For centuries, humans have used fish oils, orally or topically, to treat a wide array of ailments, from aches and pains to rickets and gout. The popularity of this supplement has shifted over the years, as have its primary uses. But over the past couple of decades, the hype around fish oil has arguably reached an all-time high. According to National Institutes of Health statistics, in 2012, at least 18.8 million Americans used about $1.3 billion dollars worth of fish oil, making it the third most widely used supplement in the nation. (Sales reportedly flattened out at about that level around 2013.)

Today, many use it because they believe it will broadly help their heart health, but others hold that fish oil can help with renal health, bone, and joint conditions, cognitive functions and mental wellness, and any number of other conditions. But is fish oil really as good for you as millions of Americans believe it is? Who should be taking it and when? We dove into the research and consulted a number of experts to try to find out.

What exactly is fish oil?

Fish oil is just what it sounds like: oil derived from processed fish, especially species like herring, mackerel, salmon, anchovies, and sardines. These oils are rich in Omega-3 fatty acids, specifically eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Although our bodies can turn alpha-linoleic acid—another type of Omega-3 fatty acid found in things like canola oil, pumpkin seeds, and walnuts—into EPA and DHA, fish oil is one of the most efficient sources of these two Omega-3s for us.

Omega-3 fatty acids “do a lot of things in our bodies,” explained Stephen Kopecky, a cardiologist at the Mayo Clinic. It's hard to summarize exactly what they do, as such, but it’s safe to say that maintaining a sufficient level of these acids is vital to our overall functioning.

Does fish oil help with heart health?

Modern beliefs in the heart health benefits of fish oil stem from observational studies of Greenland Inuit in the 1970s, who found that their diets were extremely high in oily fish and they suffered fairly low rates of heart disease. In the years since, the American Heart Association has routinely recommended that people eat fish at least twice a week to, among other things, slow the rate of plaque growth in the arteries, lower their blood pressure a bit, and generally stay heart healthy. The Federal Drug Administration (FDA) has also approved a few tightly regulated fish oil-based medicines for use in controlling high triglyceride (a type of fat you want to control) levels in patients.

Fish oil, Kopecky said, “is really the standard of our treatment now for the majority of people with high triglycerides,” alongside increasing physical activity and lowering carb intake. A number of studies also suggest that reliable doses of good quality fish oil can lower the risk of bypass surgery, chest pain, heart attack, stroke, and sudden cardiac arrest in people who have heart health issues already. On the other hand, excessive consumption of fish—over 1.6 ounces per day—has actually been linked to a higher risk of stroke in at least one study.

While a few studies suggest that fish oil supplements could help healthy people protect themselves from heart issues a little bit, that evidence seems relatively weak and inconsistent, so most doctors do not seem to recommend it as a viable preventative health tool.

Can fish oil help your mental and neural health?

Belief in the mental and neurological benefits of fish oil likely stems from the fact that “about 40 percent of the polyunsaturated fat in our brains,” or five to ten percent of our brain mass overall, is “Omega-3 DHA,” said R. Preston Mason, a professor in the department of medicine, division of cardiology, at the Harvard Medical School. This compound seems to be involved in sending signals between brain cells. And government nutritional guidelines assert that eating seafood is important for normal brain growth and development. But we don’t really know what specifically, if anything, taking fish oil supplements as a child or adult would do to boost neurological or mental health.

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Some research has suggested that fish oil might help with attention in kids with attention deficit hyperactive disorder (ADHD), or with mental functioning and behavior in children in general. A little research also suggests that fish oil might help reduce the risk of, or control, certain mental illnesses in kids and young adults who show mild symptoms of them. At least one study has even put forward the idea that fish oil could help to reduce seizures in patients with drug resistant epilepsy. But all of those findings are fairly weak and provisional—not to be trusted at face value.

Does fish oil help with eye health?

Although a few studies have suggested that eating fish regularly could result in less age-related vision loss, and that regular fish oil consumption reduced the risk of cataracts, these studies were not conclusive and have been disputed. Fish oil used to be prescribed regularly to treat dry eye based on a few studies, Kopecky said, “until three-to-five years ago, when meta-analyses showed there wasn’t actually much benefit. Ophthalmologists don’t really recommend it anymore.”

Does fish oil help your skin?

Kopecky is hesitant to back any dermatological studies that suggest fish oil can help with wrinkles. Other studies also suggest that fish oil might help with acne, eczema, and psoriasis, but these are also relatively limited and are hardly conclusive. But it is worth noting they seem less contested than other fish oil benefits findings.

Does fish oil help or prevent cancer?

While a few researchers have suggested that fish oil might help to lower one’s risk of developing all sorts of cancers—like breast, colorectal, esophageal, lower oral, ovarian, pharyngeal, and prostate cancer—studies examining these claims have been limited and their results have been all over the map. At least one study has actually suggested fish oil usage might increase risks of prostate cancer, when consumed by certain subgroups and in certain amounts over time.

Does fish oil help with weight loss?

Lifestyle magazines briefly hyped a study out of Japan arguing that fish oil stimulated the digestive tract of mice to reduce weight gain and help weight loss. Another study suggested that overweight humans with high blood pressure might reap weight loss benefits from fish oil. However these findings are exceptionally limited and there's no substantial evidence that it would help humans manage their weight.

If there's so much uncertainty around the benefits of fish oil, why do people still use it?

The vast majority of research into the purported benefits of fish oil—including many exceptionally specific ones (e.g.: gum disease, pneumonia, and assorted food allergies)—has been inconclusive. This is likely in part because studies of the effects of supplements are notoriously often poorly structured, with no placebo controls, or attempts to account for additional factors, like diet and lifestyle, that might also influence user outcomes. Many studies are also one-offs with no substantive follow-up to flesh out or disprove their claims, leaving fish oil consumers with tons of weak information.

Some of the inconsistency may also come down to the fact that not all of the hundreds of different fish oil products on the market are created equal. Studies of over-the-counter fish oil supplements have found that many of them misrepresent the amount of EPA and DHA they contain, especially relative to each other, or omit details about additional filler ingredients they use. (The supplement industry in America is notoriously under-regulated.) In most supplements “only about a third of the product is Omega-3 fatty acids, Mason said, based on his own and other watchdogs’ research. “That’s how they can afford to make them so cheap.” Mason’s research has also shown that sloppy production, transport, and storage often leads the Omega-3 fatty acids in supplements to go rancid, which ruins any benefits it might convey. A fishy, sour smell in the container would indicate that the capsules may have gone bad.

So if I decide to try it, how much Omega-3 fatty acid do I need?

We know that Omega-3 fatty acids play a vital role in human development, especially neurological development. We also know that our bodies cannot produce them on their own. However no one has really figured out exactly how much of it we need as we’re growing up, or as adults, to stay healthy, Kopecky said. A lot probably depends on each person’s individual body, Mason added, although he points out that our bodies are pretty efficient, in general, at absorbing it, so “it’s not like you need it every single day to keep it” in your physical structures.

And, although the vast majority of people fall short of eating fish two times a week as per most nutritional guidelines, they still generally get enough Omega-3 fatty acids in their everyday diets, perhaps indirectly through foods like canola oil or walnuts, to avoid an Omega-3 deficiency.

Doctors actually don’t know a ton about what happens when someone has an Omega-3 deficiency, Kopecky said, because it is so hard to become deficient. Case studies suggest it can lead to heart disease risks, mood disorders, inflammation, or even cancer, but that is hardly a definitive list of effects. One would need to have an incredibly bad and unvaried diet to get to a deficiency, Mason said.

Are there any risks associated with taking fish oil supplements?

While Mason noted that there may be risks associated with consuming some of the adulterants that supplement producers add to their fish oil, our bodies seem to tolerate a wide range of Omega-3 fatty acid doses fairly well. Consuming more than three grams per day, though, may lead to blood clotting complications or reduced immune system functioning—especially for those already on blood thinners. Some people may also experience nausea, heartburn, diarrhea, or rashes. And those who are allergic to any type of seafood should check with their physician before taking these, even though they may not cause any type of reaction.

Beyond human risks, demand for fish oil fuels overfishing that is devastating stocks of anchovies, krill, sardines, and other environmentally important fish species. Fishing for fish oil production rather than whole fish meant for human consumption also tends to pay low-wage laborers less.

So do I really need to take fish oil supplements?

Unless you have a heart disease and your doctor prescribes an FDA-approved product, Mason said, probably not. If a person’s diet is, for some reason, completely devoid of Omega-3 fatty acids and they cannot fix that, then Kopecky thinks that seeking out a fish oil product might be useful. But in that case, you should be extremely careful to make sure that what you’re getting is actually just fish oil—and that it is not spoiled or riddled with environmental contaminants, as Mason and other researchers have shown is all too possible in our modern American supplement landscape.