Interest in the health benefits of oily fish started when researchers observed that Eskimos, who mainly eat oily fish, had fewer than average heart attacks and strokes.
Oily fish such as salmon, mackerel and sardines are said to help against cardiovascular disease, prostate cancer, age-related vision loss and dementia.
It's a good source of vitamin D, protein, some B vitamins and selenium. It's also a rich source of omega-3 fatty acids, a type of fat that is good for our health.
We've teamed up with the British Dietetic Association (BDA) to examine what the evidence says about the supposed health benefits of oily fish.
The evidence on oily fish
The UK Scientific Advisory Committee on Nutrition reviewed the evidence on the health benefits of fish in 2004. It said a "large body of evidence" suggests that fish consumption, particularly oily fish, reduces the risk of cardiovascular disease.
Studies have found eating oily fish can lower blood pressure and reduce fat build-up in the arteries. The evidence is strong enough to warrant a government recommendation that we eat at least two portions of fish a week, one of which should be oily.
However, there are maximum recommended amounts for oily fish, crab and some types of white fish. There is additional advice for women who are pregnant or breastfeeding, and children and babies.
The evidence for oily fish's effect on prostate cancer is inconclusive. Some limited research suggests that eating fish may reduce the risk of developing prostate cancer. However, this is not backed up by other studies, so we can't be sure of the effect.
A 2012 review looked into whether consuming more omega-3, a type of healthy fat found in oily fish, could reduce the risk of dementia. The review looked at studies of healthy 60-year-olds who took omega-3 capsule supplements for six months.
The review concluded that there is no preventative effect of decline in brain function and dementia when healthy older people take omega-3. The review suggested that longer-term studies would offer researchers a better opportunity for identifying the possible benefits of omega-3 in preventing dementia.
A well-conducted review in 2010 found there was some evidence that eating oily fish two or more times a week could reduce the risk of age-related macular degeneration – a common cause of blindness in older people. However, the reviewers said the results should be interpreted cautiously, because of weaknesses in the research.
A further review carried out in 2015 looked at whether fish oil supplements could reduce the progression of macular degeneration in people who already had the condition. The results were disappointing, as there was no evidence of any benefit.
A 2013 study looked at the eating habits of around 32,000 middle-aged and older women to see if oily fish consumption had any influence on them developing rheumatoid arthritis. They did find that women who ate one or more servings of oily fish were 29% less likely to develop rheumatoid arthritis than women who never, or very rarely, ate oily fish.
However, this type of study can never prove cause and effect, as other factors could have been involved.
In 2013, the National Institute for Health & Care Excellence (NICE) reviewed the evidence about whether medication based on omega-3 fatty acids could improve the symptoms of schizophrenia. The results were mixed. Four out of the eight studies showed some modest benefit when compared to placebo (a dummy treatment). The other four showed no benefit.
Based on these results, it is not recommended to use omega-3 fatty acid-type drugs as an alternative to existing treatments.
The dietitian's verdict on oily fish
Alison Hornby, a dietitian and BDA spokesperson, says if there's one food that's good for your heart, it's oily fish.
She says: "The benefits of eating at least two portions of fish a week, including one of oily fish, include keeping your blood pressure at a healthy level and improving blood lipids, both of which reduce your risk of cardiovascular disease: the biggest killer in the UK.
"Remember that you can get your omega-3 from a range of oily fish. Tinned sardines and mackerel, for example, are an easy and cheap way to stock up the store cupboard. Eaten on toast with a side salad, this makes a quick, easy and nutritious meal."