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According to two meta-analyses fish oils have a significant but modest effect dose-dependent, on blood pressure in hypertension (Geleijnse et al 2002, Morris et al 1993). The DHA component is likely to have stronger effects than EPA. In contrast, a 2006 Cochrane review found no significant changes to SBP or DBP with n-3 EFA consumption of vegetable or fish origin (Hooper et al 2006).

The first meta-analysis was of 31 placebo-controlled trials involving 1356 subjects and detected a statistically significant dose-response effect on blood pressure when studies were grouped by omega-3 fatty acid dose: -1.3/-0.7 mmHg at doses <3 g/day, -2.9/-1.6 mmHg at 3.3-7 g/day, and -8.1/-5.8 mmHg at 15 g/day (Morris et al 1993). The hypotensive effect was strongest in hypertensive subjects and those with clinically evident atherosclerotic disease or hypercholesterolemia, whereas no effect was detected in healthy subjects.

The 2002 meta-regression analysis considered the results from 36 trials, of which 22 had a double-blind design, to determine whether fish oil had a significant effect on blood pressure (Geleijnse et al 2002). Fish oil intake (median dose: 3.7 g/day) was found to reduce SBP by 2.1 mmHg and DBP by 1.6 mmHg when all trials were considered. When restricted to double-blind studies only, effects were not as large, but still apparent. Overall, the effects of fish oil on blood pressure tended to be greater in older people (>45 years) and in hypertensive populations (BP >140/90 mmHg).

In contrast, the 2006 Cochrane review discussed at length in the previous section failed to detect a significant hypotensive effect for n-3 fatty acids of vegetable or fish origin (Hooper et al 2006).

Would you like methyl mercury with that?

There has been increasing public awareness and concern regarding mercury exposure due to the consumption of fish in the diet. This has been partly in response to the health warnings issued by Food Standards Australia and New Zealand (FSANZ) in March 2004 regarding maximal intake of select species of fish during pregnancy and childhood (Bambrick & Kjellstrom 2004). Interestingly, while the main public concern relates to neurodevelopmental toxicity, emerging data shows a relationship between increasing methylmercury (MeHg) exposure and cardiovascular disease, in particular Ml (Stern 2005). Mechanisms postulated for this effect include the known oxidative stress and reactive oxygen species observed with in vitro exposures to MeHg, as well as impaired calcium homeostasis and kidney function.

MeHg concentrations in fish and shellfish species, which represent 80-90% of the mercury present, range from <0.1 ppm for shellfish, such as oysters and © 2007 Elsevier Australia

mussels, to multiple parts per million in large predatory fish such as tuna, marlin, swordfish and shark. Consequently, MeHg intake depends on the species of fish consumed, as well as the quantity of fish eaten. Previous American data determined that adults consumed an average of 18^g MeHg/day, with 80-90% coming from fish and shellfish (Mahaffey et al 2004).

In contrast to the beliefs of many, it is the organic form of mercury, which is a natural geological product and not an industry byproduct, that is toxic to human beings. Inorganic mercury is readily excreted in the urine whereas MeHg accumulates in erythrocytes across a wide range of exposures (Mahaffey et al 2004). Multiple international studies have assessed levels of MeHg exposure to reveal that approximately 10% of the samples have high blood levels. American studies have identified populations who appear to be at greater risk, among them a subpopulation consuming a substantial amount of fish in pursuit of health benefits. Blood MeHg analysis revealed blood mercury levels up to 90 fjg/L (Hightower & Moore 2003). This is of concern because levels >5 /Jg/L have been reported as potentially detrimental in women of childbearing age.

Some researchers propose that the potentially cardiotoxic effects of MeHg is countered by the presence of the omega-3 oils also found within fish, and interestingly there is some overlap between those species with the highest concentrations of both (Bambrick & Kjellstrom 2004). However, there is also concern that the converse is true and MeHg could counteract the health-giving benefits of fish.

While there remains little doubt that the discriminating inclusion of fish is an important and healthy part of the diet, more research is required to better weigh up the risks and benefits of dietary fish. In contrast to the concerns of some consumers, fish oil supplements are not a major source of mercury and as such there is no need to restrict their intake (FSANZ 2004)._

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