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
Published in Norsk Fiskeoppdrett nr. 4, 2006.
Contact: Kåre Julshamn email@example.com
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.
Concentration (mg As/kg wet weight)
|Small sand eel
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
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
European upper limit (88% dry matter)
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.
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.
1: Model of a possible transfer of arsenic from industrial
fish to consumer.
Kohlmeyer et al. (2005). Determination of arsenic
species in fish oil after acid digestion. Microchim, Acta 151,
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.
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