The Risks of Fish Oil Supplements

It’s been a bad few years for fish oil, as I discuss in my video, Omega 3s, Prostate Cancer, and Atrial Fibrillation. Claims were crushed that the long-chain omega 3s in fish oil—EPA/DHA—would stop the progression of heart disease. Then, DHA was associated with increased risk of prostate cancer and “monumentally” failed to treat macular degeneration. This over-the-top rhetoric sounded a little suspicious, and, indeed, the paper was retracted because the author sells some rival supplement he failed to disclose, but he does have a point.

I covered the fish oil failure for heart disease in my Is Fish Oil Just Snake Oil? video. But, what about the increased cancer risk? Men with the highest circulating levels in their blood of the long-chain omega 3 fat DHA were found to be at higher risk for prostate cancer, though a subsequent compilation of all such studies suggested EPA, the other major long-chain omega 3 in fish and fish oil, may be more closely associated with increased cancer risk. Either way, these long-chain omega 3s have been promoted for prevention of heart disease and cancer. We now know, however, that not only does there appear to be no benefit for death, heart attack, or stroke, there may be an elevation in cancer risk. The general recommendations to increase the intake of these fats should consider its potential risks.

How could eating more fish or fish oil increase cancer risk? Well, there are some industrial pollutants, like PCBs, linked to increased prostate cancer risk, and the “primary source of exposure in the general population is believed to be through diet from fish, meat, and dairy products.” If you do a supermarket survey, the PCBs are highest in freshwater fish and lowest in plant-based foods. Vegans have been tested, and they were found to be significantly less polluted than omnivores of the PCB linked to prostate cancer.

However, the prostate cancer study was done in North America where people don’t eat a lot of fish, and indeed, even the group with the highest DHA levels weren’t that high. So, maybe the confounding factor was meat consumption in general, not just fish. Lower meat consumption may be a reason for the lower rates of prostate cancer in the lower DHA group, as the consumption of well-done meat is associated with an increased risk of prostate cancer, and intake should therefore be restricted.

We also used to think omega 3s could protect us from arrhythmias—abnormal heart rhythms, like atrial fibrillation. Millions suffer from the condition, which causes an irregular heartbeat and a higher risk of stroke and death, but fish and fish oil consumption does not appear effective for preventing it or treating it.

Other arrhythmias can be life-threatening and can cause sudden death. Despite initial encouraging results, more recent studies have not only failed to reduce sudden cardiac death with omega 3s, but have actually increased mortality in cardiac patients. For example, men with heart disease advised to eat more oily fish or supplied with fish oil capsules were found to have a higher risk of cardiac death, possibly because of the contaminants in fish, such as mercury. In either case, given the inconsistent benefits and the potential adverse effects, omega 3s must be prescribed with caution and generalized recommendations to increase fish intake or to take fish oil capsules need to be reconsidered.


I was as surprised as you to learn that fish oil doesn’t help with heart disease. Learn why in my Is Fish Oil Just Snake Oil? video.

In terms of PCB contamination, see Food Sources of PCB Chemical Pollutants and PCBs in Children’s Fish Oil Supplements. Our oceans and waterways have become humanity’s sewer—everything eventually flows into the sea. For discussion of all the other industrial contaminants that build up in the aquatic food chain, see:

For ways to decrease the risk of prostate cancer and slow its progression, see:

In health,

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Who Should Avoid Coffee?

Do coffee drinkers live longer than non-coffee drinkers? Is it “wake up and smell the coffee” or don’t wake up at all? I discuss these questions in my video, Coffee and Mortality.

The largest study ever conducted on diet and health put that question to the test, examining the association between coffee drinking and subsequent mortality among hundreds of thousands of older men and women in the United States. Coffee drinkers won, though the effect was modest, a 10-15% lower risk of death for those drinking six or more cups a day. This was due specifically to lower risk of dying from heart disease, respiratory disease, stroke, injuries and accidents, diabetes, and infections.

However, another study that amount of coffee was found to increase the death rate of younger people under age 55. It may be appropriate, then, to recommend that you avoid drinking more than four cups a day. But if you review all the studies, the bottom line is that coffee consumption is associated with no change or a small reduction in mortality starting around one or two cups a day, for both men and women. The risk of dying was 3% lower for each cup of coffee consumed daily, which provides reassurance for the concern that coffee drinking might adversely affect health, or at least longevity.

A recent population study found no link between coffee consumption and symptoms of GERD, reflux diseases such as heartburn and regurgitation. If you actually stick a tube down people’s throats and measure pH, though, coffee induces significant acid reflux, whereas tea does not. Is this just because tea has less caffeine? No. If you reduce the caffeine content of the coffee down to that of tea, coffee still causes significantly more acid reflux. Decaf causes even less, so GERD patients might want to choose decaffeinated coffee or, even better, opt for tea.

Coffee intake is also associated with urinary incontinence, so a decrease in caffeine intake should be discussed with patients who have the condition. About two cups of coffee a day worth of caffeine may worsen urinary leakage.

A 2014 meta-analysis suggested that daily coffee consumption was associated with a slightly increased risk of bone fractures in women, but a decreased risk of fractures in men. However, no significant association was found between coffee consumption and the risk of hip fracture specifically. Tea consumption may actually protect against hip fracture, though it appears to have no apparent relationship with fracture risk in general.

Certain populations, in particular, may want to stay away from caffeine, including those with glaucoma or a family history of glaucoma, individuals with epilepsy, and, not surprisingly, people who have trouble sleeping. Even a single cup at night can cause a significant deterioration in sleep quality.

We used to think caffeine might increase the risk of an irregular heart rhythm called atrial fibrillation, but that was based on anecdotal case reports like one of a young woman who suffered atrial fibrillation after “chocolate intake abuse.” These cases invariably involved the acute ingestion of very large quantities of caffeine. As a result, the notion that caffeine ingestion may trigger abnormal heart rhythms had become “common knowledge,” and this assumption led to changes in medical practice.

We now have evidence that caffeine does not increase the risk of atrial fibrillation. Low-dose caffeine—defined as less than about five cups of coffee a day—may even have a protective effect. Tea consumption also appears to lower cardiovascular disease risk, especially when it comes to stroke. But given the proliferation of energy drinks that contain massive quantities of caffeine, one might temper any message that suggests that caffeine is beneficial. Indeed, 12 highly caffeinated energy drinks within a few hours could be lethal.


To learn more about various health aspects of coffee, see my videos Coffee and Cancer, What About the Caffeine?, Preventing Liver Cancer with Coffee?, and Coffee and Artery Function.

What else can we consume to live longer? Check out Nuts May Help Prevent Death, Increased Lifespan from Beans, Fruits and Longevity: How Many Minutes per Mouthful?, and Finger on the Pulse of Longevity.

And, for more on controlling acid reflux, see Diet and GERD Acid Reflux Heartburn and Diet and Hiatal Hernia.

In health,

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: