Fish Oil: Too Much of a Good Thing?

Fish oil may have some benefit for the heart. But a study in the Journal of the National Cancer Institute links higher blood levels of the omega-3 fatty acids in fish oil to a higher risk of prostate cancer. Study author Alan Kristal says the potential mechanism is unclear, but he warns that supplements can sometimes increase the risk of the very diseases they're meant to prevent.

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IRA FLATOW, HOST:

Welcome back. I'm Ira Flatow and our next story is destined to confuse you. It's one of those stories about nutrition, an update where new research seems to contradict old ideas, so let me just get right into it. We've all heard a lot of good news about fish oil and Omega-3 fatty acids.

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UNIDENTIFIED WOMAN #1: Omega-3 fatty acids may help reduce the risk of breast cancer.

UNIDENTIFIED WOMAN #2: The fish oil you need is Omega-3 fatty acids.

UNIDENTIFIED MAN #1: Omega-3 is protective in heart...

UNIDENTIFIED MAN #2: Fish oil may even slow the aging process...

UNIDENTIFIED WOMAN #3: It also helps people with diabetes, asthma, rheumatoid arthritis, osteoporosis, post-partum depression, and cancer.

FLATOW: And the list goes on. But last year a major study in the Journal of the American Medical Association said, hey, you know what? Omega-3 supplements don't really protect you against death from heart disease. And now a new study by my next guest adds to the bad news. It says men who had a higher level of Omega-3 fatty acids in their blood also had a higher risk of prostate cancer.

No cause and effect here, just a correlation. But it does make you wonder whether all these supplements and vitamins we take might be doing more harm than good. Alan Kristal is the author of the study in the journal of the National Cancer Institute. He's associate head of the cancer prevention program at the Fred Hutchinson Cancer Research Center in Seattle, professor of epidemiology at the University of Washington.

Welcome to the program, Dr. Kristal.

DOCTOR ALAN KRISTAL: Thanks very much.

FLATOW: Tell us exactly what you found.

KRISTAL: So the - this is based on a study of 35,000 men where we did a randomized trial looking at whether selenium or Vitamin E supplements would prevent prostate cancer. And one thing we did was we measured the levels of Omega-3 fatty acids in the blood of men as they entered the study and looked at how that related to prostate cancer risk.

And we found that high levels increased the risk of high-grade prostate cancer, which is the serious disease that actually kills you by over 70 percent. And it increased your risk of low-grade cancer by about 40 percent. And I should just add that we published a similar finding from a different study two years ago and we didn't really believe it, but we thought it was important to get it into the literature.

And now that we replicate it, it has some more immediacy. We begin to believe the association is true.

FLATOW: Does it matter where the Omega-3s come from, whether it's a supplement or, you know, you're just having some salmon?

KRISTAL: Yeah, I wish I could answer that very clearly, but I can say that most of the men in our study were not taking dietary supplements of Omega-3 fatty acids, so the high levels were coming from food.

FLATOW: Are you then recommending people don't eat fish, salmon and things like that?

KRISTAL: You know, that's the most complicated question you could ask me and I'd say this: Although the data on dietary supplements really doesn't support the use of Omega-3 fatty acids for anything and I want to emphasize that. There's no good data.

FLATOW: For anything? I just went through this long list, a little clip there, of everything that it was supposedly good for.

KRISTAL: It's, oh yes, things people want to believe about nutrition versus what scientists can tell you we know. It's an interesting list. But the point would be that there probably are benefits to fish consumption. The evidence points to Omega-3 fatty acids consumed as part of foods and that would be as part of fish reducing the risk of cardiovascular disease.

But it definitely shows, I think, that taking supplements has no impact. So there are some benefits, obviously, to eating salmon and other fatty fishes and, you know, it's a balance. There's a benefit for cardiovascular disease and I think there's clearly an increased risk for prostate cancer and, as I often say, none of us get out of this alive, so it's a balance.

FLATOW: Yeah. You know, first we hear it was Vitamin E was good; then it wasn't good. And then we hear Vitamin D - are we going to hear that something about Vitamin D now also?

KRISTAL: I'm pretty sure you will. I mean, if I look back on my career, which I've been doing this work for about 25 years now, we naively thought a long time ago that high doses of micronutrients would reduce risk of cardiovascular disease or various cancers. And as we test them rigorously, what we find it has no impact at all. Or worse we find that it increases the risk of the diseases we're very trying to prevent.

And it also increases the risk of other diseases we never thought it would affect.

FLATOW: Well, then how did we get the idea in the first place that these things were helpful?

KRISTAL: You know, that's a long story, but part of it has to do with the way we do nutrition research and the difficulty in measuring what people eat. So there are certain types of study designs that have inherent balances and the way that we measure food consumption when we have very large studies is actually very poor.

And so as we - so one thing we've done is we've moved to using biomarkers of dietary intake, so we can - I don't have to ask people how much Omega-3 fatty acids they eat; I can measure it in their bloods. And as you start doing the designs more rigorously, as you start measuring food intake more carefully, then, you know, you start narrowing down on the truth.

FLATOW: Yeah. So you've ruled out that it could be contaminants in some of the supplements?

KRISTAL: No, we can't rule out contaminants, and to be honest with you, we can't rule out the problem that plagues all this sort of research and that's that there may be something that characterizes men who eat fish that also increases their risk of prostate cancer and the association that we see isn't true, it isn't causal.

But on the other side, if you look at the number of studies, and there have been quite a few that have looked at Omega-3 fatty acids and prostate cancers, they all point to an increased risk except for one. So these have been done all over the world, these have been various kinds, different kinds of people, different sources of Omega-3 fatty acids, and they all point in the same direction.

FLATOW: To me what is most fearful about your results is that it increased the risk of the worst kind of cancer, that very fast-moving cancer.

KRISTAL: It does, but you have to remember that it's actually a rare disease. High-grade prostate cancer is very rarely diagnosed. It's no more than 18 percent or so of all prostate cancers detected.

FLATOW: But even then, the other prostate cancer, that's still up 40 percent.

KRISTAL: Right. You know, definitely a cause for concern and...

FLATOW: You must be getting pushback, a lot of pushback.

KRISTAL: Goodness yes. I have received the most vituperous(ph) email I've ever received in my career on this one. You know, people like to believe that the supplements they take improve their health and they're barraged by advertisements on this. And it's all, frankly, it's just not true.

FLATOW: But you're going up against people who will say there's all this other research that X, Y, Z contradicts what you're saying. Yeah?

KRISTAL: Not really. Not really. Now, what people say and the quality of the research it's based on is something that really requires a lot of critical scrutiny. There's a lot published on nutrition and disease and a lot of it, frankly, is not high quality work.

FLATOW: Is there any kind of supplement that you've looked at that you would suggest people take?

KRISTAL: Well, you know, you can look at, for example, folate supplementation of women of childbearing age. That has had a profound impact on neural tube defects, so that's really a success story that's translated into fortification of the food supply. I honestly can't say any condition where I think taking supplements really makes any sense.

FLATOW: But you're even saying here that it's not just the supplements. It could just be eating fish that are high in Omega-3s.

KRISTAL: Right. So in this case, yes. So here we have the issues that - I'll explain it this way. So we know Omega-3 fatty acids have a long range of biological effects. Some of them we understand quite well. And the issues is some of these effects would prevent some diseases while at the same time increasing the risk of other diseases.

You know, overall there's a balance and I will say that there's a very large trial ongoing right now looking at Vitamin D supplements and Omega-3 fatty acid supplements, both on men and women. I don't expect it to publish for another five years at least, but that's a trial that will really be informative on, you know, the balance.

It will increase the risk of some things, increase some risk of the others, and the question is what's the overall impact on quality of life and survival.

FLATOW: Do you have any preliminary results from that study?

KRISTAL: Oh no, it's not my study and...

FLATOW: So how does it affect you personally? Do you now, when you sit down in a restaurant, say oh I'm not ordering this salmon?

KRISTAL: Well, you know, the strangest thing in the world is that the day this came out I went downstairs to my cafeteria at work and I had a piece of salmon, so I really believe in the balance issue. To me, if you had one or two servings of salmon or a similar kind of oily fish a week, that's fine. It's when you get to higher levels that the possibility of a risk really exists.

FLATOW: Because people are taking those fish oil tablets every single day, multiple times a day, sometimes.

KRISTAL: Multiple times a day, right. You know, just the best advice I can ever give people is just to stop taking supplements. The evidence that they're beneficial for anything is so slim and the more quality research we do, these randomized trials, we find it increases the risk of really serious kind of diseases.

FLATOW: Is there money to do randomized trials that we need to do, because supplements, they're not, you know, they're not Viagra.

KRISTAL: That's right. So that's a big policy question and I'll just say that there was a lot more money to do these trials ten years ago. The effect of the decreased federal funding for scientific research has been profound over the last couple of years and the possibility of getting a large clinical trial done to test - to examine either nutrition or test dietary supplements just doesn't exist.

They're very expensive to do. they're hundreds of millions of dollars to do and there's no excitement to do them.

FLATOW: Thanks for being the barer of good news today, Alan.

KRISTAL: My pleasure. Thank you very much.

FLATOW: You're welcome. Alan Kristal, associate head of the Cancer Prevention Program at the Fred Hutchinson Cancer Research Centre in Seattle. Also professor of epidemiology at the University of Washington talking about stuff that was published in a very tough journal, The Journal of the National Cancer Institute. They don't put stuff in there that they don't believe in, so if he's put it in there and they've accepted it, it's a very, very good study.

Thank you, Alan, for taking time to be with us today.

KRISTAL: My pleasure.

FLATOW: We're going to take a break and next up, get out your drills and saws. You might want to swing by your Radio Shack, your Home Depot, your Lowes, whatever, because it's time for our summer do-it-yourself segment. We've got projects you can so out by the pool. How about a beverage barn? Yeah, how about a water bottle rocket launcher? How about a cooler that you can turn into a heater?

All kinds of projects we've got for you. Stay with us, we'll be right back after that.

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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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