Establishing a baseline value for urinary arsenic:selenium ratio in unexposed populations in the United Kingdom

Abstract

The relationship between arsenic (As) and selenium (Se) in the human body is poorly understood. We have investigated the concentrations of urinary As and Se in three ethnic groups (n=63) in the United Kingdom and show that there is a positive correlation (r=0.62, p<0.001) between total concentrations of As and Se and that the ratio of these two elements is stable, with a mean value (±SD) of 0.7±0.4. Furthermore, concentrations of individual arsenic species methylarsonate (MA), dimethylarsinate (DMA) and arsenobetaine (AB) in the urine samples show a positive correlation with total Se (As(III) and As(V) were not detected). The intra-individual variation of the As:Se ratio also remains stable over time, as determined by monitoring a volunteer over a period of one year, and deviates only after recent seafood consumption. It appears that the ratio is also stable across diverse populations across different cultures and continents, evident from our calculation of As:Se ratio from concentrations of these elements found in urine samples from different populations published in the literature. Our study involved analysis of 63 urine samples from three ethnic groups (White Caucasian n=20, Asian n=21 and Somali n=22), 58 urine samples from 29 Ramadan fasting volunteers and 12 from one volunteer whose urine samples were collected over a period of one year. All the participants completed a lifestyle questionnaire and were asked to refrain from eating seafood or fish for three days prior to collection of the sample. Total As and Se in urine were determined by inductively coupled plasma mass spectrometry (ICP-MS). As species (AB, DMA, MA, As(III), As(V)) were determined by using high performance liquid chromatography (HPLC) combined with ICP-MS. Mean ± SD As:Se ratios of 0.8±0.4, 0.7±0.4, 0.4±0.2, 0.7±0.3 and 1.2±0.3 were obtained for the Asian, White Caucasian, Somali, fasting, and one volunteer respectively, giving an overall mean of 0.7±0.4 (SD). It is noteworthy, that when comparing ethnic differences, the Somali group shows a statistically significant lower As:Se ratio (0.4±0.2, p<0.05) compared to Asian and White Caucasian groups; this is ascribed to lower urinary As concentrations in this group. The study over one year with a single volunteer revealed that recent (within 3 days) seafood consumption results in a significantly different (p<0.05) As:Se ratio (4.0). We have calculated from the literature the value of As:Se for populations, exposed to As through drinking water, can range from 2.0–9.6. Based on our own work and the values we calculated from other studies we suggest that the baseline range for mean As:Se ratio is 0.4–1.2, provided that the urine samples are collected in the absence of recent consumption of seafood. The relatively stable As:Se ratio observed in this study suggests a link between these two elements in humans adding support to earlier studies with animals and humans exposed to inorganic arsenic in drinking water which reported interaction between these elements and that Se may play a role in counteracting As toxicity.