Oily fish protects against prostate cancer deaths

Behind the Headlines

Friday July 20 2012

Swedish men who ate lots of oily fish were less likely to die of prostate cancer

“Eating oily fish regularly ‘can significantly cut risk of prostate death’” reports the Daily Mail.

These findings come from a study of 525 men with prostate cancer in Sweden. Their diets in the year before their diagnosis were assessed, and they were followed for 20 years to identify which men died from their prostate cancer. The researchers found that men with the highest consumption of omega-3 fatty acids from fish had a lower risk of having died from their prostate cancer.

While men whose prostate cancer had not spread at the time of diagnosis and who had a higher rate of consumption of certain saturated fats (so called ‘bad fats’ that can raise cholesterol) were more likely to have died from their prostate cancer.

So can stocking up on salmon and sardines really reduce your risk of dying from a disease that, sadly, kills around 11,000 men in the UK every year? Well, while eating more oily fish cannot hurt (eating oily fish brings other important health benefits such as protecting against second heart attacks in people who’ve already had one), it is premature to claim a definitive link between fish consumption and surviving prostate cancer.

The main limitation of this study is that many statistical tests were carried out, and when the researchers took this into account many of their findings did not remain statistically significant. This means that some of the associations seen could have arisen by chance.

These findings need to be confirmed by other studies.

 

Where did the story come from?

The study was carried out by researchers from the Harvard School of Public Health and other research centres in the US, Iceland and Sweden. The study was published in the peer-reviewed American Journal of Epidemiology.

 

What kind of research was this?

This was a cohort study looking at the relationship between dietary intake of fatty acids and survival in men with prostate cancer. This study design is appropriate for looking at this question. 

The researchers say that although many studies have looked at fatty acid consumption and prostate cancer risk, few studies have looked at the effect on prostate cancer progression.

 

What did the research involve?

The researchers looked at 525 men (average age 70.7 years) in Sweden who had been diagnosed with prostate cancer and had enrolled in another study between 1989 and 1994. The men usually completed the assessment of their diets factors within three months of their diagnosis. The men also completed face to face interviews, or mailed questionnaires, assessing non-dietary factors.

The men completed questionnaires on their food consumption in the year before their diagnosis. A sample of 87 men also completed a one week dietary record four times over a year period to test whether their answers tallied with their responses in the food questionnaire. The relationship between the two methods was moderate for energy intake, saturated and unsaturated fat intake, and weaker for total fat intake. The relationship for specific fatty acids was not tested.

Deaths among the men up to March 2011 were identified using the Swedish Cause of Death Registry, and cause of death was verified by a panel of urologists who reviewed their medical records.

Based on the responses to the food questionnaire, intake of individual fatty acids was calculated. The researchers then looked at whether the level of fatty acid intake was related to the men’s risk of dying from their prostate cancer. The researchers split fatty acid intake into four groups and compared those with the lowest quarter of intakes against those with the increasing highest quarter of intakes, and also those with the two quarters of intakes falling in between the highest and lowest intakes.

The analyses took into account:

  • age at diagnosis
  • body mass index
  • smoking
  • family history of prostate cancer
  • year of diagnosis
  • alcohol intake

Treatment received did not influence the relationship between fatty acid intake and death from prostate cancer, so was not taken into account in the analysis.

 

What were the basic results?

By March 2011, 222 of the 525 men with prostate cancer (42.3%) had died from their cancer and 268 (51.0%) had died from other causes.

Overall, men who had the highest intake of omega-3 fatty acids from oily fish were 41% less likely to die from prostate cancer than those with the lowest intake (hazard ratio [HR] 0.59, 95% confidence interval [CI] 0.40 to 0.87). This relationship remained significant if the analysis adjusted for vitamin D intake.

The relationship between total fat intake and death from prostate cancer in all men with prostate cancer or in men with advanced prostate cancer was not significant. There was a trend for increased risk of death from prostate cancer with increasing total fat intake among men whose prostate cancer had not yet spread at the time of diagnosis (localised cancer). However, when comparing death from prostate cancer in men with localised cancer with the highest total fat intake against those with the lowest fat intake the relationship did not reach statistical significance.

There was no relationship between overall saturated or unsaturated fat intake and risk of death from prostate cancer. However, higher intakes of certain saturated fatty acids (myristic acid and shorter chain fatty acids) were associated with an increased risk of death from prostate cancer in men with localised cancer.

 

How did the researchers interpret the results?

The researchers concluded that high intake of total fat and certain saturated fatty acids “may worsen prostate cancer survival, particularly among men with localised disease”. In contrast, they say that high intake of omega-3 fatty acids from fish may reduce risk of death from prostate cancer in men with the disease.

 

Conclusion

This study has suggested a link between consumption of certain fats in the diet and risk of death from cancer in men with the disease over twenty years of follow up.

There are a number of limitations to this study:

  • The number of men included was relatively small, and groups being compared would be smaller once the men began to be divided by cancer stage and fatty acid intake.
  • The study carried out many statistical tests. This means we might expect to see some significant results arising just by chance. The researchers noted that some of their results would not remain statistically significant if they took the number of tests into account.
  • The men had to report their food consumption in the year prior to diagnosis and it may have been difficult to accurately recall what they had eaten. When the researchers compared a sample of men’s food questionnaire answers with a food diary, there was not very strong agreement. The men’s diets could also have changed over time, which could influence results.
  • Although the researchers took into account a number of factors that could influence the results, these or other factors could still be contributing to the differences seen.

The results will need confirmation in other studies.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.

Links to the headlines

‘Eating oily fish regularly ‘can significantly cut risk of prostate death’’. Daily Mail, July 19, 2012

Links to the science

Epstein M, Kasperzyk L, Mucci LA, et al. Dietary Fatty Acid Intake and Prostate Cancer Survival in Örebro County, Sweden. American Journal of Epidemiology. Published online July 10, 2012.

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Analysis by Bazian

Edited by NHS Choices

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