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What’s so healthy about seafood?

Section 2

More detailed information on seafood and specific conditions

Finfish and mortality

In the early 1970s, evidence was emerging that finfish intake had important health implications. Most studies focused on protection by finfish against heart disease and consequent death. It was demonstrated in a clinical controlled trial that, for those who recovered from heart attacks, consuming fatty finfish two or three times a week could reduce overall mortality by nearly one-third after two years.

Similarly, in another trial, with supplements of marine Omega-3 fats, total mortality was reduced by 20%, cardiovascular death by 30%, and sudden death by 45%. The Mediterranean diet, which includes a recommendation for eating moderate amounts of finfish, has also been shown to be more effective than a low-fat diet in protecting against sudden cardiac death.

Studies indicate that one or two serves of finfish per week can substantially lower the risk of coronary heart disease.

Finfish and heart disease

Coronary heart disease is a major cause of death and disability in Australia. While many individual factors are involved in the onset and progression of the disease, including genetics and lifestyle, the positive benefits of finfish in reducing the risk of coronary heart disease have been widely studied and are now well accepted. However, it is important to emphasise that no single food will cause or prevent coronary heart disease.

Studies have found that decreased risk of death from heart attack is related to increased finfish consumption. In one study, finfish consumers had less than half the mortality rate of people who ate no finfish. The highest rates of heart disease have been recorded for men who ate no finfish at all. These studies indicate that one or two serves of finfish per week can substantially lower the risk of coronary heart disease.

Prevention of coronary heart disease through diet relies on a combination of the following strategies:

• reducing total serum (blood) cholesterol,
• reducing serum LDL cholesterol and increasing serum HDL cholesterol,
• minimising LDL oxidation,
• optimising blood pressure,
• minimising heart arrhythmia,
• improving blood flow, and
• preventing obesity.

The following sections discuss the benefits of finfish consumption on key factors in coronary heart disease, including:

• secondary prevention of coronary heart disease,
• nitric oxide production,
• cholesterol,
• heart arrhythmia,
• blood composition, and
• high blood pressure.

Secondary prevention of coronary heart disease

After someone has had a heart attack, it is very important to prevent further damage that may bring about another heart attack. Steps taken to lower the risk of another heart attack are called secondary prevention measures.

Secondary prevention has presented many challenges to researchers and most research has been unrewarding. However, one study achieved results from increasing the intake of Omega-3 fish oils. Further studies have shown that a Mediterranean diet that includes 47 grams of finfish per day can reduce the incidence of recurring heart attack – even compared with a low-fat diet. People on the Mediterranean diet consumed less fat, saturated fat, cholesterol and linoleic acid, but significantly more oleic acid and alpha-linolenic acid (a plant Omega-3 oil). This resulted in far lower heart attack and death rates in the five-year follow-up period. In fact, only two years into the study the researchers found the results so striking that they stopped the trial and recommended that all the participants follow the Mediterranean diet.

People on a Mediterranean diet in a trial had far lower heart attack and death rates in the five-year follow-up period than people on a low-fat diet. Two years into the study, the results were so striking that the trial was stopped; all participants were recommended to follow the Mediterranean diet.

Cholesterol from seafood and blood cholesterol

Most studies confirm that increasing dietary cholesterol results in higher blood cholesterol, and is associated with increased heart disease risk. This is particularly so when combined with eating saturated fat. Only 15% of the population experience increases of blood cholesterol greater than 10%. There are a number of factors modulating this effect. Gender, age and distribution of body fat can influence the way the body handles cholesterol from food. Another is the fatty acid composition of the diet. Two classes of dietary fatty acids can raise blood cholesterol: saturated fats (mostly from animal foods, such as meats) and trans-monounsaturated fats (produced by hydrogenation of vegetable oils).

Cholesterol in the body is transported by low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Cholesterol-laden LDL is deposited in part of the wall of blood vessels. The accumulation of LDL can cause tissue damage and, as a result, the artery becomes completely blocked. If this happens to the arteries of the heart (coronary arteries), it leads to a heart attack. For this reason, LDL cholesterol is considered to be ‘bad’ cholesterol – although it is not all bad since the body requires some LDL cholesterol for body metabolism.

In contrast, HDL cholesterol is considered to be ‘good’ cholesterol, as HDL helps to remove LDL cholesterol from the body. High cholesterol, high LDL and low HDL levels are risk factors for heart disease. High triglycerides in combination with these factors multiply this risk further.

Furthermore, LDL can undergo a chemical reaction of oxidation, which is enhanced by certain reactive molecules normally produced as part of the defence system, and as the by-products of metabolic processes that utilise oxygen. This ‘oxidised’ LDL has been implicated in hardening of the arteries and arterial damage. LDL oxidation appears to be influenced by the type of fats we eat and minimised by the intake of various anti-oxidant foods. Whilst replacing saturated fat with Omega-6 fats is a healthy strategy, a healthier strategy would be to reduce the amount of dietary saturated fats and Omega-6 fats in the diet by replacing some of these with Omega-3 fats from seafood which also contains its own natural anti-oxidants such as vitamin E, carotenoids and co-enzyme Q10.

Seafood should be combined with a diet rich in fruit and vegetables.

Anti-oxidants – such as vitamin C, E, beta-carotene and certain compounds found in fruits and vegetables – are crucial in protecting the body against the process of oxidation. The amount of anti-oxidants in the body protects against unwanted oxidation of the fats in seafood. Hence, to maximise the benefits of Omega-3 and Omega-6 fats, anti-oxidants should be eaten as well. To further enhance the intake of anti-oxidants, seafood intake should be combined with a diet rich in fruit and vegetables.

The general dietary approach for lowering blood cholesterol is to limit cholesterol intake to less than 300 mg per day and reduce dietary fat, especially animal saturated fat. Low-fat diets also reduce the protective HDL levels. If the person has high blood cholesterol, intake is restricted further, to less than 200 mg per day.

People at risk of developing coronary heart disease are often advised to only rarely eat crustaceans (prawns, crabs, lobsters) because these foods contain high cholesterol. However, it should be noted that oysters, clams, mussels and crab can be found to be suitable for diets that lower blood cholesterol, in combination with a reduced saturated fat intake. Moreover, their Omega-3 fat content provides protection against heart disease by means other than the effects on blood cholesterol.

Use of seafood in conjunction with low-fat diets can increase HDL cholesterol and reduce triglycerides; therefore, use of seafood may be more suitable for reducing the risk of coronary heart disease than a low-fat diet on its own. Some of the disadvantages of a low-fat diet are avoided by supplementation with fish oil.

Fish oil may help to improve blood flow

Blood flow in injured arteries is restricted by the hardening of blood vessels caused by the deposition of fatty plaques and calcium in the arterial wall. Nitric oxide, which is a major contributor to dilation of blood vessels, can ease the blood flow. Injured parts of an artery wall produce much less nitric oxide.

Fish oils are thought to increase nitric oxide levels released by blood vessel tissue (especially when it has been injured), and to locally relax blood vessels. Other research shows that Omega-3 fish oil helps to dilate the coronary artery in heart transplant patients. In patients with non-insulin dependent diabetes mellitus, the oil also improves blood flow in the forearm – indicating protection against blood vessel constriction and thrombosis.

Finfish protects against heart arrhythmia

Many scientists consider that Omega-3 fats from finfish may help to prevent disruptions to the rhythm of the heart (heart arrhythmia or ventricular fibrillation). In one study, patients who had recently suffered a heart attack were given 5.2 grams per day of fish oil. Heart rates were examined for variability, since such variability protects against ventricular arrhythmia. It was found that fish oil had the beneficial effect of increasing heart rhythm variations in the patients.

There is considerable evidence that the consumption of Omega-3 fat-enriched diets leads to a marked reduction in the susceptibility of the heart muscle to develop heart arrhythmia and subsequent sudden cardiac death.

Finfish and high blood pressure

Early studies on the effect of fish oil supplements on high blood pressure showed significant lowering of blood pressure. However, other studies have shown no – or relatively small – effects on blood pressure. It is suggested that in severe and life-threatening situations, fish oil may be useful for rapid exchange of Omega-3 fats for Omega-6 fats to help normalise blood pressure and a range of other risk factors for myocardial infarction (heart attack).

One study has shown that in overweight people with high blood pressure, incorporating finfish into a weight-reduction diet has additive effects in reducing blood pressure, as well as beneficial effects on heart rate. These effects may be due in part to constituents of finfish other than Omega-3 fats, such as finfish protein. It was recently reported in an animal model study that Omega-3 fats supplied in the early development period can affect blood pressure later in life.

There is considerable evidence that the consumption of Omega-3 fat-enriched diets leads to a marked reduction in the susceptibility of the heart muscle to develop heart arrhythmia and subsequent sudden cardiac death.

Seafood and other diseases or conditions

Finfish and rheumatoid arthritis

Rheumatoid arthritis is a painful and debilitating illness. In its most severe forms, it can shorten life expectancy. However, recent studies have indicated that increased consumption of finfish may improve symptoms of rheumatoid arthritis. Comparisons between people who live in the Faroe Islands in the northern Atlantic and mainland Nordic countries show that people from the Faroe Islands have a much lower prevalence of rheumatoid arthritis. This lower prevalence is associated with higher finfish consumption and much lower meat, dairy and vegetable consumption. The inhabitants of the Faroe Islands also consume significant amounts of whale meat and fat, further increasing their intake of Omega-3 fats.

Supplementing diet with Omega-3 fats while patients have continued with their usual medications has improved symptoms. In addition, supplementing diet with EPA and DHA decreases the need for non-steroidal anti-inflammatory drugs. Omega-6 fat content in the patient’s diet, and drug therapy, affect how well the medication works. Therefore, Omega-3 fats may potentially allow medication dosages to be decreased.

Finfish and obesity

Obesity in Australia is generally a reflection of changing lifestyles: more Australians than ever before are obese. Being overweight is unhealthy, often leading to serious problems such as diabetes, coronary heart disease and cancer. Diet and physical activity are regarded as the most important factors in obesity.

Total dietary fat is a large contributor to obesity. Hence, low fat diets are popular for reducing weight. However, the type of fat consumed is just as important as the total fat consumed. The popular belief that all fats contribute to weight gain is not accurate – whether the fats are unsaturated or saturated significantly affects their contribution to weight gain. Some studies also show that increased finfish intake may protect against obesity and glucose intolerance.

Finfish is a protein source that has low saturated fat content and can contribute usefully to calorie-controlled weight loss diets, particularly when it replaces high-fat sources of protein.

Important: Capsules of Omega-3 oil with dosages between 4 and 10grams per day may increase blood sugar levels in people with diabetes, although other benefits, such as the reduction in certain risks for coronary heart disease, usually outweigh this consideration. People with diabetes should consult their medical practitioner.

Finfish is a protein source that has low saturated fat content and can contribute usefully to calorie-controlled weight loss diets, particularly when it replaces high-fat sources of protein.

Fish oil and Crohn’s disease

Crohn’s disease is characterised by inflammation, thickening and ulceration of any part of the intestine. Inflammation of the liver, kidneys, joints, eyes and skin can also occur. Patients suffer bouts of illness interspersed with periods of remission. In Japan, increasing incidence of Crohn’s disease has been associated with increased dietary intake of fat, animal fat, Omega-6 fats, animal protein and milk protein, increased proportions of Omega-6 compared to Omega-3, and decreased dietary intake of Omega-3 fats.

A study of Crohn’s disease patients who consumed fish oil (2.7 g per day of Omega-3 fatty acids) showed significantly higher remission. Using fish oils to treat Crohn’s patients requires substantially more fish oil than can be obtained from eating finfish. Consequently, fish oil capsules are usually required. The need for capsules rather than fresh finfish to treat Crohn’s disease patients is evident from findings showing that countries with high finfish consumption also have quite appreciable incidences of Crohn’s disease. Patients with Crohn’s disease should consult their physician for advice on the amount of fish oil they should consume.

Finfish and asthma

Dietary factors have been implicated in increasing rates of asthma in children and young adults over the last two decades. The effects of eating finfish on development and prevention of asthma require substantial research to be conducted before strong claims can be made. However, a recent study found that children who consumed fresh (not canned) finfish had a lower risk of developing asthma. The evidence is not clear as to which components of finfish might be important in asthma prevention. Studies into the effect of fish oil on patients who already have asthma have been inconclusive.

Seafood and cancer

Many foods and nutrients have been studied for good and bad effects on the risk of bowel cancer. These include red meat (especially charred), fruits and vegetables, dietary fibre, fat and alcohol. Although dietary fat is considered to be a risk factor for bowel cancer, indications are that EPA and DHA intakes are associated with a decreased risk of bowel cancer. It is thought that the long-chain polyunsaturated fatty acids in fish oil suppress the formation of inflammatory metabolites in the gut. Inflammation may decrease the immune surveillance. This immune surveillance reduces the growth of cancer cells. Thus seafood consumption may be protective against this disease.

Studies have also indicated that fish oil may also be associated with a decreased risk of developing laryngeal and pancreatic cancer. However, these findings require more substantiation before strong claims can be made.

Seafood and diabetes

People with diabetes have an increased risk of developing heart and other cardiovascular diseases. Consequently, diabetes and heart disease management require a combined strategy. Omega-3 seafood fats have a positive effect on heart disease risk factors such as platelet aggregation, blood pressure and plasma lipoprotein metabolism. In people with diabetes, caution needs to be exercised in supplementation with Omega-3 fish oil capsules (where 1 gram of fish oil is about 30% Omega-3 fatty acids). This is because dosages between 4 and 10grams per day (equivalent to or more than 1—3 serves of seafood per day) may result in increased levels of blood sugar (glucose). However, lower doses (2.5 grams per day) have been shown not to increase blood sugar levels, although they still provide useful effects on heart disease risk factors. Generally, eating seafood does not create a problem and is actually an advantage.

Omega-3 fat intake can reduce insulin resistance in skeletal muscle. A high dietary proportion of Omega-6 and Omega-3 fats has been implicated in increased insulin resistance; hence, an increase in dietary Omega-3 fats from seafood may address this.

Seafood and neural development

Essential fatty acids, particularly DHA (an Omega-3 seafood fat), are needed for cell membranes. They are a major factor for early development of nerve and brain cells in infants. It is evident from a number of studies that adequate supply of DHA is needed for brain growth and functional development of infants. Both integrity and function of nerves can be permanently disturbed by deficits of essential fatty acids. Breast-fed infants have a higher DHA level in the brain and red blood cells, and they are likely to have an enhanced neural development compared with formula-fed infants. One study indicates that infants need a continuous supply of DHA, since infants breastfed for a shorter period – that is, less than 16weeks – show poorer visual acuity scores than those receiving DHA, either from breast milk or fish oil.

Finfish and memory

Eating finfish may be favourable for brain function, since Omega-3 fats are important in the structural components of nerve cells, and play a role in the formation of brain chemicals transmitting information from one nerve cell to the next. A high prevalence of Alzheimer’s disease is associated with low consumption of finfish.

Finfish and depression

A multi-national comparison of depression prevalence shows that Japan – where the finfish consumption averages 67 kg per person per year – has a relatively low prevalence of 0.12%. New Zealand has an average finfish consumption of 18 kg per person per year, and a high prevalence of depression: 5.8%. It is clear that there is an association between finfish consumption and incidence of depression. In addition, the most consistent observations about fatty acids and depression are the low level of both Omega-3 and Omega-6 fats in the blood. It is likely that the availability of these fatty acids is important in modulating mood. It is further suggested that decreased Omega-3 fat intake may affect the central nervous system in early development or adulthood, so increasing vulnerability to depression.

Seafood and women’s health

Much of the research on heart disease has concentrated on men, since they have a higher risk of developing this disease. However, women also develop coronary heart disease. A recent study into the effects of various foods on heart disease risk in women found that eating foods such as meat, salami, butter and coffee was associated with increased coronary heart disease risk. Eating foods such as carrots, fresh fruit, green vegetables, finfish and moderate alcohol reduced risk. In addition, women who ate finfish more than once per week had a 40% lower chance of developing heart disease than those who consumed finfish less than once per week.

Research has also indicated that seafood consumption may have a range of benefits during pregnancy and lactation. During pregnancy, the foetus requires large amounts of Omega-3 fatty acids and obtains them from the mother. In general, DHA levels get progressively lower as the pregnancy becomes more advanced. Furthermore, maternal DHA levels are higher in women during their first pregnancy compared with levels in subsequent pregnancies. As a result, first-born children have a higher DHA status than that of following children. It appears that the mother’s DHA source is not easily replenished in time for subsequent pregnancies and if the mother breastfeeds, this replenishment is likely to take longer.

DHA is found in breast milk and the levels in breast milk are predicted by how much seafood the mother eats. In one study, breast-feeding mothers achieved an average DHA level in breast milk of 0.46% by consuming 400 milligrams per day of DHA (equivalent to 97 grams of Atlantic salmon). A level of 1.13% was achieved by consuming 1300milligrams per day.

It is important to ensure adequate intake of Omega-3 fatty acids during pregnancy and lactation, to top up maternal DHA stores. Increasing the intake of essential fatty acids during pregnancy and lactation will benefit both mother and child. The DHA status in premature infants has been positively related to head circumference, birth weight and birth length. Increasing the foetal DHA status could promote foetal growth, thereby improving the survival chances of premature infants.

It is important to ensure adequate intake of Omega-3 fatty acids during pregnancy and lactation, to top up maternal DHA stores.

Furthermore, fats from seafood have an important role in development of nerve cells in infants. It is therefore advisable for pregnant or breast-feeding women to increase their seafood consumption. A number of studies indicate that breast-fed infants have enhanced neural (brain) development compared with formula-fed infants. DHA is thought to contribute to enhanced neural development since breast milk contains this oil but, until recently, artificial formulae did not. In one study, the DHA content of red blood cell membranes in infants who were breast-fed or had their formula supplemented with fish oil remained the same as the levels at birth. However, infants who only consumed unsupplemented formula showed significant decreases in DHA levels.

Seafood and older people

Seafood, because it is high in nutrient density, has an important role to play in diets designed to maintain health and well-being as people get older. Studies from three countries with relatively low intakes of finfish indicate that finfish intake is associated with reduced risk of death from coronary heart disease. Of particular note are the benefits of even small levels of finfish intake. Middle-aged men who ate no finfish were at a much higher risk than those who ate finfish once or twice per week. One study showed that older people who regularly ate 24 grams of finfish per day (127 milligrams of Omega-3 fats, or 58 grams of snapper) had about half the chance of dying from coronary heart disease as those who ate no finfish.

Seafood and indigenous Australians

Indigenous Australians are at particular risk of developing type II diabetes, having up to six times the rates of Australians of European descent. Research showed that remarkable improvements to diabetes and coronary heart disease risk factors can result from changing the diet. In the research, ten indigenous Australians with diabetes changed from a modern Western diet (high in fat and low in unrefined carbohydrate) to a more traditional diet (low in fat, low in saturated fat and high in dietary fibre). The group lost an average of 8kg over the seven-week period, and had significant improvements in glucose tolerance and insulin sensitivity. Of the total energy in the traditional diet, 19% was derived from finfish.

Heavy metals in seafood

The health hazards associated with the consumption of seafood contaminated with heavy metals such as mercury have received world-wide publicity. Large carnivorous finfish such as tuna, swordfish and some species of shark eat smaller plant-eating finfish. Further up the food chain, these heavy metals can become more concentrated in the flesh of the finfish. Problems have only arisen when individuals eat large amounts of finfish that have fed in contaminated waters. There have been no reported cases of mercury poisoning in Australia as a result of seafood consumption. In Australia, two separate limits for mercury, as prescribed by the Australian Food Standards Code, 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, barramundi, ling, orange roughy, rays and shark), and
• 0.5 mg/kg for all other species of finfish, crustaceans and molluscs.