Fish oil supplements might make cancer chemotherapy less effective - but many people with cancer were taking those supplements in a recent survey.
All six of the fish oil supplements the researchers tested contained a specific fatty acid that's been found to reduce the effectiveness of chemotherapy in mice, the researchers report in JAMA Oncology.
People receiving chemotherapy should refrain from taking fish oil supplements and discuss any supplement with their doctors, said the study’s lead author from the Netherlands Cancer Institute in Amsterdam.
The researchers had previously found that even a small amount of two fatty acids reduced the effectiveness of chemotherapy in mice with cancer. These fatty acids may ultimately enable cancer cells to repair themselves faster after chemotherapy.
When the team looked for the two fatty acids in six fish oil supplements, one was not detected but the fatty acid called 16:4(n-3) was ubiquitous.
The researchers also surveyed 400 people receiving treatment for cancer in November 2011. Of 118 people who returned the surveys, 35 percent reported using nutritional supplements and 11 percent reported using omega-3 fatty acid supplements.
In another part of the study, the researchers then had 30 volunteers without cancer take 10 or 50 milliliters (mL) of the three fish oil supplements. Levels of 16:4(n-3) peaked in their blood about four hours after taking the 10 mL (2 tsp.) supplements; levels returned to normal after about eight hours. Levels of the fatty acid remained elevated longer among those taking 50 mL.
Furthermore, when 20 volunteers ate mackerel or herring, levels of 16:4(n-3) in their blood went up. Eating salmon or tuna had little or no effect, however.
The researchers conclude that until more is known, people on chemotherapy should avoid fish oil - and herring and mackerel - the day before and after their treatments.
They note that the Dutch Cancer Society and the Dutch National Working Group for Oncologic Dieticians recommend that people receiving chemotherapy avoid fish oil around the time of treatment.
People should be cautious about going overboard with avoiding all omega-3 fatty acids, because they're an essential part of the diet that the body can’t make on its own, said cancer experts.
Flaxseed, chia seeds and walnuts are also sources of omega-3 fatty acids.
It’s important discuss risks and benefits of fish oil and other nutritional supplements with your doctor and dietitian.
Source: JAMA Oncology, online April 2, 2015.
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