Oily Fish…

I have eaten a lot of tinned sardines this week. Call me crazy but a warm baguette filled with good cheese and four oily sardines makes a great lunchtime snack. I prefer the ones packed in oil to the ones packed in tomato sauce but I am prepared to be open-minded on the issue. As well as being yummy, of course, sardines are a good example of oily fish. They differ from white fish (e.g. cod, haddock & whiting) in that they have oil as an intrinsic part of their gut whereas white fish only have oil stored in their liver. White fish tend to live near the seafloor whilst sardines are pelagic fish (i.e. they live in the water column away from the sea floor). All fish are an excellent source of protein.

There are other oily fish such as swordfish, salmon and tuna (although tinned tuna is not considered oily because so much of the oil is removed in the canning process). Sardines are canned whole so don’t lose any of their nutritional yumminess. They have their heads and tails removed and are gutted (I bet they are!) before being cooked and sterilised inside the sealed can. This process can be fully automated but many producers pride themselves on the level of human involvement in the process as can be seen in the video below.

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This website called The World’s Healthiest Foods has a page dedicated to the humble sardine. It is a little bolder in its health claims than some other sites but I have no reason to doubt the accuracy of its nutritional chart showing the relative levels of different dietary nutrients. Sardines are a good source of certain vitamins, particularly the water-soluble vitamins B₂ & especially B₁₂ (which are both needed for the nervous system and help in respiration reactions) as well as niacin (B₃), which also helps out in respiration and the breakdown of harmful fats such as cholesterol in the bloodstream. They also contain the minerals phosphorous (needed for bones), potassium (which helps with muscle and nerve action) and calcium (which is also needed for calcium phosphate, a major component of bones).

The main reason that oily fish are recommended in the diet is that they contain omega-3 fatty acids. These are important polyunsaturated fats that have been linked to a reduced risk of cardiovascular disease – the UK’s biggest killer.

The double bonds are what makes the molecule unsaturated. In this case there are three double bonds so it is poly-unsaturated. This makes it much easier to break down and rearrange for use by the body. There are various beneficial health effects claimed for these fish oils but the newspapers have tended to over-interpret the evidence. It seems that taking supplements in large doses is unlikely to produce any really strong improvements in overall health above and beyond those seen by including oily fish in your diet.

Oily fish have a tendency to accumulate heavy metals from their environment. The most notable of these is mercury, which is very poisonous. Because sardines are low in the food chain, they do not accumulate much mercury unlike longer-lived predatory fish such as swordfish. This building up of materials from the environment over time is called bioaccumulation. It is one of the long-term consequences of agricultural pesticide use, and one of the things that manufacturers try to understand in their research. Infamously, an insecticide called DDT, which was very effective at protecting crops from insect damage, worked its way through the food chain and ended up in birds of prey and was closely linked to weaker egg shells. This in turn reduced survival and led to a drop in predator population. More recently produced agricultural sprays are designed to quickly break down after they have had their intended effect.

Questions…

1. Name a vitamin not mentioned in the post above, a good source of it and what deficiency disease it prevents.
2. What job does protein perform in your diet and into what is it digested in the small intestine?
3. If pelagic describes fish that live in the mid-water zone, what word describes whiting and cod that live at the bottom of the sea?
4. Swordfish live for about 15 years on average but what is the longest lived fish in the sea?