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What’s so healthy about seafood?

Section 1

Overview of 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 polyunsaturated 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 20.

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) levels, and higher ‘good’ cholesterol (high-density lipoprotein) levels, than their Danish equivalents. 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 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.

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. This has further highlighted the possibility of additional health benefits associated with eating finfish.

Beneficial effects of seafood consumption have also been reported on other diseases or conditions. These benefits have been linked to the long-chain, highly polyunsaturated Omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are found in seafood. However, the levels of supporting evidence are different.

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

Disease or	Strong evidence	Promising	Possible health health condition	of significant	preliminary	benefits (require		health benefits	results	more substantiation)
Coronary heart disease	3
High blood pressure	3
Irregular heart beat (arrhythmia)	3
Diabetes	3
Cancer
	Bowel cancer	3
	Laryngeal cancer			3
	Pancreatic cancer			3
Asthma		3
Rheumatoid arthritis	3
Crohn’s disease		3
Central nervous system
	Neural development	3
	Memory			3
Depression				3

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 polyunsaturated 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 polyunsaturated 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. Plants alone may not provide enough essential fatty acids, especially long-chain Omega-3 fats. These are best obtained from seafood.

Omega-3 fat content of Australian seafoods

Different types of seafood have different levels of Omega-3 fats. As a general rule, plant-eating finfish have less Omega-3 fats than Omega-6 fats (e.g. 30% Omega-3 and 70% Omega-6 as a percentage of total fat content), whereas carnivorous finfish have more Omega-3 fats than Omega-6 fats (e.g. 70% Omega-3 and 30% Omega-6 as a percentage of total fat content). These Omega-3 fat contents are far higher than the amounts found in plants.

The Fisheries Research and Development Corporation has published three detailed books on Australian commercial seafood and its fat content and composition:

• Seafood the good food: oil content and composition of Australian commercial fishes, shellfishes and crustaceans;
• Australian Seafood Handbook: an identification guide to domestic species;
• Australian Seafood Users Manual – making the most of the world’s best.

The contents of Omega-3 fats in various species of seafood, derived from the Australian Seafood Handbook, is shown in tables 2a and 2b.

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

Total oil content

Species	Oil	PUFA*	DHA	(%)	(mg/100g)	(mg/100g)
School shark	0.9	300	250
Swordfish	7.7	1350	550
Snapper	0.6	220	150
Barrumundi	0.9	110	50
Pink ling	0.5	120	80
Spanish mackerel	1.2	400	275
Atlantic salmon	2.7	650	425
Tiger prawn	0.8	180	75
Blue mussel	1.7	330	170
Pacific oyster	1.0	300	150
Sydney rock oyster	1.3	300	160

* 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

Grams of seafood that must be eaten to obtain the ;following levels of Omega-3 fats, fish oil and DHA

Species	1 gram of	2.5 grams	200 milligrams	Omega-3 fats	of fish oil	of DHA
School shark	333	278	80
Swordfish	74	32	36
Snapper	455	417	133
Barramundi	909	278	400
Pink ling	833	500	250
Spanish mackerel	250	208	73
Atlantic salmon	154	93	47
Tiger prawn	556	313	267
Blue mussel	303	147	118
Pacific oyster	333	250	133
Sydney rock oyster	333	192	125

(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

Finfish can be a useful source of dietary selenium, which is now recognised as an essential element for humans. In Australia, low levels of selenium in soils make this particularly relevant. Selenium plays a major role in the enzyme systems that control the accumulation of free radicals in the body. Adequate dietary selenium helps to protect against situations that create substantial oxidative damage to genetic materials, such as exposure to various chemicals and radiation. Selenium deficiency has a characteristic pathology that involves adverse changes to heart muscle.

Co-enzyme Q10

Although it has been known for more than 40 years that co-enzyme Q10 functions as an anti-oxidant at the sub-cellular level, it has only recently received attention in relation to dietary source. 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, and finfish is one of the good sources of co-enzyme Q10.

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 g 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 polyunsaturated 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’ cholesterol (HDL) but also the ‘bad’ cholesterol (LDL), 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.

Fish oil may achieve effects much more conveniently than eating finfish, but some health effects of finfish are not seen with fish oil.

Healthy ways to cook seafood

There is really no perfect oil for deep frying. Other cooking methods such as grilling or steaming should be encouraged.

The best ways to cook seafood and maintain its health benefits are by steaming, micro-waving, grilling or baking. If seafood is to be fried, it should be pan-fried in a small amount of oil rather than solid fat (these tend to be more saturated or hydrogenated, and have a higher trans fatty acid content).

Deep-frying of seafood provides few – if any – health benefits. The important consideration in deep frying is to use oils that are clean and replaced regularly.2

Most ‘fish and chip’ shops use saturated oils as they last longer and are generally cheaper than monounsaturated or polyunsaturated oil. Olive oil (monounsaturated) is generally too expensive, and polyunsaturated oil, with essential fatty acids, may undergo unfavourable chemical changes. There is really no perfect oil for deep frying. 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 monounsaturated oils in various cooking oils.

Table 3: Relative percentages of fats in common cooking oils.

2 Poly-unsaturated oils oxidise with repeated heating and cooling, and should therefore be used only two to three times before changing.

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.

Finfish do not lose their positive health benefits by being canned.

Cooking seafood with various herbs may 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, and can help to preserve the essential fatty acid value of seafood and reduce the formation of potentially harmful heterocyclic amines (derivates of amino acids in proteins) if it is over-cooked.

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.

What is the Mediterranean diet?

The Mediterranean diet consists of:

• more bread,
• more vegetables and legumes,
• more seafood,
• less meat (beef, lamb, pork) – replaced by poultry,
• no day without fruit,
• no butter or cream, and
• olive oil or other monounsaturated oil source.

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.

Other 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.

Concerns about cholesterol content of crustaceans and shellfish are hardly relevant with occasional intake, and of even less concern when saturated animal fat is avoided.

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.

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

Heavy metals and contaminants

Sea creatures near the top of the food chain and ‘filter feeders’ such as molluscs – normally accumulate certain heavy metals (notably mercury), mainly as organic compounds. Where water has been polluted by manufacturing or mining, the amounts can be unacceptable and intolerable from a human health point of view. The most vulnerable is the human foetus, so that pregnant women need to take as much care as they can to ensure that seafood comes from areas with pollution controls.

However, many components of seafood are very important for foetal development – for example protein and its amino acids, Omega-3 fatty acids, iodine and calcium. Therefore, having up to 400 grams per week of finfish during pregnancy is normally safe and beneficial to the foetus.

Message: Consider up to about 400grams of finfish a week as 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.

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.

A useful source of information on sustainability factors in the fishing industry is Investing for tomorrow’s fish: the FRDC’s research and development plan, 2000 to 2005. It is available from the FRDC on telephone: 02 6285 0400 or at 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.