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After reading this article, I looked over my last year's posts and realized that the recent studies posted all found that eating fish showed health benefits (and they did not look at supplements). Once again, a food shows benefits while the supplement is debatable. Current advice: try to eat fish at least twice a week. From the NY Times:

Fish Oil Claims Not Supported by Research

Fish oil is now the third most widely used dietary supplement in the United States, after vitamins and minerals, according to a recent report from the National Institutes of Health. At least 10 percent of Americans take fish oil regularly, most believing that the omega-3 fatty acids in the supplements will protect their cardiovascular health. But there is one big problem: The vast majority of clinical trials involving fish oil have found no evidence that it lowers the risk of heart attack and stroke.

From 2005 to 2012, at least two dozen rigorous studies of fish oil were published in leading medical journals, most of which looked at whether fish oil could prevent cardiovascular events in high-risk populations. These were people who had a history of heart disease or strong risk factors for it, like high cholesterol, hypertension or Type 2 diabetes. All but two of these studies found that compared with a placebo, fish oil showed no benefit.

In theory at least, there are good reasons that fish oil should improve cardiovascular health. Most fish oil supplements are rich in two omega-3 fatty acidseicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) — that can have a blood-thinning effect, much like aspirin, that may reduce the likelihood of clots. Omega-3s can also reduce inflammation, which plays a role in atherosclerosis. And the Food and Drug Administration has approved at least three prescription types of fish oil — Vascepa, Lovaza and a generic form — for the treatment of very high triglycerides, a risk factor for heart disease. But these properties of omega-3 fatty acids have not translated into notable benefits in most large clinical trials.

Like many cardiologists, Dr. Stein encourages his patients to avoid fish oil supplements and focus instead on eating fatty fish at least twice a week, in line with federal guidelines on safe fish intake, because fish contains a variety of healthful nutrients other than just EPA and DHA. “We don’t recommend fish oil unless someone gets absolutely no fish in their diets,” Dr. Stein said.

But some experts say the case for fish oil remains open. Dr. JoAnn Manson, the chief of preventive medicine at Brigham and Women’s Hospital in Boston, said the large clinical trials of fish oil focused only on people who already had heart disease or were at very high risk. Fish oil has also been promoted for the prevention of a variety of other conditions, including cancer, Alzheimer’s and depression. Dr. Manson is leading a five-year clinical trial, called the Vital study, of 26,000 people who are more representative of the general population. Set to be completed next year, it will determine whether fish oil and vitamin D, separately or combined, have any effect on the long-term prevention of heart disease, Type 2 diabetes, and other diseases in people who do not have many strong risk factors.

Dr. Manson says that although she recommends eating fatty fish first, she usually does not stop people from taking fish oil, in part because it does not seem to have major side effects in generally healthy people“But I do think people should realize that the jury is still out,” she said, “and that they may be spending a lot of money on these supplements without getting any benefit.”

This article describes results of a research review showing cancer prevention benefits from eating fatty fish and fish oil. From Science News:

Marked benefits found for cancer prevention with a higher intake of fatty fish

A new research review will once again have people asking for a second helping of wild Alaskan salmon at the dinner table. While several other studies have recently challenged the long-held belief of the benefits of a diet high in omega-3 fatty acids, this new study cites compelling evidence that eating the right kinds of fatty fish, in the right quantity, and prepared the right way, can in fact help prevent the body’s development of adenocarcinomas, a common type of cancerous tumor. A high proportion of the cancers arising in the breast, prostate, pancreas, colon, and the rest of the gastrointestinal tracts are adenocarcinomas.

The authors first cite evidence that the recently-demonstrated ability of daily low-dose aspirin to decrease risk for adenocarcinomas is attributable to its ability to modestly decrease the activity of cyclooxygenase-2 (cox-2), an enzyme which contributes importantly to the genesis and progression of adenocarcinomas. They then propose that an ample dietary intake of omega-3 fats -- the type prominent in fatty fish -- could also be expected to oppose cox-2 activity, and thereby reduce risk for adenocarcinomas.

The authors emphasize that it is not only the amount of fish consumed daily, but also the nature of this fish, and how it is preserved or cooked, that can have a major impact on the potential of dietary fish to lower cancer risk. "An easy way to see the benefit of omega-3 is to look at Italy," Dr. DiNicolantonio said. "The staple oil used in cooking and as a salad dressing in Italy is olive oil, which is quite low in omega-6. Meanwhile, fish -- high in omega-3 -- is a staple food in the Italian diet, and this fish is rarely salt-preserved or fried. In Italians studies, subjects who consumed fish at least twice weekly as compared to those who ate fish less than once a week, were found to be at a significantly lower risk for a number of cancers, including ovarian, endometrial, pharyngeal, esophageal, gastric, colonic, rectal, and pancreatic."

The authors also focus on several recent studies in which regular consumption of fish oil is correlated with lower subsequent cancer risk. These studies have reported lower risks for colorectal, breast, and advanced prostate cancer in those taking such supplements. And a recent study from the University of Washington, which estimated total omega-3 intakes of its subjects from both fish and from supplements, found that a high omega-3 intake was associated with a 23 percent reduction in total cancer mortality. Indeed, mortality from all causes was significantly lower in those with higher omega-3 intakes. The authors also noted that cox-2 is significantly expressed in pre-malignant and early stage adenocarcinomas, but expression is sometimes lost as cancers mature. This may be why cox-2 inhibition (via increased omega-3 intake) seems to have greater potential for cancer prevention, than for cancer therapy.