8 things you need to know before you take fish oil supplements

Fish oil is one of the most widely used supplements in the United States. A National Institutes of Health (NIH) survey found almost 19 million adults had popped a fish oil pill within the last month — that's around one in every 17 people. There's good reason why: Many supplements are plagued by inconclusive research, but fish oil isn't one of them. A solid evidence base suggests fish oil supplements can hold benefits for your health, and may even slow down the aging process.

But before you jump on this trend, here are 8 facts you need to know to decide if fish oil is right for you.

What is fish oil?

As the name suggests, fish oil is essentially fat extracted from fish. The specific components of fish oil scientists are most interested in are omega-3 fatty acids.

The number ‘3’ stands for the position of an important double carbon bond in the compound’s molecular structure. The position of the bond singles out omega-3s' behavior in the body from other omega fatty acids, like omega-6 fatty acids, which are derived from plant sources, according to Harvard Medical School.

Omega-3s come in three main varieties: alpha-linolenic acid (ALA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA). Again, these three kinds of omega-3 come from different sources — EPA and DHA come from fish oil, while ALA is found in plant sources such as flaxseed.

What food contains fish oil?

If you are an adherent to a Mediterranean-style diet, or pescatarianism, then chances are your diet is already rich in omega-3s. But there are some other natural sources of this essential nutrient not to do with fish, too.

These are the best foods to consume to ensure you get enough omega-3s:

• Seafood: Cold-water fatty fish are ideal. The best include tuna, mackerel, salmon, sardines, and herring.
• Nuts and seeds: Walnuts, flaxseeds, and chia seeds are all excellent sources.
• Plant oils: Soybean oil, flaxseed oil, and canola oil all contain omega-3s.
• Fortified foods: These can include yogurt, juice, milk, soy drinks, and eggs.

If you don't eat fish, have a nut allergy, or otherwise think you don't get enough omega-3s in your daily diet, then you may want to consider taking omega-3 supplements — typically fish oil capsules.

What are fish oil supplements?

Supplements to boost omega-3 consumption are typically made from fish oil, and most frequently cod liver oil, krill oil, or even oil from algae. Algal oil is a plant-based source of omega-3s that contains both EPA and DHA.

While scientists aren't sure whether one specific kind of omega-3 is better for you than another, there is evidence to suggest omega-3s are — at least in part — the reason why eating fish regularly, as in the Mediterranean diet, is so good for your physical and mental health.

How do omega-3s affect mental health?

Your body uses omega-3s to build cells' membranes, the basic container for all of your body's cellular processes, but some of your cells use more omega-3s than others. DHA levels are particularly elevated in brain cells, which suggests they play an important function in brain health.

Here are four key ways omega-3s may influence your brain:

• Neurogenesis and brain structure:

A 2014 review found omega-3s play a role in neurogenesis, or the creation of brain cells. People who consumed more fish oil tended to have greater volumes of grey matter in their brains. It also found evidence to suggest both grey and white matter may be less prone to the affects of aging in people who consume more fish oil.

• Cognition and aging:

According to the National Institutes of Health, research shows consuming more omega-3s from dietary sources, such as fish, may lower the risk for Alzheimer’s disease, dementia, and other cognitive health problems. But as the NIH researchers point out, the evidence on omega-3s and staving off cognitive decline is mixed.

A 2015 study showed two to four years of omega-3 supplementation in 4000 older participants didn't prevent cognitive decline. But the study did not test the effects of omega-3 intake from diet — a major limitation.

• Depression:

Researchers have found stronger evidence omega-3 supplementation help with depression, however. In a review of multiple studies involving more than 10,000 participants, researchers found using omega-3 supplementation in conjunction with conventional treatments for depression helped alleviate symptoms significantly more than the traditional therapies alone.

• Mood and behavior:

There is also evidence to suggest omega-3s may curb aggressive behavior. In Australia, researchers are testing the idea that omega-3 supplementation may reduce violent incidents in Australian prison populations. In a pilot study published in the journal PLOS ONE in 2015, the team found omega-3 levels varied between individual Australian prisoners and lower omega-3 levels were correlated with increased aggression and signs of attention deficit hyperactivity disorder, or ADHD. The jury is still out on their overarching hypothesis — a study published this year assessing the feasibility of a randomized, double-blind, placebo controlled trial of omega-3 supplements in prisoners suggests these supplements do reduce violent or aggressive behaviors among inmates — but the sample size in the feasibility study is far too small to come to any definitive conclusion.

Why are omega-3s good for the body?

In addition to being a building block of cell membranes, omega-3s play an important role in safeguarding respiratory health, the cardiovascular system, the immune system, and endocrine system function.

Here are three of the top benefits omeag-3s have for the body:

• Heart health:

According to the NIH, both dietary and supplemented sources of omega-3s support cardiovascular health.

One or two servings of seafood per week is recommended by the American Heart Association (AHA) to reduce the risk of developing cardiovascular problems. And both dietary and supplemental omega-3s have been shown to reduce triglycerides — a particle in the blood that transports fat through the body.

One large clinical trial involving 26,000 people found that omega-3 supplementation reduced heart attacks by 28 percent and deaths from heart attacks by 50 percent — there is also evidence to suggest omega-3s reduce the risk of arrhythmia — a potentially deadly condition in which the heart falls into the wrong rhythm.

For heart disease patients, the AHA recommends 1 gram per day of EPA and DHA. Ideally, these fatty acids are consumed as part of one's diet, but supplements work as well. The AHA do not recommend supplementation for people who are not high-risk for cardiovascular disease, however.

• Eyesight:

Just like brain cells, retinal cells in the eye have particularly high levels of DHA. People who have a high intake of omega-3s from food may lower their chances of developing age-related macular degeneration, a condition which eventually causes blindness. This condition, in which the retina deteriorates, is a leading cause of vision decline in older people. But you need to start as you mean to continue: According to the NIH, there is no evidence to suggest supplements prevent the problem from worsening once it begins.

• Reproductive advantages:

Brain and retinal cells are not the only ones in the body with particularly high levels of DHA. The amount of this fatty acid is particularly high in sperm cells, too. Studies suggest omega-3 supplementation can support male reproductive health, boosting sperm fitness.

Men are not the only beneficiaries. Between 8 to 12 ounces of seafood per week during pregnancy and breastfeeding may significantly benefit the unborn child, too. Omega-3 supplements may increase birth weight and the length of gestation, both of which may be beneficial for infant health, according to the NIH.

Are fish oil supplements good for you?

This depends on whom you ask. The evidence in support of omega-3s is strong, but the NIH and other health research bodies say the best way to consume omega-3s is by eating fish, rather than taking fish oil supplements.

What is not a point of contention however is whether fish oil supplements boost omega-3 levels — they do.

How long do fish oil supplements take to work?

Fish oil supplements do seem to work, but it isn't clear how long they take to work, according to Carol Haggans, a nutrition scientist at the NIH Office of Dietary Supplements. Part of this is to do with the studies we have at hand to suggest fish oil supplements are of benefit at all.

"Most studies examining the health effects of fish oil for conditions like cardiovascular disease last many years, like 5 years or even longer," she tells Inverse. "The idea is to increase your intake of omega-3's to see if they have long-term health benefits."

There are some studies which have tested omega-3 supplements over shorter time frames. For instance, one 2011 study found omega-3s lowered inflammation and anxiety in a group of 68 students after just 12 weeks.

Can you take too much fish oil?

According to the NIH, there is no set recommended daily intake for DHA or EPA, the omega-3s found primarily in fish oil. But the United States Food and Drug Administration sets an upper limit of 3 grams per day of DHA and EPA combined.

You might want to talk to your doctor before you opt for omega-3 supplements, the NIH says. These supplements can interact with prescription medications, including warfarin and other anticoagulant drugs, potentially causing bleeding problems, according to the NIH.

In low doses, however, the side effects of fish oil supplements tend to be minimal. But they can include: bad breath, an unpleasant taste in the mouth, nausea, stomach discomfort, heartburn, diarrhea, headache, and smelly sweat.

The Inverse analysis — Ultimately, the evidence in favor of fish oil supplements reflects the evidence in favor of a Mediterranean-style diet, rich in oily fish, nuts, and plants. While this diet isn't right for everyone, the evidence is good enough to suggest sticking to this style of eating will ultimately benefit your health — from your brain to your gut and your heart.