Levels of infants' urinary arsenic metabolites related to formula feeding and weaning with rice products exceeding the EU inorganic arsenic standard

Overview

This PLOS ONE study measured urinary arsenic species in 79 infants from Belfast before weaning, followed 11 infants with paired pre- and post-weaning urine samples, and measured arsenic species in 73 rice-based products purchased in Belfast in February 2016. The food analyses used IC-ICP-MS speciation and directly measured inorganic arsenic (iAs), DMA, and MMA in baby rice, rice crackers/cakes, and rice cereals. The study found higher urinary methylated arsenic species in formula-fed infants before weaning, higher urinary arsenic metabolites after rice-product weaning in the paired subset, and frequent exceedance of the source-cited EU 0.10 mg/kg iAs comparator for rice destined for foods for infants and young children.

Key numbers

Infant urine before weaning (Table 1; median with min-max, microgram/L):

Feeding mode	n	AsB	DMA	MMA	iAs	tAs excluding AsB
Breastfed	20	0.07 (0.01-0.40)	0.17 (0.03-0.61)	0.01 (0.00-0.06)	0.13 (0.02-0.76)	0.31 (0.08-1.16)
Mixed feeding	27	0.07 (0.01-0.16)	0.15 (0.03-2.70)	0.03 (0.00-0.32)	0.12 (0.06-0.59)	0.32 (0.09-3.60)
Formula fed	32	0.08 (0.03-0.42)	0.37 (0.09-3.25)	0.04 (0.01-0.83)	0.15 (0.04-0.92)	0.58 (0.25-4.33)

Formula-fed infants had higher log MMA, log DMA, and log total arsenic (tAs) than breastfed or mixed-fed infants (p<0.001, p=0.001, and p=0.001, respectively). The authors report formula-fed median urinary MMA, DMA, and tAs as 6.7-fold, 2.2-fold, and 1.9-fold higher than breastfed infants.

Paired infant urine before and after weaning (Table 2; n=11; median with min-max, microgram/L):

Timing	Infant age (months)	AsB	DMA	MMA	iAs	tAs excluding AsB
Pre-weaning	2.1 (1.3-6.5)	0.09 (0.03-0.18)	0.26 (0.07-3.24)	0.03 (0.00-0.30)	0.13 (0.06-0.93)	0.57 (0.16-4.21)
Post-weaning	7.7 (6.6-9.3)	0.05 (0.00-10.22)	2.38 (0.08-9.27)	0.22 (0.01-1.34)	0.21 (0.07-3.96)	2.81 (0.18-12.89)

Post-weaning urine had higher log MMA, log DMA, and log tAs than paired pre-weaning urine (p=0.013, p=0.005, and p=0.008). Median post-weaning MMA, DMA, and tAs were 7.2-fold, 9.1-fold, and 4.8-fold higher than paired pre-weaning values. Post-weaning iAs trended higher (1.6-fold; p=0.067). Mothers reported that 91% of the paired-subset infants consumed rice or rice-based products as part of the weaning diet.

Rice-based product iAs in 2016 (Table 3; median with min-max, mg/kg):

Product group	n	iAs	DMA	MMA	Percent above 0.10 mg/kg source comparator
Baby rice	13	0.103 (0.055-0.177)	0.080 (0.013-0.109)	0.006 (0.003-0.008)	61%
Rice crackers/cakes	29	0.118 (0.063-0.165)	0.026 (0.014-0.063)	0.004 (0.003-0.005)	79%
Rice cereals	31	0.081 (0.056-0.138)	0.025 (0.016-0.062)	0.004 (0.004-0.005)	32%

The authors also compare these 2016 values with a 2014 pre-regulation dataset from their earlier work: baby rice n=29, median iAs 0.121 mg/kg, 58% above 0.10 mg/kg; rice crackers n=36, median iAs 0.127 mg/kg, 80% above 0.10 mg/kg; rice cereals n=53, median iAs 0.076 mg/kg, 18% above 0.10 mg/kg. Across all 73 rice-based products analyzed in 2016, 56% exceeded 0.10 mg/kg and the median iAs was 0.104 mg/kg. Inorganic arsenic represented a median 74% of summed arsenic species across the rice-based products, ranging from 46% to 86%.

Belfast tap-water samples were low in arsenic species: iAs median 0.042 microgram/L (0.039-0.047), DMA 0.029 microgram/L (0.027-0.031), and TMAO 0.053 microgram/L (0.050-0.057).

Methods (brief)

Pre-weaning urine samples were collected from 79 Belfast infants born in 2015 and classified by pre-weaning feeding mode: breastfed (n=20), formula fed (n=32), or mixed feeding (n=27). An 11-infant subset had paired pre- and post-weaning urine samples; the post-weaning samples were collected at median age 7.7 months. Urine was collected with pre-tested cotton pads, squeezed from the pads, aliquoted, frozen at or below -20°C, centrifuged, treated with hydrogen peroxide to convert arsenite to arsenate, and analyzed without dilution adjustment.

The food arm purchased 73 rice-based products in Belfast in February 2016: baby rice (n=13), rice crackers/cakes (n=29), and rice cereals (n=31), covering nine popular commercial brands or manufacturers. This source page reports only aggregate product-group data and does not reproduce brand-coded results. Samples were freeze-dried, powdered, digested in 1% nitric acid using a CEM MARS 6 microwave system, centrifuged, treated with hydrogen peroxide, and analyzed by ion chromatography coupled to ICP-MS (Thermo Scientific IC5000 with Thermo AS7/AG7 column/guard column interfaced to Thermo iCAP Q ICP-MS in collision-cell mode).

Arsenic species measured were DMA, iAs, MMA, tetramethylarsonium, and arsenobetaine. Reported quality-control recoveries were ClinChek urine CRM: iAs 113 +/- 2%, MMA 95 +/- 2%, DMA 102 +/- 2%, AsB 95 +/- 2% (n=9); NIST 1568b rice flour: iAs 99 +/- 1%, MMA 126 +/- 2%, DMA 101 +/- 3% (n=4). LODs were 0.005 microgram/L for urine and 0.003 mg/kg for rice-based products, calculated from DMA calibration. Statistical analyses used R 3.2.3, ANOVA with Tukey’s range test for feeding-mode comparisons, paired t-test for pre/post-weaning comparisons, natural-log transformed arsenic species, and 1/2 LOD substitution for values below LOD.

Implications

Standards work: This source is direct iAs evidence for Baby Cereals / Grain Products, Dry (Rice-Based) and Rice snacks/crackers, with HPLC/IC-ICP-MS speciation and explicit comparison to the EU 0.10 mg/kg historical rice-for-infant-food comparator. It also provides exposure-biomarker context showing that rice-product weaning coincided with higher urinary MMA, DMA, and tAs in the paired infant subset.

Courses: Useful case study for explaining why rice-containing infant products require inorganic-arsenic speciation rather than total arsenic alone, and why product labels and use-context matter when rice products not explicitly marketed for infants are nevertheless consumed by infants and young children.

App: Supports a high-priority iAs flag for rice-based infant cereals and rice snacks/crackers when used during weaning. The data are product-as-sold concentrations, paired with infant urinary biomarker evidence.

Microbiome: Not addressed in this study.

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Download evidence packet

Institute of Contaminant Standards. (2026). Levels of infants' urinary arsenic metabolites related to formula feeding and weaning with rice products exceeding the EU inorganic arsenic standard. Heavy Metal Index. https://heavymetalindex.com/sources/signes-pastor2017-infant-arsenic-rice-weaning

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