Hanson et al (2020)’s research study, entitled 'Omega-3, omega-6 and total dietary polyunsaturated fat on cancer incidence: systematic review and meta-analysis of randomised trials,' was commissioned by the WHO Nutrition Guidance Expert Advisory Group (NUGAG) Subgroup on Diet and Health and has been published in the British Journal of Cancer.
Background of Study
Amid the prevalence of cancer as a global cause of morbidity and mortality, the researchers sought to explore the relationship between long-chain omega-3 (LCn3), alpha-linolenic acid (ALA), omega-6 and total polyunsaturated fatty acid (PUFA) absorption and cancer risk — as currently, this remains unclear.
Detailing that there were 17 million new cases and 9.6 million cancer-related deaths in 2018, the study’s researchers stated that every sixth death in the world is due to cancer. In the United States, for example, the total cancer expenditure by 2020 is estimated to reach $156 billion.
Consequently, “even small beneficial or harmful effects could be important," the study highlighted. In addition, the researchers stated that cardiovascular disease is the other major health risk impacting the world’s population; responsible for 37% of premature deaths due to non-communicable disease in 2012, with cancers accountable for 27%.
Polyunsaturated Fatty Acids (PUFAs) Prevalence
PUFAs include long-chain omega-3 (LCn3 including eicosapentaenoic acid and docosapentaenoic acid), alpha-linolenic acid (ALA, a shorter chain omega-3) and omega-6 fats (including linoleic acid, LA).
Theories outline that dietary PUFAs are a “modifiable component of lifestyle that could influence cancer risk”. Within our diets, polyunsaturated fats are common healthy eating choices, with fish oil (LCn3) and flaxseed (ALA) supplements often consumed.
As lowering dietary fat, which may include PUFAs, is often seen as a way to reduce weight in adults, the researchers indicate that reducing PUFA consumption could provide protective effects against those cancers that are associated with being overweight. By continuing this idea, it can be suggested that omega-3 may be protective, while omega-6 and total PUFA may heighten the risk of cancer.
Alternatively, evidence supports the idea that the human diet does need some polyunsaturated fats. UK dietary reference values, for example, indicate that 6.5% of our energy intake is from cis-polyunsaturated fats. In addition, raising polyunsaturated fat intake is linked with healthy eating and cardiovascular disease prevention. However, it remains “scientifically controversial."
Due to its perceived controversy and subsequent avoidance by consumers looking for nutritious and healthy diets, fish supplements are a popular choice. “The use of supplements as additions or replacements to food stuff has gained traction with the general public,” the study highlighted.
Fish oil supplements usage
Estimates reveal that approximately 38% of US adults use complementary medicines to supplement their diets. The most popular non-vitamin, non-mineral natural products taken is fish oil, omega 3 or DHA supplements — with 37.4% of US consumers stating they take these forms of supplements — followed by flaxseed at 15.9%.
Concerns over contaminants such as heavy metals are prevalent. LCn3 is often ingested in the form of oily fish or fish oil in capsules. Mercury, cadmium, chromium, nickel, lead and cobalt and toxic compounds such as dioxins have been identified in fish and fish oils, indicating a potential health risk.
The researchers confirm that due to the associated health risk, it is important to analyze both “potential benefits and harms of increasing omega-3, omega-6 and total polyunsaturated fats on cancer risk” to provide information, advice and guidance on the impact/elements of dietary change or supplementation.
Examining a range of health effects resulting from omega-3, omega-6 and total PUFAs, the systematic reviews led the researchers to actively compare health benefits and damage indicators in cancer and cardiovascular disease; the core determiners of mortality and morbidity in developed countries.
As part of the inclusion criteria, the randomized controlled trials (RCTs) that compared higher versus lower LCn3, ALA, omega-6 and/ or total PUFAs were conducted in adults aged at least 18 years, who were not pregnant or seriously ill.
The study participants permitted to take part could be free of cancer, at increased risk of cancer or those who had received a previous diagnosis of cancer. However, participants who were currently undergoing cancer treatment were excluded.
Cancer diagnosis risk
Following research into RCTs, the study included 47 RCTs, complete with a total of 108,194 participants. By analyzing and evaluating these RCTs, Hanson et al (2020) found that increasing LCn3 has little or no effect on the risk of any cancer diagnosis and “probably” has little or no effect on the risk of cancer death. In addition, increasing the amount of ALA “probably” has little or no effect on cancer death. But, the study also found that raising LCn3 and ALA may slightly increase the risk of prostate cancer.
Examining the impact of total PUFAs on the risk of cancer, the researchers noted that heightening total PUFA may “slightly increase risk” of any cancer diagnosis and cancer death.
The effects of omega-6 on all cancer outcomes are unclear from the research carried out and the evidence for these effects is of very low quality according to the report.
What does this study suggest about the relationship?
The scientists conclude that the study was the “most extensive systematic review to assess the effects of increasing PUFAs on cancer risk." In doing so, the scientists found that lifting the total polyunsaturated fatty acid (PUFA) intake “may very slightly increase cancer risk, offset by small protective effects on cardiovascular diseases”.
In identifying the presence of small harms resulting from increased LCn3, ALA and total PUFAs, the researchers urged for consideration. The rise in small harms must be “balanced against potential gains from the other major cause of morbidity and mortality, cardiovascular disease," they suggested.
As such, “increasing LCn3, ALA, omega-6 and total PUFA appear to have little or no effect on all-cause mortality." Taking into consideration both cancer and cardiovascular outcomes, the overall health effects of raising LCn3, ALA, omega-6 and total PUFA “appear small."
Source: British Journal of Cancer
"Omega-3, omega-6 and total dietary polyunsaturated fat on cancer incidence: systematic review and meta-analysis of randomised trials"
Authors: S. Hanson et al.