Nutrition Myths

SUMMARY

  • Canned tuna is one of the most nutritious foods available.
  • However, besides a wide variety of nutrients, it also contains various toxins such as methylmercury, BPA and histamine, which can be detrimental to one’s health.
  • Canned tuna can be a healthy meal if consumed within the safe limits. These limits are mainly determined by the methylmercury contents, which is the most significant toxin contained in tuna flesh.
  • Insuring that the overall weekly methylmercury consumption from all food sources and the outside environment is kept under the safe limit, canned tuna can be a healthy addition to the diet.
  • There are two main types of canned tuna: “white” – albacore tuna or “light” – skipjack.
  • Albacore tuna contains about three times more mercury than “light” tuna.
  • Most of the mercury in fish is in the form of a highly toxic methylmercury compound.
  • To avoid mercury toxicity, restrict the weekly intake depending on your body weight.
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Benefits and risks of eating canned tuna

Canned tuna is a popular food worldwide. It is used, for instance, on top of salads or as part of an afternoon snack.

On the one hand, canned tuna seems like a healthy food choice, due to its many essential nutrients and its beneficial effects on normal physiology, optimal fetal neural development, and general health in pregnancy.

On the other hand, canned tuna contains methylmercury and other heavy metals, from polluted waters, chemicals, the lining of the cans, and histamine, from inappropriate handling of fish. (1)

People often underestimate the dangers of these chemicals and metals, but consuming too much canned tuna may lead to some serious health problems.

The nutritional and toxic contents of canned tuna, and, therefore, the potential positive and negative health effects of consuming it, differ depending on the:

  • Tuna variety (e.g. light tuna has less mercury than albacore tuna.)
  • Tin content besides the fish component, such as vegetable oil versus spring water

    For instance, the fat content varies between 3% and 33%, canned tuna in water contains more omega 3 fatty acids than tuna in oil.

  • Material used for lining the cans
  • Method of storage after the tuna is caught
  • In addition, various brands produce different quality products.

There is an optimal amount of canned tuna that you can eat per week. This will enable you to get the most health benefits but at the same time won’t reach the maximum safe level of methylmercury, which is the most abundant, illness causing toxin in canned tuna.

The following sections describe the “good and bad” aspects of eating canned tuna and a summary of how much canned tuna you can eat per week and which type to pick. The calculation is based on upper safe limit recommendations of methylmercury. (2)

What types of tuna do we usually eat?

The most common type of fresh tuna available in stores comes from large and older fish varieties such as Bluefin tuna.

Canned tuna is usually found in two varieties, a small tuna variety called “light tuna” (mainly skipjack tuna, less common yellow-fin tuna) and a large tuna variety called “white tuna” (albacore tuna). 

Albacore tuna has the highest methylmercury content of all canned fish, but at the same time it is richer in omega 3s: DHA and EPA.

Studies have found that some of these species contain a higher amount of mercury than others.

The amount of mercury accumulated in fish depends on a number of factors such as the size and age of the fish. (3)

Here is a description of the various types of tuna we consume (4):

Albacore tuna is primarily sold as canned tuna and labelled as “white” tuna meat. It grows up to 80 pounds and lives up to 12 years.

Skipjack Tuna is primarily sold as canned tuna and labelled “light” tuna. It grows up to about 8 pounds.

Yellow-fin Tuna: only a small amount is canned and labelled as “light” tuna. It is primarily served as steaks and loins. It is usually mixed with Skipjack tuna. It grows up to 400 pounds and lives up to 7 years.

Bigeye Tuna is not used for canning. It is used for sashimi, sushi and served as steaks or loins. It grows up to 6.5 feet long and lives up to 10 years.

Bluefin Tuna is used mostly for sashimi or sushi. It grows up to 1200 pounds and lives up to 30 years.

Health benefits of canned tuna

There are many health benefits of tuna, due to its high nutrient contents. Canned tuna is an excellent source of many nutrients such as protein, fat, Omega 3s, vitamin A, vitamin B group (B1, B2, B3, B5, B6, B12), phosphorus and selenium and a good source of copper and magnesium. Many of these nutrients have significant positive effects on our health.

Canned tuna benefits

  • Tuna may have anti-inflammatory properties, due to high omega 3 (EPA and DHA) and selenium contents. EPA and DHA fatty acids regulate the inflammatory system and prevent excessive inflammation.

    NOTE: tuna canned in water has more omega 3s than tuna canned in oil.

  • The omega 3 and selenium contents may also decrease the risk of cardiovascular related diseases. (5)
  • Selenoneine is a selenium containing compound found in the tissues and blood of tuna.

    It has strong antioxidant properties. Selenoneine protects heme proteins from iron oxidation by binding to hemoglobin and myoglobin.

    It also reacts to methylmercury, the major contaminant in tuna, reducing the danger of mercury toxicity. (6)

    Consuming selenoneine rich tuna lowers the reactive oxygen radicals’ levels, and reduces the risk of cancer formation, and of developing various chronic diseases. It also slows the effects of aging. (7)

  • Tuna canned in water contain antioxidant peptides that are released during cooking. These peptides may protect the cell membranes from damage due to peroxidation, an oxygen-related damage. (8)

Risks of eating canned tuna

  • Methylmercury in tuna

    Methylmercury is an extremely poisonous form of mercury formed in water, soil, or plants as a result of reaction between bacteria and mercury.

    All tuna is contaminated with methylmercury to some degree. The important questions are:

    • which canned tuna products contain, on average, the least amount of mercury? Light tuna.
    • how much canned tuna can we eat weekly not to exceed the recommended upper safe limit of mercury?
      It depends on the body weight – see table below.
    • how else we can avoid crossing that upper safe limit?
      Don’t eat only tuna, consume other types of fish with lower amounts of mercury, such as sardines or salmon.
    • who is in the risk group and should lower the consumption of tuna?
      Pregnant and breastfeeding women and young children.

    The following table contains a summary of the safe weekly limits of cans of snack-size tuna per body weight. The data was taken from the FDA which gathered the main results from major studies that used a large sample of products.

    As an example, a 70 kg/154lb woman shouldn’t eat more than about 7 cans of snack size canned light (skipjack) tuna per week and no more than 2.5 cans of white (albacore) tuna.

    Weekly limits of canned tuna per body weight
    Body weightNo. of snack-size cans
    KglbLight tunaWhite Tuna
    40883.851.45
    501104.851.8
    601325.852.15
    701546.82.5
    801767.82.85
    901988.753.2
    1002209.73.55
    11024210.6753.9
    12026411.644.25
    13028612.6054.6
  • Other heavy metals

    In addition to mercury, other potentially harmful heavy metals are also found in tuna. However, these metals are found to be in acceptable quantities:  lead (Pb), cadmium (Cd), Nickel (Ni) (9)

  • Bisphenol A and alternatives

    A chemical called bisphenol A (BPA) has been used in the can lining to protect the food from direct contact with the metal surface.

    It is now prohibited in many countries due to its hormone-like properties and interference with the endocrine system, causing serious health complications.

    Many substitute chemicals have been used as alternatives, such as bisphenol B (BPB), bisphenol E (BPE), bisphenol F (BPF), bisphenol S (BPS) and 4-cumylphenol (HPP), but the safety of these chemicals has not been thoroughly researched. Some studies show that these chemicals have similar endocrine-disruptive properties to BPA, although the mechanism may be different. (10, 11)

    Although these chemicals are used in amounts that comply with the safety regulations, to insure the least exposure to these toxins, it is advisable to choose fresh products rather than canned.

  • Histamine

    Tuna meat contains varied levels of histamine. Histamine levels in tuna of above 200ppm (parts per million) may cause poisoning.

    Most countries have established either 50ppm or 100ppm as a safe limit.

    Histamine levels in tuna increase rapidly with time and inadequate temperature. To prevent unacceptable levels, tuna must be chilled on the fishing vessel to below 4 degrees Celsius (39.2 Fahrenheit) immediately after catching and be transported in chilled trucks.

    In the most developed countries, the usual levels of histamine in tuna vary between 1ppm and 30ppm, since quality procedures are very strict. However, unacceptable levels of histamine (above 200ppm or even 500ppm) may occur in tuna fish caught in small, developing countries, especially in the tropics, due to inadequate chilling facilities on the fishing boats and in the transport trucks.

    Canning or cooking the tuna does not reduce the histamine levels, so if it was badly handled, the excessive amount of histamine will remain the same.

    Histamine poisoning symptoms, such as headache, burning or tingling sensation in the mouth, skin rash, inflammation or reduced blood pressure, may develop between 10 minutes to 2 hours after consumption. It normally subsides within 24 hours, except in the cases of elderly or sick individuals, who may need hospitalization. (12)

    To avoid histamine poisoning, chose canned products from trusted brands.

  • Canned tuna in oil

    Canned tuna in oil is not a good choice for a few reasons.

    Firstly, the oil used is usually vegetable oil, which is highly processed involving toxic chemicals and contains a high proportion of inflammation-causing fatty acids such as omega 6.

    Another reason is that canned tuna in oil has lower amounts of omega 3s than canned tuna in water.

    In addition, if the olive oil used for canning is not extra virgin, it is usually of inferior quality.

  • Can you eat canned tuna when pregnant?

    Despite the fact that the consumption of fish is beneficial for a developing fetus, breast-fed infants and young children, the intake of canned tuna and other fish high in mercury should be limited.

    There are certain types of fish, such as tilefish, swordfish, shark or kink mackerel, that must be avoided when women are pregnant or breastfeeding, due to the excessive amount of mercury.

    Although Albacore tuna is high in mercury compared to the other canned tuna products, it still contains much less mercury than the above mentioned fish. (read more..)

    The official recommendation for pregnant and breastfeeding women is 6 oz. (about 170g) per week of albacore tuna. Since light tuna has only 30% of the mercury compared to albacore tuna, it is safer for pregnant, breastfeeding women and young children to choose the light tuna variety, especially in water or brine, for higher omega 3 fat contents.

    Since the methylmercury contents in light tuna are lower, the weekly limit can be higher. (13)

    NOTE: it is important to note, however, that in order to minimize any risk, rather than eating tuna or other fish with high mercury levels, it is safer to choose fish that are low in mercury. This includes sardines or salmon, which still contain high amounts of EPA and DHA for the neuro-development of the fetus and infants.

Which canned tuna is healthiest?

The healthiest canned tuna is:

  • light tuna – for minimum mercury content;
  • water packed – contains more omega 3 fatty acids than in oil;
  • packed raw and heated – to preserve most nutrients and taste;
  • purchased from trusted brands.

Although other toxins contained in the canned tuna are important to consider, the main risk of having canned tuna fish comes from the methylmercury contents.

Other tips:

  • Alternate canned tuna with other nutrient rich fish, such as sardines and salmon.
  • Use more fresh fish rather than canned ones;
  • Avoid albacore tuna variety; 
  • Limit canned tuna consumption to the above weekly limit;
  • Be aware of other mercury containing products to reduce your daily exposure.

How much canned tuna is safe to eat?

In the U.S. population, somewhere between 39% and 43% of the total mercury intake from fish and seafood comes from tuna consumption. The reason for such a high percentage is because it is the most consumed fish.

Major tuna sources of mercury are: canned tuna (28%) of which light tuna contributes 18% and albacore/white tuna which contributes 10%.

Fresh or frozen tuna accounts for 11%. (14)

The following data have been extracted from the report published by FDA from the studies done between 1990 and 2010.

Please note that other studies not included in this report may show slightly different concentrations of mercury in particular species, but the differences are not significant.

Usually there is a consistency in levels between the specific species.

The results of the study show an average concentration of mercury (middle column) and highest recorded (right column).

The term “ppm” means parts per million. For example, 1ppm means 1 milligram of mercury per kg of fish flesh.

In addition to canned tuna, I also included other types of tuna available from the report for comparison.

This additional information will be useful as a warning for those who frequent sashimi bars.

Note: Bluefin tuna was not included in the report but being a large predatory fish that lives for 30 years, mercury levels are in the high range.

Mercury Levels in Tuna (1990-2010) (15)

Tuna typeAverage (ppm)Highest (ppm)mcg/oz
Tuna (canned, light)0.1280.8893.6
Tuna (fresh/frozen, Skipjack)0.1440.264
Tuna (canned, albacore)0.350.8539.8
Tuna (fresh/frozen, Yellowfin)0.3541.4789.9
Tuna (fresh/frozen, Albacore)0.3580.8210
Tuna (fresh/frozen, Bigeye)0.6891.81619.3

As can be seen from the table, the types of tuna which are usually served as sashimi or as a steak contain much higher amounts of mercury.

The following table shows the calculation of the maximum weekly amount of tuna you can eat depending on your body weight. An example of the calculations is shown at the bottom of this article.

Weekly safe limits of tuna

Body
Weight (kg)
Limit of mercury
(mcg/week)
Weekly Limit (oz)
Light Canned
Weekly Limit (oz)
White Canned
Weekly Limit (oz)
Skipjack
Weekly Limit (oz)
Yellowfin (oz)
Bigeye (oz)
50359.73.68.83.51.8
604211.74.310.54.22.2
704913.6512.34.92.5
805615.65.7145.72.9
906317.56.415.86.43.3
1007019.47.117.57.13.6

How to read the table above:

If your weight is 80kg, your weekly safe limit is 56 mcg of mercury. This means that you will reach your safe weekly limit of mercury by eating either:

  • 15.6 ounces of canned “light” tuna, or
  • 5.7 ounces of canned “white” albacore tuna, or
  • 14 ounces of Skipjack tuna, or
  • 5.7 ounces of Yellow tuna, or
  • 2.9 ounces of Bigeye tuna.

Any extra fish that week may exceed the safe level of mercury recommended since almost all fish contain some traces of mercury. (3)

Calculation example:

If you weigh 50kg how much of white/albacore tuna you can eat per week?

  • White/albacore canned tuna contains on average 0.35 ppm of mercury.
  • 0.35 ppm (parts per million) means that there is 0.35 micrograms (mcg) of mercury in one gram of tuna flesh.
  • 50 kg person’s safe weekly limit for mercury is 35 mcg.
  • 1 ounce = 28 g
  • 0.35 x 10-6 of mercury per gram of tuna times 28g of tuna =0.0000098
  • This means that there is 9.8*10-6 or 9.8 mcg of mercury in 1 oz. (28g of tuna)
  • 35 mcg weekly mercury limit divided by 9.8 micrograms (in 1 oz.) = 3.6 ounces
  • Answer: 35 mcg of weekly safe limit for a 50kg person is reached by consuming 3.6 ounces of canned white/albacore Tuna.

NUTRITION FACTS VS NUTRITION MYTHS

You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.

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