Arsenic species in human milk

Background:
Breast milk is an incomparable food for infants, containing all the micro- and macro- nutrients that an infant needs for healthy growth. Although breastfeeding of infants is recommended globally at least for the first six months of their lives, the fact that maternal toxic metal stores are mobilised into breast milk implies that infants, whose mothers are exposed to toxic metals, are at risk of multiple exposure through breast milk intake. For infants in particular, these exposures may have adverse effect on the developing central nervous system, the immune system or other functionality, leaving a life-long defect.
Arsenic is one of the metal(oid) of concern whose toxicity depends heavily on its chemical form. Inorganic arsenic (iAs) is a known carcinogen that has also been associated with several additional health problems, including heart disease and diabetes. A valuable risk assessment therefore has to be based on the arsenic species being present rather than on the total arsenic concentration. Speciation analysis that can provide such data has rarely been reported up to now for human milk and was limited with respect to detection power.

The new studies:
A group of researchers from Austria and Norway now has developed a method for the determination of arsenic species being present in human milk.  First, the milk was fractionated into the water-soluble fraction and the lipid fraction by using trifluoroacetic acid for protein precipitation and dichloromethane for liquid-liquid extraction of the lipid fraction. The aqueous phase was used for the determination of water-soluble arsenic species by HPLC-ICP-MS using anion- and cation exchange chromatography. Polyatomic interferents such as ArCl+ resulting from the use of the dichloromethane were overcome by using an ICP-QQQ-MS for detection. Quantification was performed by external calibration against standard arsenic species (AB, DMA, MA, and As(V)) based on peak areas in the calibration range of 0.02 to 5 µg/L As depending on the species. Application of the method to two breast milk samples from Norwegian mothers suggested that iAs was not a significant species, and that arsenobetaine, dimethylarsinate, and a currently unknown As species constituted most of the water-soluble arsenic.

While the authors had their focus on the determination of iAs as the most toxic species, they discovered in an initial testing of 10 samples that a significant portion of the arsenic in the human milk was lipid-soluble. Since it was recently shown that some arsenic-containing lipids (arsenolipids) are highly toxic to human cells and that they have the potential to cross the blood–brain barrier, these compounds should be included into the risk assessment. The researchers therefore investigated in detail the lipid-soluble arsenic in five of the samples by purifying the major compounds and using high-performance liquid chromatography coupled to both elemental and molecular mass spectrometry to identify them as arsenic hydrocarbons and arsenic fatty acids. The level of lipid-soluble arsenic varied notably between samples, ranging from 0.05 to 0.98 μg of As/kg corresponding to approximately 2–61% of the total arsenic. Dietary fish is a likely source of the arsenolipids detected in the milk.  The authors conclude, that the alarming results related to the toxicity of some of the arsenolipids together with the sensitivity of the developing brain to toxicants warrants further investigations leading to a complete risk assessment.

Agilent Technologies Inc. - 7900 ICP-MS

June 16, 2005: Arsenic concentration in groundwater may be affected by bacteria