Fishy facts: a comprehensive guide to all things fish


If only it were as simple as telling your clients to ‘eat more fish’ for their health. The nutritional content, type, source and freshness of fish, as well as cooking methods all make a huge difference to the nutritional quality of meals. Explaining the health benefits of fish to your clients may also help to motivate them to make important dietary changes.

The health benefits of fish providing high levels of omega-3 EPA and DHA

We all know that fish is extremely good for us and most of us should be eating more of it, but it is relevant to understand what it is about fish that offers such health benefits. Studies clearly show correlations between high fish consumption and reduced risk of disease (1;2), but to understand why this may be, should help us to determine what types of fish we should be recommending and why.

The nutritional content of fish clearly differs from most other foods in that fish provides a high dose of omega-3 fatty acids EPA and DHA. Fish is also a brilliant source of vitamin B12 and protein.

Not only does fish have an excellent nutritional profile, it can also help to modulate inflammation by balancing omega-6 AA with omega-3 EPA, affecting anti-inflammatory or pro-inflammatory signalling molecules in the body. (3) A high ratio of AA:EPA is associated with a range of chronic inflammatory conditions, including arthritis. (4)

In addition to reducing inflammation in the body, omega-3 EPA in fish supports immune health and has also been shown to reduce blood pressure, platelet aggregation and risk of stroke and heart attack. (5) Brain function is another well researched area for omega-3; DHA in particular provides brain structure and EPA is well known for its ability to support neurotransmitter function such as dopamine, and is therefore excellent for anyone suffering from conditions such as depression and/or anxiety. Concentration levels of omega-3 actually appear lower in individuals suffering from depression. (6) Chronic low-grade inflammation and a high omega-6 AA : omega-3 EPA ratio is also associated with depression. (7)

Who actually needs more fish?

It is easy to recommend by default, that everyone eats more fish, but as practitioners we should really avoid this blanket approach. Yes, up to around 95% of our clients could probably do with increasing their fish intake, but perhaps we should consider their symptoms initially and consider their current intake of fish to determine what to recommend.

Initial questions to clients covering omega-3 deficiency symptoms such as dry skin, nails, hair and eyes are possibly the most useful initially. Delving into other areas of health such as immune health, brain function and joint health may also help to give you an idea of their fatty acid status.

It is useful to be aware that common deficiencies exist in individuals with eczema and mood and neurodevelopmental disorders. Children with ADHD have very low blood levels of essential fatty acids, in particular the long-chain omega 3 fatty acid EPA. (8)

Are fish oils ‘essential fats’?

We are often told that fish oils are essential fats, but strictly speaking this is not entirely true, as the body can produce omega-3 EPA and DHA by metabolising plant-based short-chain fatty acids such as Alpha Linolenic Acid (ALA) from flaxseeds and chia seeds. As this conversion in the body is quite limited, particularly if someone has a poor intake of zinc, magnesium and vitamin B6, then their delta-6 desaturase and delta-5 desaturase enzymes may not be converting fats efficiently.

More often than not, if clients present with omega-3 deficiency symptoms, even when eating high levels of the essential fats omega-3 ALA and omega-6 LA, it is likely that the fats are not being converted to the long-chain fatty acids EPA and DHA. Given these limitations in conversion of fats in many individuals, this is why fish oils are by some considered to be essential.

If someone appears to have quite a high intake of oily fish, but they still present with omega-3 deficiency symptoms, consider their digestive health. To support the digestion of fats, consider recommending the digestive enzyme lipase.

Types of fish


Whether a fish is oily or white is possibly the most common way to differentiate between their nutritional content. Asking the question ‘how many times do you eat fish per week’ should really be replaced with ‘how many times do you eat OILY fish per week’ if you are considering inflammatory conditions, skin health and brain health, for example.

Some of the oily fish providing highest levels of omega-3 EPA and DHA include salmon, mackerel and herring. In the UK, the most commonly eaten fish still tend to be white fish such as cod and haddock. White fish is of course still very nutritious, but a combination of both white and oily would help to improve fatty acid status in the body. It is important to note that the canning process can considerably reduce the fatty acid content of oily fish, so a tuna steak may provide a good level of EPA and DHA, whereas canned tuna may only provide a very small amount.

The size of fish may also have an effect on health due to differing levels of toxins. Larger fish generally have much higher quantities of methyl mercury, PCBs and dioxins, due to the cumulative effect of larger fish eating smaller fish. Smaller fish such as anchovies, herring and mackerel are therefore much more favourable when it comes to keeping intakes of contaminants low while still including plenty of omega-3 fats.

The stability of omega-3 fatty acids in fish

Fish which is openly displayed in shops can significantly increase rancidity of the delicate oils in fish. Frozen fish may actually provide more stable oils compared to those which are considered fresh at the fish counter in supermarkets (which may have previously been frozen and then thawed out to appear to be fresh in many cases). Unless you live by the sea in a fishing village with a regular fresh supply of fish (lucky you!), fish in cities are most likely not freshly caught.

Processed fish based products

Ready meals such as ‘salmon with lemon butter’ may sound very healthy to most people, but when looking into the ingredients of such finished products, they are most commonly over-processed vegetable oils combined with refined sugar. Keeping it simple with a plain fish fillet and organic butter with herbs and a slice of lemon when cooking is your best bet.

Cooking methods

As the fatty acids in fish are easily prone to oxidation, it is always best to recommend cooking fish at lower temperatures if possible. The worst possibly cases would be frying fish on a high heat in vegetable oil, or deep frying in reused vegetable oil. If fish is to be fried, this should be done on a medium heat in a stable fat such as coconut oil. Baking fish in the oven at around 160-180 deg. C is also fine as the fish is usually kept below 140 deg. C, keeping the delicate oils protected. The thick white residue you often see when baking fish is not actually fat, but the amino acids seeping out of the fish when heated for a long time, so this is nothing to worry about in terms of keeping the fats intact.

Another tip to reduce oxidation of EPA and DHA when cooking is to add sulphur-rich antioxidant foods such as onions and garlic. Adding sliced garlic and lemon works well when baking fish wrapped in foil, and finely chopped onions, herbs and garlic is great when frying.

Wild vs farmed fish

The majority of people do not even think about the source of their fish, but understanding the nutritional quality of wild versus farmed fish is actually quite important. Wild fish have a natural diet consisting mostly of algae or smaller fish, whereas farmed fish are often fed additional foods, (un)surprisingly including corn and soy, which are much higher in omega-6 fatty acids. As a result of the variation in food eaten by wild and farmed fish, the two source types have very different fatty acid profiles. Wild fish tend to have higher levels of fats, of which a higher proportion will be from omega-3 EPA and DHA. Farmed fish on the other hand have lower levels of fats, and a higher proportion of omega-6 fatty acids when compared to wild fish.

Practically, it may be quite difficult for your clients to only choose wild fish as the vast quantities of fish provided in supermarkets comes mostly from farmed fish. Restaurants are also most likely to serve farmed fish because of its comparatively reduced price. When determining whether a fish is farmed or wild, look for wild on the label, but also look for terms such as line caught for example, which should also indicate that the fish is wild.

Fish sustainability

Although the sustainability of fish is not of nutritional relevance, being able to make decisions which may affect our future supplies of fish in the sea is definitely something to consider when recommending that people increase their fish consumption. Wild fish is by far nutritionally superior, however not all fish caught wild is sustainable if endangered. This explains why most fish labelled ‘sustainable source’ are actually farmed. For information on fish sustainability, see for a full list of types of fish, area fished and sustainability at different times of the year.

Diagnostic tests to determine fish intake requirements

Opti-O-3 visual logo - square low resIf you are unsure as to whether your client may have an omega-3 deficiency, the easiest way to determine this is to carry out a diagnostic test to directly measure levels of a range of fatty acids in red blood cell membranes, such as finger prick test Opti-O-3. Results from such a test will help to identify both the ‘omega-3 index’ i.e. the percentage of omega-3 in red blood cell membranes and also the AA:EPA ratio to indicate inflammatory levels.

Reference List

(1)    Takata Y, Zhang X, Li H, Gao YT, Yang G, Gao J, et al. Fish intake and risks of total and cause-specific mortality in 2 population-based cohort studies of 134,296 men and women. Am J Epidemiol 2013 Jul 1;178(1):46-57.

(2)    Qin B, Plassman BL, Edwards LJ, Popkin BM, Adair LS, Mendez MA. Fish intake is associated with slower cognitive decline in Chinese older adults. J Nutr 2014 Oct;144(10):1579-85.

(3)    de BJ, Sauleda J, Balcells E, Gomez FP, Mendez M, Rodriguez E, et al. Association between Omega3 and Omega6 fatty acid intakes and serum inflammatory markers in COPD. J Nutr Biochem 2012 Jul;23(7):817-21.

(4)    Patterson E, Wall R, Fitzgerald GF, Ross RP, Stanton C. Health implications of high dietary omega-6 polyunsaturated Fatty acids. J Nutr Metab 2012;2012:539426.

(5)    Ohnishi H, Saito Y. Eicosapentaenoic acid (EPA) reduces cardiovascular events: relationship with the EPA/arachidonic acid ratio. J Atheroscler Thromb 2013;20(12):861-77.

(6)    Pottala JV, Talley JA, Churchill SW, Lynch DA, von SC, Harris WS. Red blood cell fatty acids are associated with depression in a case-control study of adolescents. Prostaglandins Leukot Essent Fatty Acids 2012 Apr;86(4-5):161-5.

(7)    Conklin SM, Manuck SB, Yao JK, Flory JD, Hibbeln JR, Muldoon MF. High omega-6 and low omega-3 fatty acids are associated with depressive symptoms and neuroticism. Psychosom Med 2007 Dec;69(9):932-4.

(8)    Bloch MH, Qawasmi A. Omega-3 fatty acid supplementation for the treatment of children with attention-deficit/hyperactivity disorder symptomatology: systematic review and meta-analysis. J Am Acad Child Adolesc Psychiatry 2011 Oct;50(10):991-1000.


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Kyla Newcombe

About Kyla Newcombe

Kyla is a highly qualified clinical nutritionist with a master’s degree in Nutritional Medicine. Kyla runs her own private practice, offering personalised dietary and supplement advice. Kyla has extensive experience in weight management, skin disorders and digestive issues. Her website is at Kyla regularly contributes to articles for leading consumer magazines, and blogs about healthy cake ingredients and recipes at