A public health perspective...
The consumption of fish has long been associated with several health benefits. Fish is an important source of many nutrients, including protein and long-chain omega-3 polyunsaturated fatty acids (n-3 PUFAs), as well as several vitamins and minerals including selenium, iodine, potassium, vitamin D and B vitamins. Intakes of some of these micronutrients, including iodine and vitamin D, are low in some subgroups of the UK population and increasing fish consumption could help to improve intakes within these groups. It has been suggested that the long-chain n-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), present in oily fish and fish oil, are responsible for some of the beneficial health outcomes associated with fish consumption. The BNF carried out an in-depth review of the scientific evidence investigating associations between fish consumption and various health outcomes. They also reviewed the effects of long-chain n-3 PUFAs on cardiovascular disease and other health parameters to determine whether any positive health effects of fish are in fact due to the presence of these fatty acids.
Overweight & Obesity
Dietary patterns that typically include good amounts of fish, such as the Mediterranean diet, are generally associated with lower risk of overweight and obesity. There is some evidence from randomised controlled trials (RCTs) that long-chain n-PUFAs may positively affect body composition. In two studies long-chain n-3 PUFAs supplement use was associated with reduced whole-body fat mass, and in another study supplementation with long-chain n-3 PUFAs led to greater weight loss and a fall in waist circumference compared to a control group in men but not in women. It has been suggested that a shift in fuel metabolism, away from storage and towards oxidation of fat, may be responsible for these findings. However, not all studies have demonstrated an effect of long-chain n-3 PUFAs on body composition, and more studies are needed to draw firm conclusions.
Nutrients present in fish, including protein, calcium and vitamin D, play a critical role in bone health and development. Oil-rich fish is an important source of vitamin D, and fish that are consumed with bones are a good source for dietary calcium. A role for long-chain n-3 PUFAs in bone metabolism has also been suggested, but evidence is too limited to draw any conclusions. There is some evidence from cohort and cross-sectional studies that fish intake may be associated with better bone health, but not all studies have found such an association.
Evidence from population studies suggests that there may be some protective effect of fish on risk of colorectal cancer, but the evidence is too inconsistent to draw any firm conclusions. Potentially, any protective effects observed may be due to other factors, such as lower meat intakes by fish eaters (red meat, particularly processed meat, has been associated with a higher risk of colorectal cancer). There is no conclusive evidence to support an association between fish intake and other types of cancer, including breast, pancreatic, prostate, bladder, stomach and ovary cancer.
Asthma, Eczema and Other Atopic Conditions
There is some evidence that eating fish during pregnancy may be associated with a lower risk of atopic conditions in the offspring, particularly for eczema. However, the findings remain inconsistent and there is not clear evidence on fish type or potential mechanisms. There is also some evidence that fish intake during early childhood is associated with a decreased risk of atopic outcomes, but again the findings have again not been consistent. Supplementation with long-chain n-3 PUFA is not effective in reducing the risk for asthma in adults or children. The evidence on fish and atopic conditions is promising and further research in this area is warranted.
Population studies have generally shown a protective effect of fish intake on risk of cardiovascular disease (CVD), including stroke. However, the strongest association has been found with coronary heart disease (CHD) mortality, whereas no significant effect has been found with risk of non-fatal CHD. The most recent meta-analysis of studies of fish intake and CVD found that the risk of fatal CHD was 13% lower in those from the highest versus the lowest intake category. However, total fish intakes in the highest and lowest fish categories differed between studies, depending on the habitual fish intakes of the study populations (e.g. intakes are typically high in countries such as Japan, whereas in many European countries they are much lower). These differences in habitual fish intake of study populations may account for some of the differences in findings between the single studies and makes it difficult to quantify the amount of fish required for a beneficial effect on health.
Observational studies suggest that fish consumption is associated with a moderate, but statistically significant, decrease in risk of cerebrovascular disease, including stroke. The most recent systematic review found that those eating 5 or more portions of fish per week had a 12% lower risk, and those eating 2-4 portions had a 6% lower risk of suffering a cerebrovascular event compared to those eating fish less than once a week. It was suggested that the observed effect could not be explained solely by long-chain n-3 PUFAs, but that other nutrients or dietary and lifestyle factors associated with fish intake may contribute to this effect. Most evidence suggests that the association is stronger for ischaemic than haemorrhagic stroke, but there is some conflicting evidence.
Observational studies are useful to reveal associations between certain dietary patterns and health outcomes but randomised controlled tials (RCTs) are needed to demonstrate a causal association. Evidence from RCTs on the effect of long-chain n-3 PUFA on CVD risk has come mainly from secondary prevention studies and is not consistent, and there is some uncertainty as to whether supplementation with long-chain n-3 PUFA has an overall protective effect on CVD. In particular, recent studies do not support the hypothesis that long-chain n-3 PUFA supplementation is beneficial in secondary prevention of CVD, although older studies do support the hypothesis. Different study outcomes are likely to be due to variations in study designs and the characteristics of study populations (e.g. stage of heart disease at baseline), and more detailed analysis is required to identify under what conditions long-chain n-3 PUFAs may be beneficial. In particular, more modern treatment of CVD is also available now, which may be having an impact on study outcomes. For example, a beneficial effect of long-chain n-3 PUFA may be ‘masked’ in patients taking statins (as the effect of long-chain n-3 PUFA is more subtle than that of statins).
The uncertainty of the effect of long-chain n-3 PUFA on CVD risk factors in primary (and secondary) prevention, coupled with evidence that fish intake is associated with health benefits in the general population (in primary prevention), suggests that there may be factors other than the fatty acid profile of fish that contribute to the associated health effects. For example, it may be that other components of fish play a role in a preventative effect, or simply that people who eat more fish have this in place of foods with a less beneficial effect on cardiovascular health (e.g. foods that are high in saturated fatty acids).
Cognitive Development and Function
Oil-rich fish is a source of DHA, which is necessary for brain development. Although DHA can be formed in the body to a limited degree, it has been suggested that amounts of pre-formed DHA in the diet during pregnancy and lactation are important. On the other hand, some fish contains contaminants (e.g. mercury) that may be harmful for fetal brain development. However, experts believe that, for most fish species, the benefits of DHA for cognitive development clearly outweigh any potential harmful effects of contaminants present. Some observational studies have shown that there may be benefit of fish consumption for cognitive development, but the evidence is not consistent. Supplementation with long-chain n-3 PUFA during pregnancy or breastfeeding or via formula feeding has not improved cognitive development. However, it is worth noting that effects of diet on cognitive development are difficult to establish as cognitive development is influenced by a large array of factors.
There is some evidence to suggest that higher fish intake, dietary DHA intake and DHA blood levels may be associated with a lower risk of dementia and Alzheimer's disease, and a slower rate of cognitive decline, but not all studies have found an association. Data from intervention studies do not suggest that long-chain n-3 PUFA supplementation is effective in preventing cognitive decline.
Some sufferers of inflammatory diseases such as rheumatoid arthritis report symptom relief with supplements of long-chain n-3 PUFAs. However, evidence on fish intake and risk of developing rheumatoid arthritis is too limited to draw conclusions. A small number of studies have investigated a potential protective effect of fish and long chain n-3 PUFAs on risk of chronic obstructive pulmonary disease but these are too limited to draw any conclusions.
Fish and Sustainability
Based on current recommendations, most people in the UK should eat more fish, but increasing intakes on a population level puts more strain on fish stocks. The concern that current fishing practices are unsustainable has led to the development of considerable work to ensure that fish stocks are not depleted and there have been some promising results. Action is being taken at government and EU level, and also by the fishing industry, as healthy fish stocks are crucial for survival of fishing businesses. Aquaculture research is currently underway to study effects of farmed fish fed on different diets on human health.
The official recommendation from the UK Department of Health is to eat at least 2 portions of fish (140g each) per week. The current UK recommendation is thought to relate specifically to fin fish but other species of fish; Plaice, Lemon Sole, Halibut, Monkfish and shellfish such as prawns, crab, mussels and squid also supply modest amounts of n-3 fatty acids as well as other nutrients. Benefits at higher levels of consumption are likely, but the evidence was insufficient to draw firm conclusions at the time of the review by the SAC on Nutrition (almost 10 years ago). A larger number of studies are now available and more accurate estimations of a benefit at higher intake levels may be possible.
Evidence suggests that including fish in the diet is associated with several health benefits. To what extent long-chain n-3 PUFAs contribute to this protective effect of fish consumption has yet to be established, with evidence about their role in protection against CVD being somewhat less consistent now compared with several years ago. Other components of fish, such as selenium, potassium, vitamin D or B vitamins, contribute to the positive health effects.
The UK population should be encouraged to increase average fish consumption. However such advice needs to consider the sustainability of fish supplies to ensure a sufficient supply for future generations.