Section 1:
Overview of the health benefits of eating seafood

Finfish and other foods from oceans, rivers and lakes have long been recognised as nutritious. They are an excellent source of protein and are rich in essential poly-unsaturated fatty acids.

Seafood is the best food source of iodine; salt water seafood contains about twice the iodine found in freshwater varieties. It also provides an excellent source of selenium and fluoride. Other minerals which are provided in moderate amounts are iron, zinc and magnesium. The iron content is about a third to a half that in red meat.

Shellfish is similar in food value to finfish, but crustaceans contain about twice as much cholesterol as other seafoods. Molluscs used to be classified as foods high in cholesterol, but it is now known that most of the sterols in these foods are compounds other than cholesterol. Moreover, cholesterol in food is not the main predictor of blood cholesterol. The issue of cholesterol from seafood and blood cholesterol is discussed on page @.

From the early days of nutrition science, finfish in particular has been acknowledged for being a high protein, low calorie food. In recent years, the importance of finfish in the diet has extended from its image as a cornerstone of a healthy diet, to more specialised roles in disease prevention. Scientists working on coronary heart disease in Nordic countries during the 1970s observed that Greenland Inuits (Eskimos) had one-tenth to one‑third the heart attack rate of Danes. Subsequent studies found that Inuit people have much lower blood cholesterol, triglyceride and ‘bad’ cholesterol (low-density lipoprotein, LDL) levels, and higher ‘good’ cholesterol (high-density lipoprotein, HDL) levels, than their Danish counterparts. Similar results have been observed with Japanese people from Kohama Island — where people have the lowest incidence of heart disease in Japan and much higher serum levels of certain fatty acid due to higher intake of fresh finfish.

In the last decade it has been observed that the consumption of two or more serves of finfish per week is associated with a lower prevalence of heart disease.

Beneficial effects of seafood consumption have also been reported for other diseases or conditions. These benefits have been linked to the long-chain, highly poly-unsaturated omega‑3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are found in seafood. This has further highlighted the possibility of additional health benefits associated with eating finfish. However, the levels of supporting evidence differ.

The physiological conditions which may benefit from optimal finfish consumption and the level of supporting evidence are summarised in table 1. Further details on each condition are provided in section 2.

Table 1: Health benefits associated with finfish consumption and levels of supporting evidence

Seafood is high in beneficial fats

Humans can manufacture some types of fatty acids in the body, but must obtain those essential for good health (the poly-unsaturated omega-3 and omega-6) from the diet. Omega-3 fats are best found in seafood and plants such as soybean, canola, flaxseed and purslane. Omega-6 fats are also found in seafood of all kinds: crustaceans, molluscs, shellfish and, to a variable extent, finfish. However, plant foods such as sunflower seed, corn and soybean are usually a better source of omega‑6 fats. In recent times, the emphasis in affluent diets has been on poly-unsaturated fatty acids of the omega-6 type, so that the ratio of the omega-3 to omega-6 has been too low. Regular seafood intake plays an important role in allowing a healthy ratio of omega-3 to omega-6 fatty acids.

To prevent essential fatty acid deficiency, nutritionists generally recommend that humans must consume at least 2.4% of total fat intake as omega-6 fats, and 0.5–1.0% of total fat as omega-3 fats.

Table 2a: Omega-3 oil contents of various Australian seafood species

* PUFA = omega-3 poly-unsaturated fatty acids

(Nichols et al. 1999, in Australian Seafood Handbook: Domestic Species, eds G.K. Yearsley, P.R. Last & R.D. Ward).

Table 2b: Amount of seafood required to provide various intakes of fish oils

(Nichols et al. 1999, derived from Australian Seafood Handbook: Domestic Species, eds G.K. Yearsley, P.R. Last & R.D. Ward).

Seafood is also a good source of other nutrients

Selenium

Selenium is now recognised as an essential element for humans; it plays a major role in the enzymes that make up part of the body’s antioxidant defence and other systems. It helps to prevent DNA damage caused by various chemicals and radiation. Dietary deficiency of selenium has not been demonstrated in humans — although animals on a deficient diet develop a number of symptoms including retarded growth, muscular dystrophy and necrosis of the heart, kidney and liver.

Finfish can be a useful source of dietary selenium. In Australia, low levels of selenium in soils make this an important factor.

Co-enzyme Q10

Finfish is one of the good sources of co-enzyme Q10. Although more than 40 years ago co-enzyme Q10 was known to function as an anti-oxidant at the sub-cellular level, it has only recently received attention in relation to its food sources. Co-enzyme Q10 concentrations rise under the influence of oxidative stress (e.g. physical exercise) and in degenerative conditions of the brain, such as Alzheimer’s disease, whereas it is reported that its concentrations drop in several diseases, including degenerative muscle diseases and liver carcinomas. Although co-enzyme Q10 can be synthesised in the body, additional intake from food is required.

Taurine

Seafood contains a large amount of taurine. This amino acid is long known for its role in the formation and excretion of bile salts, which are the breakdown products of cholesterol. It also plays a role in the function of the neonatal retina and in cognitive function.

Which one is better — finfish or fish oil supplements?

On the basis of omega-3 fatty acid content, fish oil is an attractive commodity, because it has a higher concentration of these than the whole finfish. Fish oil can be a useful option for people who are unable to eat seafood, or who cannot eat it in quantities sufficient for their health needs. For example, to achieve the required effects of omega-3 fatty acids, some people may need to have the equivalent of 2–3 serves (at 100 grams per serve) of finfish a day, or about 6–9 grams of finfish fat per day (an average finfish has about 3 grams fat per 100 grams). Such effects may be to reduce the concentration of blood triglyceride (a blood fat), decrease the risk of an abnormal heart rhythm or perhaps correct a mood disorder (depression).

Fish oil may achieve these effects much more conveniently than eating finfish. However, some health effects of finfish are not seen with fish oil. Reduction of high blood pressure is an example; this is thought to be due to the relatively higher proportion of DHA to EPA in finfish compared with that in fish oils. Again, factors from the flesh (muscle) of finfish, other than fatty acids — such as protein and its amino acid content, micronutrients, or co-enzyme Q10 — may be important to health. In finfish there are also factors that are anti-oxidants, which protect the poly-unsaturated fatty acids from oxidation — whether before or after ingestion. These may not be as well represented in fish oil, although this can be addressed during processing.

There is a natural constraint as to how much finfish we can eat, but less so (unless because of taste or after-taste) for fish oil. To this extent, it is theoretically possible to have an excessive amount of omega-3 fats from fish oil. However, the larger amounts are normally given under medical supervision, where effects can be carefully managed. Such effects can increase bleeding tendency or increase not only the ‘good’ HDL cholesterol but also the ‘bad’ LDL cholesterol, especially in diabetes, while lowering the triglyceride (a favourable outcome).

On the whole, it can be said that finfish (and seafood in general) are to be preferred to fish oil, but where larger amounts of omega-3 fatty acids are needed and finfish intake is a problem, fish oil can play a valuable health role.

Healthy ways to cook seafood

The cooking method could markedly alter the fatty acid content of a seafood meal. The best ways to cook seafood and maintain its health benefits by minimising omega-3 fat losses are steaming, micro-waving, grilling or baking. A recent study of older people showed that eating broiled or baked finfish provided more protection against fatal myocardial infarction and death from coronary heart disease than eating fried fish or fish burgers. It appears pan-frying and deep-frying seafood, with very high temperatures, could destroy some omega-3 fats, although some studies show no effect. Frying also causes more cooking oil to be absorbed into the finished product than with cooking by other methods. It is recommended that if seafood is to be pan-fried or deep-fried, it’s preferable to use cooking oil that is high in mono-unsaturated fats, such as olive or canola oils.

If seafood is to be fried, it should be pan-fried in a small amount of oil rather than solid fat, which tends to be more saturated or be hydrogenated with a higher trans fatty acid content. Trans fatty acids are similar in effect to saturated oils, and raise ‘bad’ cholesterol and decrease ‘good’ cholesterol levels. There is really no perfect oil for deep frying.  Dominantly, mono-unsaturated oils (such as olive oil) or poly-unsaturated oils (such as sunflower oil) are preferable to solid fat.  But mono-unsaturated oil is generally too expensive, and poly-unsaturated oil, with essential fatty acids, may undergo unfavourable chemical changes. Most ‘fish and chips’ shops use saturated or hydrogenated oils because they last longer and are generally cheaper. The important consideration in deep frying is to use oils that are clean and replaced regularly.  Other cooking methods such as grilling or steaming should be encouraged.

The cooking temperature of oil is important. For example, with chips that are often served with finfish, if the cooking oil temperature falls below 180–185°C, up to 40% more fat is absorbed into the chip (or about 3 teaspoons of extra fat — amounting to about 100 calories or 420 kilojoules — in a single serve of chips). Table 3 shows the relative contents of saturated, unsaturated and mono-unsaturated oils in various cooking oils.

Table 3: Relative percentages of fats in common cooking oils

Cooking seafood with various herbs could also be beneficial to health — for example, supplementing seafood with garlic can significantly lower cholesterol and triglyceride levels. Herbs are normally anti-oxidants as well. They can also help to preserve the essential fatty acid value of seafood and reduce the formation of potentially harmful chemicals such as heterocyclic amines (derivates of amino acids in proteins) if seafood is over-cooked.

Finfish do not lose their positive health benefits by being canned. Finfish canned in fish oil have a particular advantage, but are now less available in the market place. Finfish canned in olive or canola oil bring with them the health benefits of these oils; canned in brine or spring water, there are less calories.

Nutritionists increasingly advocate the use of a Mediterranean diet (which contains seafood) as a healthy diet for people at risk or suffering from coronary heart disease. Seafood is important in disease prevention but should not be considered in isolation. It has a premium place in a healthy diet for most people.

Concern about seafoods — heavy metal content

The health hazards associated with the consumption of seafood contaminated with heavy metals (notably mercury) have received world-wide publicity. The levels of heavy metals vary in different seafood species because each has different habitats and feeding patterns. Large predatory species of finfish such as tuna, swordfish and some species of shark eat smaller plant-eating finfish, and therefore they tend to accumulate higher levels of mercury. Further up the food chain, the mercury can become more concentrated in the flesh of the finfish. Where water has been polluted by manufacturing or mining, the amounts can be unacceptable and intolerable from a human health point of view. Problems have only arisen when individuals eat large amounts of finfish that have fed in contaminated waters.

Mercury is a highly reactive heavy metal with no known physiologic activity.  Exposure to toxic levels of mercury results in neurologic and renal damage. It has been put forward that a high dietary intake of mercury from seafood consumption increases the risk of coronary heart disease.  Therefore, the risk of heart disease in a population may depend on the balance between omega-3 fatty acids and mercury in seafood consumed. 

Presently the majority of the population are exposed to mercury at levels that are not associated with harmful effects. In Australia, there have been no reported cases of mercury poisoning as a result of seafood consumption. The Australian Food Standards Code currently prescribes maximum levels for mercury in food.  Two separate limits are imposed for seafood:

• 1.0 mg/kg for finfish that are known to contain high levels of mercury (such as swordfish, southern bluefin tuna[2], barramundi, ling, orange roughy, rays and shark/flake), and
• 0.5 mg/kg for all other species of finfish, crustaceans and molluscs.

Some research indicates that human foetuses are more sensitive than adults to the effects of mercury from food consumption, especially to the nervous system. Hence there are concerns about whether pregnant women should reduce their intake of seafood to limit the exposure of the foetus to the toxic effects of mercury. But a recent study in the Republic of Seychelles, an island nation in the Indian Ocean, suggests that mercury exposure from seafood consumption during pregnancy does not have significant cognitive or behavioural effects in later childhood. In the Seychelles, the concentration of organic mercury in seafood is similar to diets consumed during pregnancy in most of the world, but the seafood consumption rate is much higher (average 12 meals per week). This study indicates that small amounts of mercury are not a problem, although two other studies in the Faeroe Islands and New Zealand found detrimental effects. In these latter studies, whale blubber and shark provided large  amounts of mercury in short periods of time, which may be more harmful than lesser amounts when finfish are eaten more frequently and regularly.

Although findings from various studies are inconsistent, there is no reason for pregnant women to avoid or reduce seafood consumption. Moreover, many components of seafood are very important for foetal development — for example, protein and its amino acids, omega-3 fatty acids, iodine and calcium.  In addition, low consumption of seafood in early pregnancy could be a risk factor for pre-term delivery and low birth weight.  Therefore, given the concerns regarding heavy metal exposure, pregnant women need to avoid large and old predator finfish and take as much care as they can to ensure that seafood comes from areas with pollution controls.

Food Standards Australia New Zealand (www.foodstandards.gov.au) recommends that consumption of some types of finfish by pregnant women, women planning pregnancy and children up to six years of age should be limited, as shown in table 4.

Key messages

One to four serves a week are probably best

Several studies now demonstrate that even a limited finfish intake — say one serve (about 100 g) per week — is better than none, especially in relation to heart disease. Up to four serves a week may be useful in some health respects, such as blood pressure control. The amount recommended will depend not only on benefits but also on risks. For example, heavy-metal toxicity may be avoided with modest intake while the nutritional benefits of finfish are preserved.

With crustaceans, shellfish and molluscs, the concerns felt by some people about cholesterol content are hardly relevant with occasional intake (say weekly), and of even less concern where the cooking technique and food habits avoid the use of saturated animal fat (as with deep frying and fatty spreads).

Message: Have between one and four serves (about 100 grams per serve) of finfish a week, because increasing health benefits may be seen across this range of intake. This amount is considered to be safe in relation to heavy metals (such as mercury) or other contaminants, unless the finfish is harvested from an area without pollution controls. This guideline is relevant to pregnancy, as well as for adults in general; children will generally have somewhat smaller portions.

Benefit of small portions

When preferred, convenient, more affordable or culturally appropriate, serving sizes smaller than 100 grams of finfish can be used in a cumulative way for health over the day or over a few days. Examples would be sushi, marinated herrings or small tins of finfish such as sardines, salmon or tuna.

Message: Small serves of finfish as a snack or meal adjunct are healthful.

Sustainability

As the health evidence for regular seafood intake becomes clearer and more widely known, demand will increase and wild-caught seafood stocks will be under greater pressure. Optimising intake for the greatest majority globally will be important, as will the methods of harvesting and farming seafood. Another consideration will be the maintenance of aquatic biodiversity to which the interest in, and acceptability of, a variety in seafood intake will also contribute. A more contemporary science-based education program about the health benefits of finfish, crustaceans, shellfish and sea plants will promote biodiversity and, with it, sustainability.

Useful sources of information on sustainability factors in the fishing industry are Investing for tomorrow’s fish: the FRDC strategic plan for fisheries research and development, 2005 to 2010 and FRDC annual reports, available from the Fisheries Research and Development Corporation (telephone 02 6285 0400 or visit www.frdc.com.au).

Message: Eat a variety of seafood as part of a varied diet for its health benefits — both direct and indirect — which are achieved by encouraging biodiversity and sustainability.