Potential bowel cancer benefit of fish oils – Expert reaction

A diet high in fatty acids from oily fish might reduce the risk of dying from bowel cancer, but experts caution the findings are preliminary.

multimediaThumbNew US-based research indicates there might be a link between high dietary omega-3 fatty acids from oily fish and a lowered risk of death in bowel cancer patients.

The findings, published in the BMJ journal Gut, examined participants of two longitudinal studies, who were questioned every second year on the frequency of foods in their diet.

Over 1600 participants from the two studies developed bowel cancer: those who consumed at least 0.3 grams of omega-3 fatty acids a day had a 41 per cent lower risk of dying from the disease, compared with patients who reported they ate less than 0.1g daily.

The researchers note that because the findings were observational, they cannot draw firm conclusions about cause and effect. Study participants who ate more omega-3 from oily fish were also more likely to be physically active, take multivitamins, drink alcohol and to consume more vitamin D and fibre.

The SMC collected the following expert commentary from New Zealand experts. The research paper is available to registered journalists on scimex.org.

Professor Ann Richardson, Professor of Cancer Epidemiology, School of Health Science, University of Canterbury, comments:

“The Nurses’ Health Study and the Health Professionals Follow-up Study are well-regarded cohort studies.

“Because these are observational studies, they are vulnerable to uncontrolled confounding (this is where other factors can distort the relationship between omega-3 and bowel cancer mortality – randomised controlled trials are less vulnerable to this than cohort studies).

“The press release makes this very clear, and states that ‘this is an observational study so no firm conclusions can be drawn about cause and effect’.

“The investigators also state that ‘further studies are needed in a larger population. Finally, we cannot exclude the possibility of residual confounding from other dietary or lifestyle factors’.

“While alcohol consumption has been included in the press release as a factor ‘associated with a lower risk of bowel cancer’, this could be misleading, as alcohol increases the risk of colorectal cancer, and there is a dose-response effect, with the risk increasing with the amount of alcohol consumed.

“The most recent estimate of the effect of alcohol on bowel cancer comes from a pooled analysis of 66 epidemiological studies, where heavy drinking (>50 g or >4 drinks per day) was associated with a 44% increase in the risk of bowel cancer.

“Lifestyle factors that could lower people’s risk of developing bowel cancer are reducing obesity, hazardous drinking, smoking, and consumption of red and processed meats, and increasing physical activity.”

Professor Wayne Cutfield, Professor of Paediatric Endocrinology, Liggins Institue, University of Auckland, comments:

“In two large U.S. cohorts (female nurses and male health professionals) omega-3 PUFA (polyunsaturated fatty acid) intake was associated with a reduction in bowel cancer deaths. No distinction could be made between omega-3 PUFAs from fish (canned or fresh) consumption versus fish oil supplementation and there was no reduction in all-cause mortality. The association between omega-3 intake and bowel cancer occurrence was not reported.

“Although the cohorts were studied prospectively data was obtained by subject recall biennially. This raises the possibility of unrecognised bias and imprecision in determining omega-3 PUFA intake. Further, participants who consumed more omega-3 PUFAs also had a healthier lifestyle, with greater physical activity, lower BMI and lower consumption of alcohol and lifetime tobacco use.

“The relationship between omega-3 PUFAs and mortality was present even after adjusting for these factors. However, it is still possible that the better outcomes in people with greater omega-3 PUFAs were not caused by the omega-3 levels themselves.

“The possible mechanism by which omega-3 supplementation protects against bowel cancer mortality is not clear. Omega-3 PUFAs regulate eicosanoids and may reduce levels of prostaglandin E2 which has been shown to reduce tumour growth in vitro. This is the proposed mechanism of reduction in recurrence of bowel cancer with aspirin.

“Historically the omega-3 PUFA literature has begun with large association studies most commonly showing a reduction in heart disease events or recurrence.

“Subsequent intervention studies have largely discredited these association studies. A previous study has reported a reduction in bowel cancer incidence in those with higher omega-3 PUFA intakes.

“However, as the authors of this interesting study suggest, further large intervention studies are required to evaluate whether there is indeed a causal relationship between omega-3 intake and reduction in bowel cancer mortality. It is too early to include omega-3 supplementation in the treatment arsenal of bowel cancer.”

The UK SMC collected the following comment from experts.

Dr Elizabeth Lund, Independent Consultant in Nutrition and Gastrointestinal Health, and former Research Leader at The Institute of Food Research, comments:

“This is a very interesting study building on the already established concepts that fish consumption seems to be protective in relation to bowel cancer development. Now we have provisional evidence that it may be protective in relation to disease prevention. It is generally assumed that the protective effects of fish consumption are due to intake of omega-3 fatty acids and there are a number of potential mechanisms by which these may work. Having both observational data such as reported here and plausible mechanisms of action make the idea more convincing than just looking at life style in cohorts of people association with disease risk. Although it is generally believed fish is protective due to its fatty acid composition this is not certain and it is probably best that people consume fish rather than take supplements.

“This study was based entirely on US populations, who in general eat relatively little fish, therefore it would be interesting to see a similar analysis of European populations to see if the pattern is seen this side of the Atlantic. The top category of intake in the study was people eating greater than 0.3g of fish oil per day which is equivalent to about one small portion of oily fish a week. Current UK guidelines are to eat at least one portion of oily fish a week so for now British people should all be trying to follow this advice.”

Dr Alister McNeish, Lecturer in Pharmacology (and researcher into cardiovascular effects of n-3 PUFA), University of Reading, comments:

“The paper provides interesting and robust evidence that high omega-3 (n-3; fish oils) intake is associated with improved colorectal cancer survival in those who have been diagnosed with the condition. It is worth noting that those who had a high intake of n-3 also had reduced risk factors and other behaviours that may be beneficial for colorectal cancer survival (e.g. more likely to be physically active, not smoke, and consume more vitamin D and fibre) so this could potentially affect the conclusions. It is also important to note that this study does not demonstrate causality and definitely does not provide a mechanistic insight into how an effect on colorectal cancer might occur.

“Omega-3 “Fish oils” are generally considered to improve blood flow so the idea that fish oils reduce tumour blood flow seems paradoxical; this is likely due to reduced vascularity (blood vessel formation aka angiogenesis) of the tumour rather than reduced blood flow per se. Therefore people consuming omega-3 for cardiovascular benefits should not be concerned by this report.”

Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, comments:

“This is an observational study and it is not possible to attribute the lower risk of bowel cancer specifically to omega-3 fatty acids because vitamin D may also have protective effects against colorectal cancer. People who eat fish may eat less of other foods linked to risk such as red and processed meat. Intervention trials with advice to eat more oily fish in patients with early bowel cancer failed to show benefit. This study provides no support of taking omega-3 supplements but does support to the notion that eating oily fish once/twice a week is good for health especially if it replaces red and processed meat.”