Cadmium oral bioavailability is affected by calcium and phytate contents in food: Evidence from leafy vegetables in mice

Highlights

• A strong negative correlation between Cd bioavailability and Cd concentration.

• Pakchoi with higher Cd concentration was lower in health risk due to low Cd-RBA.

• Higher Ca concentration was associated with lower Cd bioavailability.

• High phytate in leafy vegetables contributed to high Cd bioavailability.

• Calcium biofortification was effective to reduce Cd bioavailability.

Abstract

To test high cadmium (Cd) concentration may not be high in health risk when considering Cd bioavailability, we assessed variation of Cd relative bioavailability (RBA, relative to CdCl₂) using a mouse assay for 14 vegetables of water spinach, amaranth, and pakchoi. Cadmium concentration varied from 0.13 ± 0.01–0.37 ± 0.00 μg g^–1 fw. Cadmium-RBA also varied significantly from 22.9 ± 2.12–77.2 ± 4.46%, however, the variation was overall opposite to that of Cd concentration, as indicated by a strong negative correlation between Cd-RBA and Cd concentration (R² = 0.43). Based on both Cd concentration and bioavailability, the identified high-Cd pakchoi variety resulted in significantly lower Cd intake than the high-Cd varieties of water spinach and amaranth (4.74 ± 0.05 vs. 10.1 ± 0.54 and 8.03 ± 0.04 μg kg^–1 bw week^–1) due to significantly lower Cd-RBA (22.9 ± 2.12 vs. 77.2 ± 4.46 and 51.3 ± 2.93%). The lower Cd-RBA in pakchoi was due to its significantly higher Ca and lower phytate concentrations, which facilitated the role of Ca in inhibiting intestinal Cd absorption. This was ascertained by observation of decreased Cd-RBA (90.5 ± 12.0% to 63.5 ± 5.53%) for a water spinach when elevating its Ca concentration by 30% with foliar Ca application. Our results suggest that to assess food Cd risk, both total Cd and Cd bioavailability should be considered.

Introduction

Cadmium (Cd) contamination in agricultural soils is a major global health concern (He et al., 2013, Yang et al., 2019). A recent Chinese national survey showed that 7.0% of soils contained elevated Cd concentrations (Chen et al., 2014, Zhao et al., 2015). Soil Cd can be taken up by crops and enter the food chain (Liu et al., 2013, Larson, 2014), threatening human health by causing various diseases including cancers (Riederer et al., 2013, Garcia-Esquinas et al., 2014). Concerns regarding Cd health effects have promoted massive assessment of Cd concentration in food including rice, wheat, and vegetables (Williams et al., 2009, Fu et al., 2013a, Fu et al., 2013b, Meharg et al., 2013a, Meharg et al., 2013b, Norton et al., 2015, Hu et al., 2016). While rice contributes to the majority (81%) of Cd dietary intake, vegetable consumption is a non-negligible contributor (19%) (Chen et al., 2018). Among vegetables, leafy greens are of the greatest concern since they tend to accumulate higher Cd concentrations than rootstalk and legume vegetables (Luo et al., 2011, Huang et al., 2014, Chen et al., 2018).

To assess and mitigate Cd exposure risks via the food chain, previous studies mainly focused on Cd accumulation in the edible portions without considering Cd oral bioavailability (Arao et al., 2009, Fu et al., 2013a, Fu et al., 2013b, Meharg et al., 2013a, Meharg et al., 2013b). Limited studies evaluated to what extent Cd in food can be absorbed by humans following ingestion and what factors drive this process. This process is termed as the bioavailable process where a fraction of total Cd dissolves in gastrointestinal fluids and is then absorbed across the intestinal epithelium into the blood circulation system (Reeves and Chaney, 2008, Zhao et al., 2018, Sun et al., 2020). This part of Cd is termed as the bioavailable Cd, determining human internal Cd exposure and health effects to a greater extent than total Cd. To measure Cd bioavailability, in vivo assays such as mouse models have been developed (Juhasz et al., 2010). Previous studies have shown that Cd accumulates mainly in the kidneys following uptake, which were often used as the endpoint to reflect Cd bioavailability (Zhao et al., 2017b, Zhao et al., 2017a, Zhao et al., 2018, Li et al., 2019). In mouse assays, Cd accumulation in mouse kidneys from test materials was compared to that from a soluble Cd compound such as CdCl₂. This gives an estimation of relative bioavailability (RBA) of Cd in food, which can be used for human health risk assessment. To the best of our knowledge, only Zhao et al. (2017b) reported Cd-RBA for 6 species of leafy vegetables using the mouse assay, showing considerable variation (17.7–78.0%) with the influence of Cd-RBA on Cd health risk remaining unexplored.

Traditionally, without considering Cd oral bioavailability, people assume that high-Cd foods are high in Cd health risk (Wang et al., 2019, Zhao and Wang, 2020). However, Cd health risk depends not only on total Cd concentration, but also on Cd bioavailability. High-Cd food may be low in Cd bioavailability and health risk because variation in food calcium (Ca), iron (Fe), and zinc (Zn) concentrations may strongly influence the Cd absorption in the gastrointestinal tract. Studies have shown reduced Cd absorption in animals with elevated Ca, Fe, and Zn via competition between these mineral nutrients and Cd for shared absorption channels (Garrick et al., 2006, Martinez-Finley et al., 2012, Reeves and Chaney, 2002). In addition, high phytate contents in food especially in some leafy vegetables such as amaranth may result in Ca, Fe, and Zn precipitation in the intestine (White and Broadley, 2009). By mediating bioactivity of Ca, Fe, and Zn, phytate in food may also affect Cd bioavailability in the intestine. Given considerable variation in composition (Ca, Fe, Zn, and phytate) among foods, it is conceivable that Cd bioavailability may vary greatly and high-Cd food may not always be high in Cd bioavailability.

To test the above viewpoint, Cd-RBA in 14 leafy vegetables of 3 species varying in genotypes were studied. Vegetables were selected due to their relatively higher mineral and phytate contents than cereals such as rice. The vegetables were grown in a greenhouse in a Cd-contaminated soil (7.30 μg Cd g^–1). After 45-d growth, vegetables were harvested and analyzed for Cd, mineral (Ca, Fe, and Zn), and phytate concentrations and Cd-RBA using a mouse assay. Surprisingly, we observed that Cd-RBA in vegetables was high up to ~80%. Further, vegetables of higher Cd concentration showed lower Cd-RBA. To explain these findings, the mechanisms of vegetable composition (minerals and phytate) affecting Cd bioavailability were illustrated by performing a Ca biofortification trial and in vitro Ca bioaccessibility test. Results will inspire further studies that aim to reduce Cd bioavailability in addition to Cd concentration to protect humans from Cd exposure via the food chain.