People who are on chemotherapy for cancer should avoid taking fish oil supplements or eating fatty fish like mackerel until they talk with their physicians, according to a new study. Fish oil and fatty fish contain a fatty acid that can neutralize the action of chemotherapy drugs.
Researchers surveyed 400 cancer patients undergoing chemotherapy at the University Medical Center Utrecht in the Netherlands. The patients were asked if they were taking fish oil supplements or if they ate mackerel, herring, salmon, or tuna. Thirty-five percent of the 118 patients who responded to the survey said they were taking nutritional supplements and 11% said they were taking omega-3 fatty acid supplements, usually through fish oil.
The researchers also tested several brands of fish oil supplements and fish to determine levels of the fatty acid 16:4(n-3), which has been shown to reduce the effectiveness of chemotherapy in mice. A third arm of the study evaluated blood levels of 16:4(n-3) in 30 healthy volunteers who took 10 or 50 milliliters of fish oil supplements and in 20 volunteers who ate mackerel, herring, salmon, or tuna. Levels of 16:4(n-3) peaked in their blood about four hours after taking the 10-milliliter supplements; levels returned to normal after about eight hours. Levels of the fatty acid remained elevated longer among the volunteers taking 50 milliliters . Volunteers who ate mackerel or herring had levels of 16:4(n-3) in their blood, but the ones who ate salmon or tuna did not.
The study concluded that patients undergoing chemotherapy avoid taking fish oil supplements or eating mackerel or herring on their treatment day, and the days before and after. All cancer patients should tell their oncologists and other physicians of any supplements that they take.
The researchers noted that about one-fifth of Americans with cancer say they take omega-3 fatty acid supplements. Flaxseed, chia seeds, and walnuts are also good non-fish sources of omega-3 fatty acids.
The study was published in JAMA Oncology.
