Fish oils come from fatty fish, also known as oily fish, specifically the tissue of fatty fish, such as trout, mackerel, tuna, herring, sardines, and salmon.

Fish oils are of interest to nutritionists and health care professionals because of two main ingredients: DHA (docosahexaenoic acid), and EPA (eicosapentaenoic acid) - both types of Omega-3 fatty acids.

The fillets of oily fish contain up to 30% oil; this figure may vary. White fish, on the other hand, only contain high concentrations of oil in the liver, and have much less oil. Apart from omega-3 fatty acids, oily fish are also good sources of vitamins A and D. Whitefish also contain these nutrients, but at much lower concentrations.

Health experts commonly tell people that oily fish have more health benefits than white fish. However, their recommendations have never been compellingly proven scientifically in large population studies.

Many health authorities around the world advise people to consume either plenty of oily fish or to take supplements, because of their supposed health benefits. Studies over the last ten years have produced mixed results regarding the benefits of the dietary intake of fish oils.

Some people confuse fish oils and cod liver oil - they are different. Fish oils are extracted from the tissue of deep sea oily fish such as tuna, mackerel, herring and salmon. Cod liver oil, by contrast, is extracted solely from the livers of cod. Fish oils contain higher amounts of omega-3 fatty acids than cod liver oil, but lower amounts of vitamins A and D.

Possible health benefits of fish oils

Over the last ten years, there have been dozens of studies on fish oils and omega-3 oils. Some have backed up these claims, while others have not.

Fish oils are said to have a number of health benefits if they are included in a human diet, including:

1) Multiple sclerosis (MS)

Fish oils are said to help people with MS. However, a study carried out by researchers from University Hospital in Bergen, Norway, in April 2012 found that omega-3 fatty acids do not help people with MS.1

2) Prostate cancer

Fish oils may reduce a man's risk of developing prostate cancer if they follow a low-fat diet, one study found, while another linked omega-3 levels to a higher risk of aggressive prostate cancer2.

A study published in the Journal of the National Cancer Institute found that omega fish oils raise prostate cancer risk. The authors, from the Fred Hutchinson Cancer Research Center reported that high fish oil intake raises the risk of high-grade prostate cancer by 71% and all prostate cancers by 43%.3

3) Post-natal (post-partum) depression

Fish oils consumed during pregnancy may help protect mothers from post-partum depression - Dr. Michelle Price Judge, of the University of Connecticut School of Nursing, said after carrying out a study in 2011 "DHA consumption during pregnancy at levels that are reasonably attained from foods has the potential to decrease symptoms of postpartum depression."4

4) Mental health benefits

A pilot study carried out in 2007 suggested that fish oils may help young people with behavioral problems, especially those with ADHD. The eight-week study demonstrated that children who consumed between 8 and 16 grams per day of EPA and DHA (the long chain omega-3 fatty acids found in fish oil) showed significant improvements in their behavior (rated by both their parents and the psychiatrist working with them).5

5) Memory benefits

Omega-3 fatty acid intake can help improve working memory in healthy young adults, researchers reported in the journal PLOS One (October 2012 issue).6

The benefits of fish oils in aiding cognitive function in older populations may be less beneficial, however. A study by researchers at the University of Iowa suggested that high levels of omega-3 are of no benefit to cognitive decline in older women.10

6) Heart benefits

Omega-3 fatty acids found in fish oils may protect the heart from mental stress. A study study published in the American Journal of Physiology - Regulatory, Integrative, and Comparative Physiology revealed that people who took fish oil supplements for over a month experienced less mental stress in measurements of cardiovascular health than those who did not.11

We look closely at all of the studies related to fish oils and the heart on the next page.

7) Protection from Alzheimer's disease

Claims were made for many years that regular fish oil consumption would help prevent people from developing Alzheimer's disease. However, a major study in 2010 found that fish oils and a placebo were no different in Alzheimer's prevention.

In contrast, a study published in Neurology in 2007 reported that a diet in fish, omega-3 oils, fruit and veggies reduces dementia and Alzheimer's risk.7

8) Protection from vision loss

Adequate dietary consumption of DHA protects people from age-related vision loss, Canadian researchers reported in the journal Investigative Ophthalmology & Visual Science.8

9) Epilepsy

A 2014 study published in the Journal of Neurology, Neurosurgery & Psychiatry claims epilepsy patients could reduce seizure frequency by consuming low doses of omega-3 fish oil every day.

The research team at the University of California-Los Angeles (UCLA) School of Medicine, says their findings may be particularly useful to epilepsy patients who no longer respond to medication

10) Schizophrenia and psychotic disorders

In what was believed to be the first study of its kind, research has revealed the omega-3 fatty acids found in fish oil may be effective for reducing the risk of psychosis.

The study, published in Nature Communications, details how a 12-week intervention with omega-3 supplements substantially reduced the long-term risk of developing psychotic disorders.

11) Benefits for the fetus

Omega-3 consumption may help boost fetal cognitive and motor development. In a study published in 2008, scientists from L'Université Laval Laval found that omega-3 consumption by the mother during her last three months of pregnancy improved her baby's sensory, cognitive and motor development.9

On the next page we look at what omega-3 fatty acids are, whether fish oil supplements offer heart benefits, we examine whether low Japanese heart disease rates are linked to high fish oil consumption and we list which foods are rich in omega-3 fatty acids.