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Accumulation of Engineered Nanoparticles in Belowground Vegetables: Nutritional Bioaccessibility and Dietary Exposure Risks

Ebbs, Stephen; Ma, Xingmao
Southern Illinois University
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The central hypothesis of this work is that plant foods that develop belowground (e.g., root, tuberous, and bulb vegetables) in direct contact with substrate containing engineered nanoparticles (ENPs) accumulate higher concentrations of ENPs that are more bioaccessible during digestion. Consequently, dietary exposure from consumption of these edible plants represents a greater risk of ENP exposure to human health.

This research will evaluate the accumulation of six metallic ENPs in edible tissues of ten common vegetables grown in either hydroponics or soil. These efforts will determine the extent of accumulation and how soil factors mitigate that accumulation. These results will be contrasted with the accumulation of the ionic metal counterpart to the ENP to provide a frame of reference to evaluate the ENP accumulation. The hydroponic experiments allow for the development of worst case scenarios for ENP accumulation for each ENP type. The comparison to the accumulation of the corresponding ionic metal(s) also provides a critical frame of reference to interpret the magnitude and relevance of the ENP accumulation. Efforts will also assess the spatial distribution of the ENPs in the edible plant tissues and whether the basic preparation through peeling removes tissues with the highest concentration of the ENPs.

The fundamental information provided by this evaluation will contribute to our understanding of ENP transport into plant tissues, the corresponding risks to food safety, and how simple preparation techniques may reduce human dietary exposure to ENPs. Tissues from these experiments and from another ENP project funded by AFRI will be used in a physiologically-based extract test to evaluation potential release of the ENPs during digestion. The data obtained will be used to generate dietary exposure models to assess the possible impact to human health. Those models and the information on bioavailability of ENPs in the soil have direct benefits for sustainable agriculture in the United States, specifically the goals for satisfying human food needs and maintaining the environmental quality on which the US agricultural economy depends.

The results will provide a risk-based approach to make sound decisions on agricultural land use. Understanding the behavior of ENPs in agricultural soil will allow growers, extension agents, and USDA to make sound decisions on choice of crops for particular ENP-impacted soils. The specific accumulation and dietary exposure scenarios associated with particular ENP and plant combinations would allow for recommendations or restrictions to be offered concerning which ENP-containing commercial products can be safely applied to food crops. The dietary exposure models will be of importance in predicting potential human exposures to ENPs. For those exposure scenarios that may present a tangible risk to human health, a monitoring program may have to be initiated to check plant foods for ENP contamination. Evidence arguing against significant dietary exposures could illustrate scenarios where few to no monitoring and/or food warnings are warranted.

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Non-Technical Summary:
Engineered nanoparticles (ENPs) are emerging contaminants that potentially impact food safety and human health. The number of consumer products and industrial uses for ENPs is increasing steadily. Not surprisingly, the environmental release of ENPs is expected to increase, leading to the potential release of ENPs onto arable land. What's more, an increasing number of agricultural products include nanoparticle materials in their formulation and therefore represent a direct source of ENP release. As there is no evidence that the use of ENPs will decrease, a proactive approach is needed to anticipate the potential impact of ENPs in the food supply on the general population. The empirical research conducted here will deliver information on the tendency for ten common belowground vegetable crops grown in the US to accumulate six common ENPs. From this information, it will be possible to derive information on which of those ten crops are susceptible to ENP accumulation and toxicity. The efforts to examine the spatial distribution of those ENPs in the tissues and the extent to which simple preparation techniques contribute to the removal of those ENPs will also be provided. The study here will also examine whether those ENPs are likely to be released during digestion and present a danger of human exposure through the diet. Collectively, the results here will have direct benefits for sustainable agriculture in the United States, specifically the goals for satisfying human food needs and maintaining the environmental quality on which the US agricultural economy depends.

This research is divided into six specific aims. For Specific aim #1, hydroponic screening studies will be conducted with six ENPs and ten tuberous, root, and bulb vegetables across a range of concentrations to examine the accumulation of ENPs or the corresponding ion (e.g., ZnO v. Zn2+) in the edible tissues. While this hydroponic approach minimizes variables that might influence ENP bioavailability in the soil that may influence ENP accumulation in edible tissues, this approach provides a conservative over-estimate of ENP accumulation. The impact of the ENPs and metal ions on the growth, development, vegetative yield, and physiological status will also be assessed.
For Specific aim #2, experiments will also be conducted with the selected ENPs, their ionic counterparts, and the array of belowground vegetables grown in silt-loam soils with low or moderate clay content. These experiments will demonstrate how growth in soil modulates ENP accumulation in the belowground plant tissues as compared to the hydroponic experiments. Efforts will also examine the bioavailability and chemical form of the ENPs in each soil type and ENP accumulation in the selected vegetables over repeated croppings of the ENP-amended soil.
Specific aim #3 will use parallel approaches to examine the spatial distribution and characteristics of the ENPs in the vegetable tissues using imaging and sectioning techniques to assess the extent of ENP penetration into and throughout the edible tissues. In the latter case, a turnip exposed to ENPs might be peeled and the remaining flesh cut into concentric rings from the outer surface to the center. Analysis of the peels and each ring of tissue would provide a general picture of radial penetration. Similarly, edible plants with a largely longitudinal axis, such as carrot and parsnip could be segmented along the length of the vegetable, with each segment analyzed separately.
Specific aims #4 and #5 will assess the nutritional bioaccessibility of ENPs from edible tissues from this project and a previously funded AFRI food safety project (PI J. White) using a physiologically-based extraction test (PBET). The full scope of plants tested would include the ten root and tuberous vegetables here as well as several leafy vegetables, fruits, and cereal grains.
Through Specific aim #6, the results for PBET tests on tissues from the two projects will be used to generate a series of age- and gender-specific dietary exposure models for each ENP and plant food. These models will vary from simple exposure scenarios based upon the consumption of a single edible plant food containing a single ENP to more complex exposure models based upon consumption of combinations of plant foods containing single or multiple ENPs. The models will also provide a comparison of peeled plant tissues versus unpeeled.

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Nat'l. Inst. of Food and Agriculture
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Sanitation and Quality Standards
Natural Toxins
Viruses and Prions
Bacterial Pathogens
Chemical Contaminants
Food Defense and Integrity