Something fishy is going on in the fish oil industry, scientists say. According to research conducted in the past 10 years, fish oil didn't show any health benefits in participants when compared with a placebo.

"There's a major disconnect. The sales are going up despite the progressive accumulation of trials that show no effect," said Andrew Grey, an associate professor of medicine at the University of Auckland in New Zealand and the author of a 2014 study on fish oil in JAMA Internal Medicine.

Fish oil is now the third most popular dietary supplement in the United States, after vitamins and minerals, according to the National Institutes of Health. Almost 10 percent of the population take fish oil regularly and believe in the cardiovascular health benefits of omega-3 fatty acids, reports the NYTimes.com.

The problem is that although it is known that the good fats in fish and fish oil are important parts of a healthy diet because they increase blood flow, reduce blood pressure and inflammation, and give neurons structural strength, the health benefits of supplementing diets with them has become much harder to measure.

According to Grey's study, from 2005 to 2012, at least 24 different studies were conducted on the matter, looking to find whether fish oil supplements could improve cardiovascular health in high-risk populations with a history of heart disease or strong risk factors for it.

The results were surprising. Only two of the more than two dozen studies showed any signs of health improvements in the participants. The rest indicated that fish oil supplements were no better than placebos.

The reason for the media hype concerning fish oil benefits dates back to the 1970s, when Danish scientists determined that northern Greenland Inuits had low levels of heart disease, which they attributed to a diet high in omega-3 fatty acids. The case for its benefits was then made stronger in the 1990s, when other studies seemed to indicate that a daily dosage of fish oil led to a decrease in mortality rates in heart attack survivors.

After the American Heart Association started to endorse fish oil as a way to get omega-3 fatty acids, the notion that it was a must-have dietary supplement was already entrenched in the mind of the American consumer.

"But since then, there has been a spate of studies showing no benefit," said Dr. James Stein, the director of preventive cardiology at University of Wisconsin Hospital and Clinics.

According to the United Nations Food and Agriculture Organization, per capita fish consumption has doubled since 1961, which might explain why supplementing already omega-3-rich diets is not showing any added health benefits in clinical trials, reports Scientific American.

Others, however, point out that because all these studies involved high-risk participants, the results cannot be generalized to the whole population. This means that very little research has focused on how fish oil benefits healthy people.

"The jury is still out on whether omega-3 supplements can prevent a first cardiovascular event in people at usual risk," says JoAnn Manson, an epidemiologist at Brigham and Women's Hospital in Boston who is conducting a trial to answer this question, which she estimates will be finished in 2016.