal absorption. Interestingly, sprouting wheat for up to 120 h did not affect Fe Caco-2 bioavailability, expressed relative to the total mineral content in wheat, while that of Zn was slightly higher than that of non-sprouted wheat. These findings indicate that, although sprouting increased Fe and Zn concentrations in in vitro digests, their uptake by Caco-2 cells, expressed as a fraction of bioaccessible mineral, was negatively affected. As non-labile Fe and Zn complexes may negatively affect their uptake by Caco-2 cells, the lability of bioaccessible Fe and Zn was examined using diffusive gradients in thin films (DGT). It was concluded that sprouted wheat contains non-labile Zn complexes, and that sprouting for more than 72 h leads to the formation of substantial fractions of non-labile Fe complexes. This may partly account for the lack of enhancement in Fe and Zn bioavailability to Caco-2 cells observed in sprouted wheat. Hydrothermal processing (60 °C, pH 4.0) of wheat using a citrate buffer increased Caco-2 bioavailability of Fe and Zn, by 6 and 12-fold, respectively, expressed relative to the total mineral content in wheat. This hydrothermal process did not affect the uptake of Fe and Zn by Caco-2 cells, expressed as a fraction of bioaccessible mineral. Analyses of the DGT-lability of in vitro digests showed that Fe chelates, such as Fe-citrate chelates, present in in vitro digests of hydrothermally processed wheat were sufficiently labile to contribute to increased Fe bioavailability. Most Zn in the in vitro digests was present as free ions or in labile chelates, although a small fraction remained non-labile, which may partially limit enhancements in Zn bioavailability. In the final part, the actual Zn bioavailability of breakfast flakes produced from untreated, sprouted, and hydrothermally processed wheat was measured in humans using a dual-isotope tracer technique with extrinsically labeled stable isotopes. Despite a 1.4-fold increase in Zn bioaccessibility following steeping (36 h, 15 °C) and sprouting (48 h, 26 °C), neither fractional Zn absorption (expressed relative to the total Zn content in wheat flakes) nor total Zn absorption (i.e. the amount of absorbed Zn) increased in human subjects (n = 24). Hydrothermal processing (pH 4.0, 60 °C) of wheat using citrate buffer increased fractional Zn absorption by a modest factor 1.1. However, total Zn absorption did not differ significantly between regular flakes and those made from hydrothermally processed wheat. These limited increases in Zn absorption were likely attributable to residual phytate levels, physiological regulation of Zn absorption in non-deficient human subjects, and the presence of non-labile Zn complexes. In conclusion, steeping and sprouting, and hydrothermal processing of wheat using citrate buffer effectively increase Fe and Zn bioaccessibility. However, steeping and sprouting did not enhance Fe Caco-2 bioavailability, and only prolonged sprouting modestly improved Zn Caco-2 bioavailability. Similarly, the fractional and total Zn absorption measured in human subjects was not increased by steeping (36 h, 15 °C) and sprouting (48 h, 26 °C). Hydrothermal processing (pH 4.0, 60 °C) of wheat using citrate buffer largely enhanced Fe and Zn bioaccessibility and Caco-2 bioavailability, likely due to the formation of soluble and labile mineral-citrate chelates. The human intervention study confirmed that the treatment resulted in a modest increase in fractional Zn absorption and no increase in total Zn absorption. Overall, phytate content is a major, but not the sole, determinant of mineral bioaccessibility and bioavailability in wheat. Even when phytate content is substantially reduced, other factors may still limit mineral bioavailability. Furthermore, combining the DGT technique with Caco-2 cell models provides valuable and complementary insights into mineral bioavailability. This work provides in-depth insights into how unit operations impact Fe and Zn bioavailability, contributing to the production and consumption of foods with improved nutritional quality, ultimately benefiting public health.