Cadmium bioavailability and bioaccessibility as determined by in vitro digestion, dialysis and intestinal epithelial monolayers, and compared to in vivo data

Abstract

The objective of the study was to compare in vivo estimates of Cd bioavailability in two diet materials (lettuce and durum wheat grain) with bioaccessibility estimates from three in vitro methods. For both dietary materials, the Cd was either incorporated during growth or applied topically as a soluble salt just prior to experimentation. Simulated gastric/intestinal digestion using a physiologically based extraction technique (PBET) solubilized less than 56% (lettuce) or 13% (grain) of the Cd that was either incorporated into the plant tissues during growth, or added to the plant tissues before experimentation, as Cd(NO₃)₂ · H₂O. Amended diets could not be distinguished from incorporated diets. More of the Cd solubilized from amended lettuce than from incorporated lettuce moved to the outside of MWCO 10 kD and 25 kD dialysis sacs; no difference between the amended and incorporated diets was observed for grain. The percentage of lettuce-Cd solubilized by the PBET and sorbed by Caco-2 cells was greater for incorporated than for amended lettuce; for Cd in grain, the reverse occurred. As expected, none of the in vitro estimates of bioaccessibility were the same percentage of Cd in the lettuce or grain as was measured as bioavailable in vivo. The in vitro assays all predicted that substantially less than 100% of the Cd in the foods would be bioavailable, as was identified in vivo, and simulating intestinal selectivity improved the comparison to in vivo. Some of the in vitro assays identified subtle differences between the diets (i.e., amended vs. incorporated) that were consistent with in vivo studies, and with speculated differences in Cd speciation; this suggests their potential usefulness for the study of modifiers to dietary Cd bioavailability.

Keywords:

• CD
• dietary bioaccessibility
• in vivo
• in vitro
• PBET
• dialysis

Acknowledgments

The authors gratefully acknowledge funding from Canada's Natural Science and Engineering Research Council, and the Canadian Institutes for Health Research.

Notes

^a Each value in this column is the ratio resulting from dividing the mass of Cd in the chyme resulting from digestion of the food sample that was digested by the mass of Cd in the original food; parenthetic value is the standard error of the mean.

^a Each value in this column is the mass of Cd outside the dialysis sac at the end of the experiment, divided by the mass of Cd inside the dialysis sac at the end of the experiment; parenthetic value is the standard error of mean.

^b Each value in this column is the mass of Cd outside the dialysis sac at the end of the experiment divided by the mass of Cd that was put into the dialysis sac at the beginning of the experiment (i.e. mass of Cd in the chyme); parenthetic value is standard error of mean. Bold values denote difference (P ≤ 0.05) between amended and incorporated, within diet type and MWCO.

^a Each value in this column is the mass of Cd in the cells divided by the mass of Cd in the volume of chyme to which the cells were exposed; parenthetic value is the standard error of the mean. Bold values denote difference (P < 0.05) between amended and incorporated, within diet.