Arsenic in fish oil - a new challenge?

Industrial fish species contain naturally high concentrations of arsenic consequently so do fish meal and fish oil. Considerable knowledge exists regarding arsenic and its various chemical species in fish and fish meal, but little is known about arsenic in fish oil.

By Heidi Amlund, NIFES, Jens J. Sloth, NIFES and Danish Institute for Food and Veterinary Research, Marc H. G. Berntssen, NIFES, Anne-Katrine Lundebye Haldorsen, NIFES and Kåre Julshamn, NIFES
Published in Norsk Fiskeoppdrett nr. 4, 2006.
Contact: Kåre Julshamn kju@nifes.no

Arsenic is an element which occurs naturally in marine organisms. Marine fish may contain relatively high levels of arsenic, however the level varies between and within species, as shown in Table 1.

Table 1: Arsenic concentrations in industrial fish. The concentrations are given as mg/kg wet weight. Analyses of polled samples of 10 kg fish. Analyses carried out at NIFES. The data is a part of the database "Sjømatdatabase".


Blue whiting and Norway pout contain more arsenic than capelin and sand eel. As a consequence of these naturally high concentrations of arsenic in industrial fish species, marine feed ingredients and complete feedingstuffs for fish also contain high background levels of arsenic (Table 2). Considerable knowledge exists regarding arsenic and the various water soluble chemical forms of arsenic in fish and fish meal. The major form of arsenic in fish and fish meal is arsenobetaine, a water soluble arsenic compound, which is non-toxic. On the other hand, little is known about arsenic in fish oil. Analyses of fish oil carried out at NIFES show that fish oil contains considerable amounts of arsenic (Table 2). The table shows concentrations from 10 to 15 mg As/kg oil. Thus fish oil contains considerable amounts of lipid bound arsenic, also called arsenolipids. Arsenolipids is a collective term for lipid soluble arsenic compounds.

Table 2: Arsenic concentrations in fish meal, fish oil and complete feedingstuffs produced in 2003. The concentrations are given as mg/kg. Analyses carried out at NIFES as a part of the surveillance programme of Norwegian Food Safety Authority on feed ingredients and feed to aquatic animals. Data from Måge et al. (2005).

Why is knowledge regarding arsenolipids important? The high levels of arsenic in fish oil may contribute significantly to the total level of arsenic in complete feedingstuffs. A complete feedingstuff for salmon typically contain 200-350 g fish oil per kg feedingstuff and hence more than a third of arsenic found in complete feedingstuffs may originate from fish oil. Knowledge regarding the chemical forms of arsenolipids, their occurrence, toxicity and metabolism is limited, however such knowledge is required for assessing the significance of arsenolipids with regards to food safety as and as a basis for legislation on this undesirable substance in marine feed ingredients and complete feedingstuffs. Early research on arsenolipids The first investigations of arsenic in marine fish oils were carried out in the late 1960s and the early 1970s by the Norwegian researcher Gulbrand Lunde (Lunde 1968, 1972). He investigated oils from herring, mackerel and capelin, and found that the arsenic concentrations ranged from 8 to 20 mg As/kg sample. Part of his work focused on the identification of arsenolipids. In one study fish oil (from herring) was separated into neutral lipids and phospholipids, and the arsenic content in the various fractions was determined. Two of the phospholipid fractions contained high levels of arsenic. These results indicate that fish oil from herring contains two arsenolipids which are similar to phospholipids. In a later study fish oils were saponified and subsequently fractionated. The arsenic content in fish oil and the corresponding lipid fractions was determined and compared. The results showed that at least two organic arsenic compounds are present in fish oil; one arsenic species which follows fatty acids in the saponification, and one arsenic compound which is converted into a water soluble compound during saponification.

Arsenic is an element which occurs naturally in marine organisms. Marine fish may contain relatively high levels of arsenic, however the level varies between and within species. Photo: Fish oil.

Recent research on arsenolipids

After the work by Lunde on fish oil and arsenolipids most research focused on the water soluble arsenic compounds in marine organisms. It is not until recently that arsenolipids have been in focus again. A group at Karl-Franzens University, Graz, Austria, has recently developed a method which separates and quantifies various arsenolipids in fish oil by the use of HPLC-ICPMS (Schmeisser et al. 2005). Analysis of the chemical forms of arsenic in ten fish oils with different origin (world wide) revealed that fish oil contains 4-6 major arsenolipids and several minor arsenolipids. The relative amount of the arsenolipids in the ten fish oils analysed varied from oil to oil. The chemical structure of the arsenolipids could not be identified by this method, but the results suggest that the major arsenolipids may be divided into two groups; non-polar and polar lipids. In another study by Kohlmeyer and colleagues (2005) fish oil was separated into neutral and polar lipids. The neutral lipids accounted for more than 90% of the weight of the sample, but the level of arsenic was very low. Most arsenic was found within the polar lipids. Schmeisser and colleagues (2006) focussed, in a later study, on human metabolism of arsenolipids. A volunteer ate cod liver and the arsenic metabolites in urine was the followed. The study showed that arsenolipids in cod liver were metabolised into four compounds, two of which were fatty acids containing arsenic.

Further research

The above mentioned studies, and in particular the analytical technique developed by Schmeisser and colleagues, has enabled considerable progress within the field of research related to arsenolipids. Meanwhile, the major breakthrough awaits the structural identification of the major arsenolipids. Then the analytic methods can be further developed, the toxicity of arsenolipids can be established, their occurrence and levels in fish, fish oil and feed can be documented, and their stability, metabolism and transfer within the production chain from feed to fish to consumer (figure 1) can be investigated.

Figure 1: Model of a possible transfer of arsenic from industrial fish to consumer.

References

Kohlmeyer et al. (2005). Determination of arsenic species in fish oil after acid digestion. Microchim, Acta 151, 249-255
Lunde, G. (1968). Analysis of arsenic in marine oils by neutron activation. Evidence of arseno organic compounds. J. Am. Oil Chem. Soc., 45, 331-332
Lunde, G. (1972). Analysis of arsenic and bromine in marine and terrestrial oils. J. Am. Oil Chem. Soc., 49, 44-47
Måge et al. (2005). Overvåkningsprogram for fôrvarer til fisk og akvatiske dyr. Årsrapport 2004. NIFES
Schmeisser et al. (2005). Direct measurement of lipid-soluble arsenic species in biological samples with HPLC-ICPMS. Analyst, 130, 948-955
Schmeisser et al. (2006). Arsenic fatty acids are human urinary metabolites of arsenolipids present in cod liver. Angew. Chem. Int. Ed., 45, 150-154