Food and diet

Can fish oil prevent heart attack deaths?

‘Why fish oils hold all-powerful key to healthy heart’ is the headline in the Daily Express, which goes on to report, somewhat over-enthusiastically, that ‘tens of thousands of lives a year could be saved if people ate more fish’.

These wildly optimistic claims are actually based on an small experimental study involving 59 people, looking whether our genetic makeup affects how dietary fats can influence the way in which our blood vessels constrict (narrow) and dilate (widen). The question of whether eating fish gives us a healthy heart, or saves lives, was not considered by the researchers.

On two separate occasions the participants were given either a drink high in saturated fats, or a drink with some saturated fats combined with fish oils.

The researchers then used ultrasound to look at how their subjects’ blood vessels dilatated again after being briefly blocked by a blood pressure cuff.

In general, the researchers found that the response of the blood vessels varied:

  • according to the drink given
  • between men and women
  • between people with the two different gene types known to affect blood vessel dilation

There was greater blood vessel dilatation after the drink that contained the fish oils was consumed, particularly in women with a gene type known as Asp298, which is thought to apply to around 10% of the population.

Very limited conclusions can be drawn from this study due to its size.

A healthy balanced diet and regular exercise are known to be key to good health. Whether fish oils have any particular effect on heart health cannot be answered by this study.

Where did the story come from?

Researchers from the University of Reading conducted this research which was published in the Journal of Lipid Research. Funding was provided by the Biotechnology and Biological Sciences Research Council, Unilever PLC, and FRST – Foundation for Research, Science and Technology (New Zealand). The saturated fat (palm stearin) used in the study was donated by Aarhuskarlshman, UK and the fish oil by Croda Healthcare, UK.

The media has greatly overestimated the implications of this small experimental study which did not specifically aim to assess whether fish oils affect heart or vascular health (the health of blood vessels). Instead, it aimed to look at whether a person’s genetic makeup affects how their blood vessels respond to dietary fats.

It appears that the media’s reporting of the story has been influenced by a number of quotes from one of the lead researchers, Professor Christine Williams, who made the case that fish oil could widen the blood vessels that supply blood to the heart. This in turn, could help prevent heart attacks (heart attacks are triggered when the muscles that make up the heart are starved of blood).

However, this is due to a process of atherosclerosis, where the blood vessels become clogged due to a build up cholesterol and other fatty deposits – it is not directly linked to the temporary constriction and dilation of blood vessels as this study observed.

It is a gross extrapolation of the data presented in the study to claim that ‘tens of thousands of lives a year could be saved if people ate more fish’.

What kind of research was this?

The researchers describe that reduced blood vessel reactivity – how they constrict and dilate – is an early modifiable step in the development of atherosclerosis – the thickening of the arteries due to build-up of fatty deposits. They say that there is increasing evidence that dietary factors can have an effect on blood vessel reactivity, and that dietary fat in particular may be an important modulator. Long-chain polyunsaturated fatty acids (PUFA) are particularly supposed to have beneficial effects. These fatty acids include omega-3 fatty acids, which are found in fish oils and some plant sources.

The fatty acids are believed to have a possible effect on the chemical nitrous oxide, which causes blood vessels to dilate. Nitrous oxide is produced by the cells lining the blood vessels using an enzyme called endothelial nitric oxide synthase (eNOS).

This experimental study aimed to see whether variations in the eNOS gene have an effect on how blood vessels respond to fat in the diet. It did this by looking at what happened when people with different forms of the eNOS gene were given either saturated fats or PUFA. It did not aim to look at the long term effects of fatty acids on blood vessels or the heart.

What did the research involve?

The researchers recruited:

  • 29 people who had two copies of the Asp298 variant (form) of the eNOS gene (each person carried two copies of each gene – one from each parent)
  • 30 people who had two copies of the Glu298 variant of the eNOS gene (the more common type)

All the adults were healthy non-smokers, aged 18-65 with body mass index (BMI) from 18-32, and had no cardiovascular diseases or metabolic diseases and weren’t taking medications that could affect blood clotting or blood pressure. All of their blood fats were in the normal range.

Participants attended the study centre on two separate occasions. On one occasion they received a test drink high in saturated fats (0.52g/kg body weight), and on the other they received a drink with the same total amount of fat, but made up of a combination of saturated fats (0.45g/kg body weight) and PUFA (0.07g/kg body weight).

As an example the researchers say:
“A 70kg individual would therefore receive 36.4g palm stearin; or 31.5g palm stearin and 4.9g of fish oil concentrate, which contained 3.8g docosahexaenoic acid (DHA) and 0.4g eicosapentaenoic acid (EPA) (equivalent to 1.5 times a standard 140g portion of oily fish).”

The drinks were consumed over 240 minutes and were identical in protein and carbohydrate content.

Before and after taking the drinks, the participants had blood tests taken, and various measures of vascular reactivity were measured using ultrasound. This included measuring flow-mediated dilatation, where a blood pressure cuff was inflated to temporarily occlude (block) the blood vessels.  The cuff was then rapidly released to see how fast the blood vessel responds by returning to its original shape.

What were the basic results?

Before the start of the experiment, the researchers found that the people with the two different forms of the eNOS gene in the study were fairly similar. An exception to this was women with two Asp298 gene variants (genetic variants are often referred to as alleles). In these women it was found that they had both higher blood levels of fatty acids and higher flow-mediated dilatation (their blood vessels responded more quickly in returning to normal and allowing blood to flow after the cuff was removed).

The researchers found that there were differences in flow-mediated dilatation in response to the two fat drinks, with responses differing according to gender, and according to gene type. Following the saturated fat drinks there was a decrease in flow-mediated dilatation, and this was similar in both men and women. Following the drink of saturated fat combined with PUFA, there was an increase in flow-mediated dilatation, with women having greater increases than men.

Generally, the response was fairly similar for the people with the two different eNOS gene types – those with two Asp298 alleles and those with two Glu298 alleles. However, those with the less common Asp298 alleles showed greater differences in their flow-mediated dilatation when given the two different drinks.

How did the researchers interpret the results?

The researchers conclude that dietary fatty acids have an effect on the dilation of blood vessels, and that the effects of eating different compositions of fats appears to be dependent on both the type of eNOS gene, and gender. The greatest difference in vascular response to two fat loads was in women with two Asp298 variants of the eNOS gene.


The media has taken the implications of this experimental study a little too far. The study found differences in the dilatation of the blood vessels depending on type of fatty acids consumed, the person’s gender, and what form of the eNOS gene a person had. However, this was a very small study involving only 29 people with one form of the gene and 30 with another.

The study cannot tell us whether the changes seen would persist in the long term if a person followed a diet high in fish oils. Most importantly, it also doesn’t tell us whether the small changes in blood vessel flow would have any effect at all on the cardiovascular health of the person.

A healthy balanced diet and regular exercise are known to be key to good health.

Whether fish oils have any particular effect on heart health cannot be answered by this study.

So despite any claims, taking fish oil supplements alone, without improving your diet or increasing your levels of exercise, is not going to give you a short-cut to a healthy heart.

NHS Attribution