Distinct arsenic metabolites following seaweed consumption in humans

Abstract

Seaweeds contain arsenic primarily in the form of arsenosugars, which can be metabolized to a wide range of arsenic compounds. To characterize human exposure to arsenic from seaweed consumption, we determined concentrations of arsenic species in locally available seaweeds, and assessed urinary arsenic compounds in an experimental feeding study. A total of 11 volunteers consumed 10 g per day of three types of seaweeds (nori, kombu, and wakame) for three days each, while abstaining from rice and seafood following a three-day washout period. Urinary arsenosugars and their metabolites (including dimethyl arsenate (DMA), thio-dimethylarsinoylethanol (thio-DMAE), thio-dimethylarsinoylacetate (thio-DMAA), and thio-DMA) were measured in spot urine samples prior to seaweed consumption, and in 24-hour urine samples while consuming seaweed. Commercial products made from whole seaweed had substantial concentrations of arsenic (12-84 µg/g), dominated by arsenosugars. Intact arsenosugars along with DMA, thio-DMAA, thio-DMAE all increased in urine after ingesting each type of seaweed, and varied between seaweed types and between individuals. Only trace levels of the known toxic metabolite, thio-DMA, were observed, across individuals. Thio-DMAE and thio-DMAA are unique products of arsenosugar breakdown, thus assessment of these compounds may help to identify dietary intake of arsenic from seaweed from other exposure pathways.

Figure 1

Arsenic speciation for seaweeds used in feeding study (A–C at the top of the table) and in 23 seaweeds and seaweed products marketed for consumption. Concentrations of total arsenic, extractable arsenic (50% methanol (MeOH); for seaweeds (A–C): 1% HNO₃ heated to 90 °C, 65% dichloromethane (DCM) also shown) and arsenic species are all reported in µg/g.

Figure 2

Feeding experiment design.

Figure 3

Concentrations of arsenic species (µg/L) in urine samples, normalized to specific gravity. Samples labelled D0 (day 0) are spot samples on the day prior to seaweed consumption, and D1 (day 1), D2 (day 2) and D3 (day 3) are 24 h urine collection samples for the days following consumption of each seaweed portion. Seaweeds A, B and C were nori, kombu and wakame, with arsenic concentrations of 17.1, 45 and 46 µg/g respectively. Major arsenic species (DMA, thio-DMAE, thio-DMAA), intact arsenosugars (sum of As sugar-GLY, -PO₄ and -SO₃), and thio-DMA are shown; see legend for color codes. Figure 2 Inset Mean before ingestion-after ingestion differences in urinary concentrations of arsenic species and total arsenic across individuals for each seaweed type. Arsenic species are color-coded according to the legend. Before-after differences were adjusted for age, gender and BMI. Symbols represent statistical significance (⁺represents p < 0.05; *represents p < 0.001).