Mercury in Canned Tuna in Spain. Is Light Tuna Really Light? ()
1. Introduction
Methylmercury (MeHg) is a widespread and particularly toxic form of Hg that results from the conversion of inorganic Hg to a methylated form by aquatic microorganisms and can bioaccumulate in the aquatic food web. Methylmercury accounts for more than 90% of total mercury content in fish and is attached to the thiol group of the cysteine residues in fish protein, and is thus not eliminated by cleaning or cooking the fish. Nevertheless, although fish may contain harmful compounds, it is also a very important source of nutrients, especially longchain n-3 fatty acids, high quality protein, selenium and vitamin D, and is relatively low in cholesterol [1].
Dietary intake of MeHg through ingestion of contaminated fish is a public health concern, primarily due to its neurodevelopmental toxicity in fetuses and children. Transplacental exposure is particularly dangerous, as the fetal brain is very sensitive. Neurological symptoms include mental retardation, seizures, vision and hearing loss, delayed development, language disorders and memory loss. In addition, a growing body of evidence suggests that MeHg exposure may also lead to an increased risk of adverse cardiovascular impact in exposed adult populations [2,3].
At the international level, the JECFA (Joint FAO/ WHO Expert Comittee on Food Additives) expresses risk as the provisional tolerable weekly intake (PTWI), which has been 1.6 μg MeHg/kg body weight since 2003. At the end of 2012, the European Food Safety Authority (EFSA) updated advice on risks for public health. At the request of the European Commission, EFSA’s Scientific Panel on Contaminants in the Food Chain (CONTAM Panel) has reported that, for MeHg, new studies indicate that beneficial effects related to long chain omega 3 fatty acids present in fish may have previously led to an underestimation of the potential adverse effects of MeHg in fish. The Panel has therefore proposed a TWI for MeHg of 1.3 µg/kg/bw, which is lower than JECFA’s 1.6 µg/ kg/bw [4]. However, this newly proposed TWI is still higher than the TWI in the United States (US). The US Environmental Protection Agency (EPA) and the National Academy of Sciences established a reference dose of 0.7 µg MeHg/kg/bw in 2000, which led to the US EPA’s value of 0.5 mg/kg as an acceptable standard of Hg in fish for human consumption (this corresponds to the MeHg limit of 5.8 µg/L in blood) [5]. However, the US Food and Drug Administration (FDA)’s action level for Hg in tuna is 1 mg/kg [6].
The European Union (EU) has established a Hg limit of 0.5 mg/kg for fresh fish, except for certain fish such as tuna, which has a limit of 1 mg/kg [7].
Spain is one of the countries of the EU with a highest consumption of seafood and, according to different population-based studies carried out in recent years, it is also one of the countries with the highest blood mercury concentrations [8]. However, fish consumption in Spain has decreased in recent years, down to a consumption of 26.37 kg per capita in 2012. In spite of this overall decrease, consumption of tuna fish increased 14.6% from 2011 to 2012. Taking into account only canned tuna, it is the second most consumed seafood product (behind hake) in Spain, with a consumption of 2.25 kg per capita in 2012, representing 8.5% of total fish consumption [9].
In the US, canned tuna is also the second most frequently consumed seafood product, at 1.2 kg per person per year in 2011. Light tuna, which consists mostly of skipjack (Katsuwonus pelamis) and small amounts of yellowfin (Thunnus albacares) is the least expensive product and represents the largest portion of canned tuna sales in the US. Albacore tuna (Thunnus alalunga) is the only species authorized to be labeled “White tuna” in the US [10].
The average size is different among the types of tuna. White tuna (Thunnus alalunga) has a maximum weight of around 40 kg at 15 years, while the maximum for Skipjack (Katsuwonus pelamis) is about 30 kg at 15 years. The maximum weight of Yellowfin (Thunnus albacares) is about 175 kg at 8 years and bigeye (Thunnus obesus) has a maximum of about 210 kg at 15 years. Bigeye tuna are similar to Yellowfin tuna, and in fact they are sometimes hard to distinguish [11]. These designations are standardized by the FDA with reference to a Munsell gradient of darkness. White tuna is not darker than Munsell value of 6.3 and light tuna ranges from Munsell 6.3 to not darker than 5.3 [12].
With respect to canned tuna, in 2004 the US EPA and FDA recommended that vulnerable groups such as pregnant women and small children eat up to 12 ounces (2 average meals) a week of canned light tuna and stated that albacore (“white”) tuna has more mercury than canned light tuna. So, when choosing the two meals of fish and shellfish, it is recommended to eat only up to 6 ounces (one average meal) of albacore tuna per week [13].
In Spain, in April 2011 the Spanish Agency for Food Safety and Nutrition (AESAN) published its recommendations on seafood consumption by pregnant women and small children, which made no mention of canned tuna. However, the FAQ section of its webpage states the following: species that are used for canned tuna are much smaller, thus their mercury content is considerably reduced [14].
Light tuna is recommended in other countries because of its lower mercury content, but in Spain it is made up of different species of tuna. Thus, cans of skipjack (Katsuwonus pelamis) in Spain are simply called canned tuna or striped tuna. In addition, new legislation approved in August 2009 stated that the previous Royal Decree of 1984 had provided the name light tuna only for Thunnus albacares (yellowfin). However, the Spanish market has admitted that since 2002 other species of tuna have been labeled as light tuna, particularly bigeye tuna (Thunnus obesus). Thus, this new legislation declared it convenient to allow bigeye tuna to be labeled as both tuna and light tuna [15]. Therefore, given the different species of tuna labeled as light tuna in Spain, and taking into account the high consumption of canned tuna by the Spanish population as well as the high blood mercury concentrations found by different studies, it would be desirable to know the concentration of mercury in the varieties of canned tuna most frequently consumed in Spain before making recommendations to vulnerable populations.
The objective of this study is to measure mercury concentrations in the most popular brands of canned tuna in Spain and to examine the influence of type of tuna, packaging medium, store brand, price and expiration date.
2. Material And Methods
2.1. Canned Tuna Samples
The present descriptive study used a non-probability sampling method which included 36 cans of the most popular brands of tuna in Spain purchased from 4 different grocery stores in Madrid and 4 in Cartagena (Spain) during September 2012.
We examined the type of tuna: light tuna, white (albacore) tuna (Thunnus alalunga), skipjack (Katsuwonus pelamis), frigate tuna (Auxis thazard) and Pacific (chub) mackerel (Scomber japonicus). In addition, in the group of light tuna, information regarding the type of tuna was also collected: yellowfin (Thunnus albacares), bigeye (Thunnus obesus) or unspecified. We analyzed other variables such as packaging medium (olive oil, sunflower seed oil, water and marinade), brand of tuna (store brand or not), price and expiration date.
2.2. Mercury Analyses
Total mercury concentrations (Hg) were measured using a direct mercury method based on the EPA 7473 analytical procedure: thermal decomposition, catalytic reduction, amalgamation, desorption and atomic absorption spectrometry. The analyses were performed in the Trace Element Laboratory of the hospital Clínico San Carlos of Madrid in a Perkin Elmer SMS-100 analyzer and in the Laboratory of the Chemical and Environmental Engineering Department of the Universidad Politécnica de Cartagena in a Milestone DMA-80 spectrophotometer. We carried out comparisons of samples between the two laboratories and the concordance was excellent, with an intraclass correlation close to 1. Two calibration graphs of 0 - 50 ng and 50 - 500 ng of mercury were generated from aqueous standards in 10% nitric acid. A detection limit of 0.005 ng of mercury was achievable while a maximum concentration of 20 µg of mercury was allowed.
All the analyses were done on individual cans. The contents were drained and a small amount of fresh tissue of approximately 0.1 g was weighed into a nickel sample boat, avoiding contamination in both the collection and handling of samples. Duplicated samples were measured from all cans with a mean reproducibility of 4.5% (range 0.1 - 13.5), and the samples with high results (>0.500 mg/kg) were analyzed again.
All analyzed samples were within the calibration range. Total mercury concentration was reported as mg/kg on a wet weight basis, rounded to three significant decimal points.
Internal quality controls (Seronorm® trace element levels 1 and 2) were assessed in every series of samples to check the reproducibility and accuracy of the measurements. In addition, the Madrid laboratory took part in two external quality assessment schemes with excellent performance: the European Occupational and Environmental Laboratory Medicine (OELM) program and the one organized by the University of Surrey, Guildford (Surrey, UK).
2.3. Statistical Analyses
Qualitative variables were summarized as a frequency distribution and continuous non-normally distributed variables were summarized as median and interquartile range (IQR).
The non parametric Kruskall-Wallis test was used to compare quantitative variables between more then to two independent groups, or the Mann-Whitney test for to two independent groups. Spearman correlation coefficient was used to analyse the correlation between continuous variables.
The null hypothesis was rejected by a type I error <0.05 (α < 0.05). Statistical analyses were performed using SPSS 15.0 (SPSS Inc., Chicago, Illinois, USA), and figure using GraphPad Prism 5.00 (GraphPad Software, San Diego, California, USA).
3. Results
Table 1 summarizes the different mercury concentrations in each of the types of tuna. An overall median mercury concentration of 0.298 mg/kg (IQR: 0.191 - 0.470) was obtained. The limit of 0.500 mg/kg of mercury was exceeded by 22.2% of the total of canned tuna.