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Should I eat more fish?
BEYOND just eating fish, it is also important to understand what fish to take, how it should be cooked and the quantity that should be consumed.
Among the fatty acids that are present in seafood, the long-chain n-3 polyunsaturated fatty acids (P3UFAs), namely, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are the PUFAs most closely associated with heart health.
The content of these 3PUFAs vary with the type of seafood, the highest content being seen in coldwater fatty fish such as salmon, anchovies, herring, mackerel, tuna and sardines.
One fatty fish serving of about 100 gm per week provides the recommended daily intake of 3PUFAs. Other types of seafood such as shrimp, lobster, scallops, tilapia, and cod have lower levels of 3PUFA.
Currently, supermarkets stock a wide variety of foods enriched with 3PUFAs including eggs, peanut butter, orange juice, margarine, bread, yogurt, and milk. However, seafood remains the only class of food which has been researched extensively.
Presently, the 2015-2020 Dietary Guidelines for Americans and the Scientific Report of the 2015 US Dietary Guidelines Advisory Committee recommend at least two servings per week to provide a daily average of 250 mg of 3PUFAs per day in place of other animal sources of protein.
Effect of 3PUFAS in studies
Ingested seafood-derived 3PUFAs eventually become part of the cell membranes and have been shown to have favourable physiological effects on the electrical activity of the heart cell membrane which make the cell "less irritable" and more electrically stable.
Hence, adequate consumption of seafood has also been associated with positive clinical findings such as lower heart rate, slower conduction of electrical impulses between the upper and lower heart chambers, lower likelihood of abnormal recovery of the electrical activity of the heart muscle cell and less heart rate variability.
The combined effects contribute to more stable heart muscle cell electrical activity and have been associated with a lower risk of developing life-threatening heart rhythms and sudden heart-related deaths.
Studies have shown that 3PUFA intake is associated with improvement in the elasticity of arterial walls and lower arterial stiffness. Seafood-derived 3PUFAs have been shown to reduce serum triglycerides (TG) concentrations in those with high serum triglycerides, most likely by increased liver removal of TG and reduced liver production of TG rich protein particles VLDL (or very low-density lipoprotein) .
Preventing sudden heart death
Many studies including the Physicians' Health Study have shown that while diets high in seafood 3PUFA have not been associated with prevention of heart attacks, there is a strong inverse association with sudden cardiac death. A possible explanation of this is the association of 3PUFA intake with reduction in life-threatening heart rhythms which can occur after a heart attack.
Higher seafood intakes have been associated with greater electrical stability of the heart muscle cell, decreased risk of fatal and non-fatal ventricular arrhythmias (abnormal heart rhythm originating from the lower heart chambers), lower heart rate, and improved heart rate variability, each of which is a risk factor for sudden heart death.
It does not mean that the higher the intake of seafood 3PUFA, the lower the risk of sudden cardiac death.
While there is an incremental risk reduction benefit seen in those with increasing intake of seafood, this risk reduction plateaus. Hence, while consuming up to 2 fatty fish meals per week is associated with a 50 per cent lower risk of sudden heart death compared with little or no seafood intake, no further reduction was seen with higher intake.
Beyond the quantity of seafood consumed, the method of cooking also affects the benefits. While seafood 3 PUFA intake was associated with reduction in sudden cardiac death, this benefit was not seen when the fish was fried. While the studies showing a reduction of sudden heart death with seafood 3PUFA intake were mainly studies conducted in the United States, other studies done in Japanese populations have failed to demonstrate this benefit. One explanation could be that the baseline dietary seafood intake is much higher and 95 per cent of adults eat seafood more than once weekly.
Coronary heart disease
Many large studies reported that increased seafood and dietary 3PUFAs intake was associated with a lower risk of heart artery disease in a healthy cohort.
In studies in which all participants in the study reported on their seafood intake, participants who consumed seafood at least four times a week had a 22 per cent lower risk of heart artery disease, compared with those who consumed seafood less than once a month. In addition, the risk reduction could potentially be greater if the seafood intake replaced processed meat.
This is illustrated in two large US cohorts, where substitution of 3 per cent of total protein calories in processed meat with 3 per cent of total protein calories from seafood was associated with 31 per cent lower risk of cardiovascular death.
Results from the various prospective studies which examined the relationship between seafood 3PUFA intake and stroke have shown that intake of seafood was associated with a lower risk of thrombotic (due to occlusion of artery) stroke but has no association with haemorrhagic (due to bleeding ) stroke.
One study which examined the way the fish was cooked, Cardiovascular Health Study, found that while intake of broiled and baked fish was associated with a 40 per cent lower risk of ischaemic stroke, intake of fried fish or fish sandwiches was associated with a higher risk of ischemic stroke.
Recent analysis of combined studies have suggested that consumption of one serving of seafood a week was associated with a 14 per cent lower risk of ischaemic stroke compared with no or infrequent consumption of seafood.
Large fish such as shark, swordfish, golden bass, king mackerel, tuna, marlin, and orange roughy are a significant source of methylmercury.
While current evidence does not show any adverse impact on heart disease, it is possible that high mercury levels can negate the effect of seafood 3PUFAs intake. In a Finnish study of 1,857 men, a 0.5 per cent increase in blood 3PUFAs was associated with a 23 per cent lower risk of sudden heart death among men with low hair mercury but this did not translate to any benefit for men with high hair mercury.
The current evidence suggests that the benefits of one to two servings a week outweigh the risks of methylmercury, especially if a variety of seafood is consumed. However, caution should be exercised against consuming multiple servings of seafood which may potentially contain mercury, as higher fish intake does not translate to further risk reduction as the benefits plateau and have a threshold beyond which there is no incremental benefit.
Current evidence supports the following conclusions :
* Consumption of non-fried seafood with high 3PUFA, one to two times per week is associated with a reduction in the risk of sudden heart death, heart artery disease and ischaemic stroke;
* The beneficial effects of seafood 3PUFA peaks at about 250mg of seafood intake per week and higher intakes are generally not beneficial or harmful.
* There may be further beneficial risk reduction if the seafood is substituted for unhealthy meats such as processed meats.
This series is produced on alternate Saturdays in collaboration with Singapore Heart, Stroke & Cancer Centre.