Oily fish


Oily fish have fats throughout their whole body rather than only in the liver like non-oily (white) fish. Oily fish fillets may contain up to 30 per cent fat, although this figure varies both within and between species. See table 3 for a list of oily and non-oily fish.  

How much oily fish?

In 2004, in response to a specially commissioned report on the benefits and risks of oily fish consumption, the Food Standards Agency (FSA) issued guidelines on oily fish intake. Table 4 shows the recommended weekly limits. They suggest a limit on the maximum number of portions of oily fish different groups can safely consume each week (a portion is about 140 grams). However, the FSA says that health benefits are greater than the risks as long as you don't eat more than the recommended maximums. Hardly reassuring!

EFA deficiency

For EFA deficiency to develop, dietary intake must be extremely low as even small amounts of EFAs can prevent deficiency. The 2003 National Diet and Nutrition Survey showed that the average daily total fat intakes were close to UK recommendations (FSA, 2003). In fact, most adult Western diets provide 8-15 grams of EFAs per day and healthy people have a body reserve of 500-1000 grams in adipose tissue (Department of Health, 1991). EFA deficiency in adults is only seen when they make up less than 1-2 per cent of total dietary energy (Department of Health, 1991). Where EFA deficiency does occur, it is most often seen in infants fed very poor diets.

Typical symptoms of EFA deficiency include excessive thirst, frequent urination, dry or rough skin, dry hair, dandruff, brittle nails, headaches, stomach ache, diarrhoea and constipation (Stevens et al., 1995). Low omega-3 fatty acid status is also associated with behavioural problems such as hyperactivity-impulsivity, anxiety, temper tantrums, sleep problems and learning difficulties in some children (Stevens et al., 1996). More specifically, low levels of DHA have been associated with several neurological and behavioural disorders such as depression, schizophrenia, Alzheimer's disease and attention deficit hyperactivity disorder (ADHD) (Davis and Kris-Etherton, 2003).    

These findings are consistent with evidence that omega-3 fatty acids are important for brain function and that low omega-3 intake may have negative effects on behaviour, learning and mood. It is important then to make sure that there are adequate levels of these important nutrients by supplying sufficient 'parent' fatty acids for conversion. Most people can obtain all the EFAs they require from a well-balanced plant-based diet including flaxseed oil, nuts and seeds as well as plenty of green leafy vegetables. In most cases, dietary replenishment of EFAs will usually reverse any deficiency.

Table 4: UK Food Standards Agency recommended limits for oily fish per week
Two portions Four portions
Women who are pregnant or breastfeeding and
girls and women who might have a baby one day
Other women and men and boys
Source: FSA, 2009.

Table 5 shows which plant-based foods can be used to boost your intake of omega-3 fats. Note that the fats and oils are easily damaged by light or heat so are best kept refrigerated. Furthermore, these oils should not be used in cooking; use them on salads, in dressings and dips.

Table 5: Plant sources of EFAs
Food Omega-3, ALA g/100g Omega-6, LA g/100g
Flaxseed (linseed) oil 53.30 12.70
Hempseed oil * 20.00 60.00
Walnut oil 10.40 52.90
Rapeseed (canola) oil 9.13 18.64
Wheat germ oil 6.90 54.80
Soya oil 6.79 50.42
Corn oil 1.16 53.23
Sunflower oil 0.00 65.70
Nuts and seeds
Flaxseeds (linseeds), ground 22.81 5.90
Walnuts 9.08 38.09
Hempseeds * 7.50 22.50
Vegetables, Fruits and Pulses
Soya beans, cooked 0.60 4.47
Tofu firm 0.58 4.34
Broccoli (cooked) 0.11 0.05
Soya milk 0.08 0.59
Strawberries 0.07 0.09
Peas (frozen and cooked) 0.02 0.08
Source: USDA National Nutrient Database for Standard Reference, 2008.
* Adapted from Leizer et al., 2000.

People with diarrhoea, irritable bowel syndrome, diverticulitis or inflammatory bowel disease (Crohn's disease or ulcerative colitis) should use caution if taking flaxseed (but not flaxseed oil) due to its possible laxative effects. If you have a medical condition, or are taking other drugs or supplements you should consult a doctor or nutritional therapist before changing your diet.

Balancing the fat

Research suggests that the ratio of omega-6 to omega-3 fatty may be more important than the amounts of individual fatty acids consumed (Lands, 1992). This is because omega-6 fats compete with omega-3s for use within the body. So excessive intakes of omega-6 fats can cancel out the positive effects of omega-3s and lead to health problems.

Recent changes in the diet have led to a massive rise in our intake of omega-6 fats from plant-based oils (such as sunflower oil) and most processed foods. This has increased the ratio of omega-6 to omega-3 fats in our diets.

Typical Western diets may provide ratios of up to 30:1, dramatically skewed toward omega-6 fats. The health consequences of this could be far-reaching. Dr. Hibbeln, lead clinical investigator on nutrition in psychiatry at the National Institutes of Health in Bethesda, Maryland suggests that these major changes in the essential fatty acid composition of the food supply may be linked to a range of psychiatric disorders and may even contribute towards an increased risk of homicide, suicide and social instability (Hibbeln, 2007).

The WHO/FAO joint recommendation is a ratio of LA to ALA of 5:1-10:1 (WHO/FAO, 1994). Other recommended ratios vary but a cautious approach would be to aim for a ratio between 2:1 and 4:1 making sure not to exceed 6:1 (Davis and Kris-Etherton, 2003). This ratio can be achieved by reducing the use of sunflower, safflower and corn oils and increasing the use of ALA-containing oils such as flaxseed, rapeseed and soya bean oils. Of course, optimising omega-3 conversion rates can help improve your omega-6:omega-3 ratio too.

Optimising conversion

The body converts ALA into the longer chain omega-3s (EPA and DHA), otherwise found in fish and some species of algae. However, we are repeatedly told (particularly by fish oil supplement companies) that conversion rates are low and inefficient. The claim that we cannot convert enough ALA to EPA and DHA is simply not true, but does serve to protect the fish oil market. The better health statistics for vegetarians and vegans - who don't eat fish - provides proof that they don't go short. 

Several studies have measured the rate at which we convert ALA to EPA and DHA. A conservative estimate is that about 5-10 per cent of ALA is converted to EPA, and 2-5 per cent to DHA (Davis and Kris-Etherton, 2003). However, as the body's fat deposits usually contain ALA, even a very low conversion rate of just 2.7 per cent, would allow an average person to make the same amount of EPA as would be found in 18 large (1,000mg) capsules of the omega-3 richest fish oil (Erasmus, 1993). Even a person with no omega-3 fats in their body, (perhaps after a long-term dietary shortfall) who takes two tablespoons of flaxseed oil a day, can produce more EPA than contained in two large fish oil capsules.

Other studies show much higher levels of conversion. Research from the Institute of Human Nutrition at the University of Southampton found ALA conversion rates in young women of 21 and nine per cent for EPA and DHA respectively (Burdge and Wooton, 2003).

Furthermore, a recent study from the University of Cambridge revealed that vegetarians convert more ALA to EPA and DHA than fish-eaters (Welch et al., 2008). In this study, the ratio of circulating EPA and DHA to dietary intake of ALA was used as an indicator of conversion rate; the higher the ratio, the greater the conversion rate. Results showed the ratio was 22 per cent higher in vegetarians than fish-eaters, indicating a higher conversion rate. This may explain why fish-eaters and non fish-eaters have a smaller than expected difference in circulating omega-3 levels.

Taken together, the research indicates that we can convert as much or as little as we need, providing we have enough starting material (ALA).

In addition to this, we are capable of 'retroconversion', which means we can make EPA from DHA. Research from the Department of Nutritional Sciences at Pennsylvania State University states that up to 10-11 per cent of DHA is retroconverted to EPA. The authors of this study conclude that if sufficient ALA and DHA are consumed, total EPA production would be expected to be adequate (Davis and Kris-Etherton, 2003).

So if you take an algal supplement that contains DHA you can boost both DHA and EPA levels without exposing yourself to toxic pollutants found in fish oils. Furthermore, the EPA we make in our bodies has the advantage of being fresher and more stable than that found in fish oils.

A range of dietary factors can affect efficiency of conversion too. Poorly designed diets are thought to reduce the rate of conversion as both energy and protein are needed to drive the enzymes that convert EFAs. In addition, a wide range of nutrients are required to optimise conversion including biotin, calcium, copper, magnesium, niacin (vitamin B3), pyridoxine (vitamin B6), vitamin C and zinc (McKevith, 2005; Siguel and Lerman, 1994; Horrobin, 1992). A well-balanced plant-based diet will provide all these nutrients and more.

Excessive intakes of trans fatty acids can limit conversion rates. These unhealthy fats, found in hydrogenated fat in margarine and products made with it (such as biscuits and pastries), are also found in meat and dairy products. Alcohol inhibits activity of the enzymes that convert ALA into EPA and DHA and depletes tissues of these long chain omega-3 fatty acids (Davis and Kris-Etherton, 2003).

Consuming excessive amounts of omega-6 fatty acids (associated with some vegetable oils, particularly safflower, sunflower and corn oils) can have a detrimental effect on conversion rates too (Uauy and Castillo, 2003). Indeed, a high omega-6 intake may reduce omega-3 conversion by as much as 40 per cent (Davis and Kris-Etherton, 2003). There is no need to avoid or cut down on omega-6-rich wholefoods (sunflower seeds, pumpkin seeds, sesame seeds, wheat germ and soya foods) as they contribute much less to overall omega-6 intake and contain an abundance of other healthy nutrients.

Davis and Kris-Etherton recommend that the primary fat in the diet should come from foods and oils rich in monounsaturated fat, as when monounsaturated fats predominate, saturated fats, trans fats and omega-6 fats are kept in check (Davis and Kris-Etherington, 2003). Monounsaturated fats are found in nuts, peanuts, olives and olive oil, avocados and rapeseed oil. As stated, wholefoods are superior source of fats as they contribute many other nutrients to the diet.

Lastly, it is important to optimise the intake of ALA for conversion into EPA and DHA. As stated above, ALA should provide 1-2 per cent of total energy intake in the diet. Good sources include ground flaxseeds and their oil, hempseed, rapeseed, walnuts, soya beans and products made from them, green leafy vegetables and some species of algae (which contain DHA). Table 6 shows the amount of omega-3-rich food you can take to achieve the recommended daily intake (along with a well-balanced plant-based diet).

Table 6: Suggested daily intake of foods rich in omega-3
Source One of the following.
Flaxseed (linseed) oil 1-2 tsp
Ground flaxseed 2 tbsp
Rapeseed oil 2 tbsp
Hempseed oil 1 tbsp
Walnuts 8 halves (28 grams)

If more people knew that they could produce sufficient amounts of EPA and DHA from ALA they might be less inclined to turn to potentially decaying and contaminated fish oils. The research indicates that the only thing vegetarians and vegans miss out on by avoiding fish is the toxic pollutants that contaminate them.

Omega-3s from algae - cut out the middleman

Whilst omega-3 is present in fish oil, fish do not produce omega-3 in their bodies. Fish obtain their omega-3 from their diet of algae. People who are concerned about intake of DHA can use algal supplements as a source of this long chain omega-3 too. There are even brands available online that can provide both DHA and EPA.

Algae used for omega-3 supplements is usually grown in controlled conditions away from the sea, so it doesn't interfere with the marine ecosystem and doesn't impact on fish stocks nor their natural food source. So, in addition to the numerous health benefits of this vegan-friendly source of long chain omega-3s, it is a more environmentally friendly choice.

By switching from fish to a plant-based source of omega-3 you'll be doing yourself a favour and the environment too, whilst getting the safest, toxin-free omega-3 fats.