A recent case control study (1) carried out at the University of California suggests that oily fish could help men survive prostate cancer, possibly by reversing the effects of an inherited gene. Although the results are encouraging, the evidence is not conclusive but highlights the importance of dietary factors in the aetiology of cancers. There are also strong links between oily fish and the prevention of cardiovascular disease (2), which emphasise the role of essential fatty acids in sustaining the immune system, regulating blood clotting and lowering blood pressure (3). Despite this, our annual consumption of fish within the UK is below par according to one National Dietary Survey (4), with most of us falling short of the target of two portions per week.
All Fish are Not Equal
A distinction needs to be made between oily fish such as fresh tuna, mackerel and salmon, and white fish such as cod, haddock and plaice. While all are likely to be good for you, the inclusion of the former category boosts the intake of essential omega 3 fatty acids, otherwise found hiding in dietary items such as cod liver oil, dark green leafy vegetables, nuts, seeds, eggs and soya. Sadly, tinned tuna is not a great source, although herring, sardines and pilchards are a great choice - if you like that sort of thing. Shrimps and prawns are shellfish (or crustaceans) and contain large amounts of cholesterol, most of which the body digests safely away (5). Cholesterol is a sterol - not an oil.
Good News and Bad News
Unfortunately, there is also a downside to this advice based on the known presence of minute amounts of environmental toxins such as heavy metals, dioxins and PCBs (polychlorinated biphenyls), which accumulate in most marine animals. Eating a variety of fish in sensible amounts reduces the risk, but young women including pregnant and nursing mothers are advised against regularly consuming more than two portions per week. However, omega 3 oils are needed for normal neurological development in the growing foetus.
The benefits certainly outweigh the risks according to the Food Standards Agency, who constantly review the latest nutritional research behind food claims. Even the flesh of white fish contain a small amount (< 2%) of omega 3 fatty acids, the remainder being stored in the liver - hence cod liver oil.
Getting the Right Balance
Ironically, over-consumption of seeds, nuts, soya, eggs and soft margarines raises the intake of another type of essential fatty acid (omega 6), which interferes with the metabolism of omega 3 fat molecules absorbed from the diet. Strict vegetarians and anyone obsessed with muesli bars could fall short of the latter unless fish oil supplements are included. Currently, there is concern that the ratio of omega 6:3 fats in the UK diet is too high at approx 16:1 (6). The solution is to increase the intake of oily fish along the guidelines indicated and try to moderate the consumption of soft margarine - used widely in bakery products. Before the advent of supermarkets and fast foods, this 6:3 ratio was much lower, and reducing it to the target level of 3:1 will need some very powerful nutritional messages.
It all boils down to the sensory appeal of the fish counter versus the cake stall on a Friday evening, so perhaps the answer is to combine the two and re-invent the fish-cake?
References:
- Fradet V, Cheng I, Casey G, Witte JS. 2009. Dietary Omega-3 Fatty Acids, Cyclooxygenase-2 Genetic Variation, and Aggressive Prostate Cancer Risk. Clinical Cancer Research.
- UK Department of Health 1994. Nutritional Aspects of Cardiovascular Disease. London TSO.
- Barasi M. 2003. Human Nutrition pp 95 - 96. Arnold.
- UK National Diet and Nutrition Survey 2000 - 01. Average Oily and White Fish Consumption.
- Johnson A. 2009. Good Eggs and Bad Eggs. Suite101.com. Food Facts.
- Shepherd A. 2007. The Omega 3/6 Dietary Ratio. Functional Nutrition Vol 7, No 4.
Allan Johnson - With 26 years experience in the field of Education I enjoy sharing my interest in things scientific, whether they crawl, glow in the dark ...
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