In recent years there has been much talk about the health benefits of omega-3 fats.  These have been associated with lower risk of heart disease and inflammatory joint problems as well as systemic inflammatory diseases such as Rheumatoid Arthritis.  They’ve also been found to be important for our brains, since polyunsaturated fats, including omega-3, make up 60% of the brain and are vitally important for its development and function throughout life.  Bearing that in mind it isn’t surprising to learn that omega-3 fats have been associated with lower likelihood of depression and research has shown benefits for those with autism spectrum disorders and attention deficit disorders such as ADHD. 

There have been suggestions that fish oil supplements, containing omega-3 fats, may be used to improve a child’s IQ and performance at school, but there haven’t been sufficient research trials to fully support this idea.  However, you need only look at the shelves in supermarkets and health food stores to see there are many supplements targeted at this market. 

Are you eating enough fish?

It seems that omega-3 is a good bet for all of us, so where do we get it from?  Fish.  As a nutritionist I usually recommend that people should eat oily fish two or three times a week, such as anchovies, sardines, salmon, mackerel, pilchards, trout.  White fish also contains omega-3 fatty acids but in much lower quantities than oily fish.  Not only fresh fish, but tinned fish is also good and is often a cheaper and easier option.  In much of the literature I read it has always said canned fish is good for omega-3, except for tuna which should always be fresh.  For example, the NHS Choices website states “Canned tuna does not count as oily fish”.  I have repeated this advice to numerous people and only recently asked myself why?  What happens to canned tuna that doesn’t happen to canned salmon or canned sardines?

Let’s look at some figures

Most information about the omega-3 content of fish refers only to its fresh, uncooked, unprocessed state.  Here’s a table to show some examples which also includes some canned fish figures:

 

Type of fish

Total omega-3 fatty acids in grams per 100 g

Fresh salmon

2.7

Tinned and smoked salmon

1.54

Fresh mackerel

1.93

Fresh tuna

1.5

Fresh herring

1.31

Fresh trout

1.15

Tinned sardines

1.57

Adapted from Nicolle L, Woodriff Beirne A (2010) Biochemical Imbalances in Disease, Singing Dragon, London, p. 119

We can see from the figures for fresh salmon compared with canned and smoked salmon that some of the omega-3 has been lost. 

From research carried out on fresh tuna it is reported that oven or hob cooked tuna or microwaved tuna only loses a small amount of its omega-3, but fried fresh tuna loses a far greater amount. However, the canning process destroyed these fats completely.  Most research has been carried out on fresh raw fish but, despite sushi being very delicious, most people tend to usually cook their fresh tuna or prefer the canned option.  It’s also worth bearing in mind that the proportions of these various fats vary to begin with, depending on the species of fish, its diet and the environment in which it grew and developed.

What’s involved in the processing?

From watching a number of YouTube clips it is interesting to see slightly different canning processes depending on where in the world the factory is located.  For some the tuna is caught in South East Asia where it is deboned and cooked, then frozen and transported to the USA.  It is then defrosted and chopped up, canned, cooked and sterilised at 110˚C/230˚F for two hours (Bumble Bee, USA). It doesn’t seem surprising that these fatty acids are damaged and often completely destroyed during this kind of processing.  It may then be transferred from the can straight to a salad, for example, or cooked again in some other dish.

On the other hand, salmon and sardines don’t appear to be so heavily processed.  This may be due to their much smaller size and perhaps to the distance from where they are caught to the factory.  For example, salmon caught offshore around Alaska or the north Atlantic may be transported fresh to a factory in Washington State.  The video clip I watched on YouTube showed the salmon to only be cooked once and, unlike the tuna, wasn’t frozen at all.  The same applied to sardines caught in the Pacific coastal waters off Costa Rica which were stored on ice to keep them fresh, they were sorted, packed manually into cans, cooked and sterilised.  So, with both these kinds of fish they weren’t frozen and defrosted and they weren’t cooked twice.  For the easily damaged long chain omega-3 fats it seems likely that this additional processing is enough to completely destroy them.

But this isn’t the whole story … beware of mercury!

The NHS Choices website advises the following:

  • Oily fish: if you are trying for a baby, or are pregnant or breastfeeding you should have no more than two portions of oily fish a week. A portion is around 140 g.
  • Canned tuna: if you are trying for a baby or are pregnant, you should have no more than four cans of tuna a week. This is because tuna contains higher levels of mercury than other fish.

I was surprised to read a recommendation of four cans of tuna week.  Personally, I think that’s quite high and I’d also recommend for people to vary their sources of protein to include other types of fish, meat, seafood and vegetarian sources such as nuts, beans and tofu for example.

Anyway, the thing to remember is that the larger the fish, the greater the likelihood of mercury contamination.  This is all to do with the food chain, i.e. the bigger fish eat littler fish and the smaller fish eat tiny fish – so by the time you’ve got to a fish as large as tuna it’s got the accumulated mercury from masses of little ones.

But it’s not all bad news

Although canned tuna has lost its omega-3, it contains less mercury than fresh tuna – so there are pros and cons.  There’s still a lot to recommend tuna as a healthy food option, it is a great source of protein, Vitamin B12, potassium, selenium, niacin and phosphorus – just bear in mind the mercury aspect and don’t eat it too frequently (maybe just two or three times a month), and don’t eat lots of canned tuna thinking you’re getting your omega-3 that way.

And then there’s sustainable fishing to consider … but that’s a whole different matter for another article!

 

References

Lapis TJ, Oliveira ACM, Crapo CA, Himelbloom B, Bechtelz PJ, Long KA (2013) Supplementing long chain n-3 polyunsaturated fatty acids in canned wild Pacific pink salmon with Alaska salmon oil. Science and Nutrition, 1: 15-26.

Murray M, Pizzorno J (2012) The Encyclopaedia of Healing Foods, Piatkus, London

NHS Choices website:  www.nhs.uk

Nicolle L, Woodriff Beirne A (2010) Biochemical Imbalances in Disease, Singing Dragon, London

Nimish Mol S, Jeya SR, Jeyasekaran G, Sukumar D (2010) Effect of different types of heat processing on chemical changes in tuna. Journal of Food Science and Technology, 47: 174-181.

Webb GP (2008) Nutrition: a health promotion approach, 3rd edn., Hodder Arnold, Spain

 

 

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