Date of Award

1-2023

Document Type

Thesis

Degree Name

Master of Public Health (MPH)

Department

School of Public Health

First Advisor

Nicole C. Deziel

Second Advisor

Mary H. Ward

Abstract

Nitrate levels are increasing in our environment, and epidemiological studies have found evidence of adverse health risks from this exposure. Therefore, it is important to characterize the magnitude of nitrate exposure from food and drinking water sources. This study assesses the magnitude of nitrate exposure for 47 farmers from the Agricultural Health Study (AHS) in 2018-2019. The participants in our study use private wells as their main drinking water source and live in agricultural area of Iowa (10 counties) where ground water is vulnerable to nitrate contamination. The extent of nitrate exposure is determined by several factors, including the types of food consumed, the application of nitrate-based fertilizers in agriculture, and the quality of drinking water. The association between urinary nitrate, and nitrate consumed from diet and drinking water sources were evaluated using linear regression models based on spot urine samples, participant questionnaires, and the National Cancer Institute (NCI) Automated Self-Administered (ASA) 24-Hour Dietary Assessment tool. Total 24-hour nitrate intake (water and diet combined) was the strongest predictor of urinary nitrate concentrations (β = 0.177, 95% CI: 0.060-0.295, R2 = 0.53). One example of this result is that a twenty percent increase in total 24-hour nitrate intake is associated with a three percent increase in urinary nitrate concentrations (β = 3.28%, 95% CI: 1.09%-5.53%). Nitrate intake from water in the prior 24-hours was a moderately significant predictor (β = 0.067, 95% CI: 0.018-0.116). This result tells us a twenty percent increase in 24-hour nitrate intake from water is associated with a one percent increase in urinary nitrate concentrations (β = 1.22%, 95% CI: 0.33%-2.14%). Dietary nitrate intake in the prior 24-hours was a borderline significant predictor of urinary nitrate concentrations (β = 0.119, 95% CI: -0.011-0.250, R2 = 0.53). A twenty percent increase in 24-hour nitrate intake from diet is associated with a two percent increase in urinary nitrate concentrations (β = 2.19%, 95% CI: -0.20%-4.66%, R2 = 0.53). When using the NCI ASA-24 dietary recall assessments to estimate exposure, the majority of 24-hour nitrate intake was from dietary sources (97.2%) rather than drinking water sources (2.7%). The relatively low median contribution of nitrate intake from water was mainly due to a high percentage of participants having nitrate levels in their wells below the detection limit (<0.04 mg/l). Our study adds to existing literature for the AHS cohort and increases understanding of dietary and drinking water sources, nitrate predictors, and the role of drinking water and nitrate excretion for private drinking water sources in Iowa. Both dietary and water and nitrate intake can contribute to overall nitrate exposure, and it is important to consider both sources when assessing the potential health risks associated with nitrate consumption. To our knowledge, there is only one study (unpublished) that compared estimated intake of nitrate from diet and drinking water sources to urinary nitrate excretion.

Comments

This thesis is restricted to Yale network users only. It will be made publicly available on 11/15/2023

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