Introduction

My wife and I rescued a dog at the beginning of the summer. The best dog, it turns out. And in an effort to improve her already-beautiful black and tan coat, we bought fish oil capsules that can be opened and squeezed onto her food before she eats it.

An online recommendation suggested that fish oil supplements could help make her coat smooth, soft, and silky. It seems to have worked, as petting her has become irresistible.

But fish oil supplements aren’t only for dogs. Fish oil supplements have been widely recommended for a variety of health ailments including cardiovascular disease, arthritis, cancer, depression, anxiety, diabetes, and of course, weight loss.

Are these recommendations based on science? Is there any evidence that fish oil supplements actually help with these conditions?

What Is Fish Oil?

Fish oil, as you might have surmised, is in fact oil derived from fatty fish. This oil is typically high in omega-3 fatty acids, which are essential nutrients that your body needs to work properly. Omega-3s are a subcategory of a broader nutrient called polyunsaturated fatty acids (PUFA). Other PUFAs include omega-6 fatty acids and omega-9 fatty acids.

There are a few different types of omega-3 fatty acids. Eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are the ones most commonly found in fish oil, and alpha-linolenic acid (ALA) can be obtained from other sources. (71).

There is a biochemistry lesson to be learned here about these compounds, but I will spare you the pain. It isn’t necessary to go through the details just yet, except the fact that these acids have many different roles in the body and are involved in a variety of different functions.

Your overall intake of these acids, as well as the ratios between them, do have health implications and make studying it very complex.

Omega-3s can be obtained from your diet, and you may not need to use supplements. Fish such as salmon, mackerel, tuna, herring, and sardines, as well as other fortified foods including flaxseed, chia seeds, and their oils, can have plenty of omega-3s (72).

The American Heart Association has recommended having two servings of fish per week to meet this dietary requirement (35) but people still supplement anyway for all the proposed health benefits.

Is There Any Research?

Fish oil supplementation has been widely studied for a variety of different conditions. Quantifying it can be difficult, but a preliminary search on Pubmed, a website indexing medical research, reveals over 30,000 matches. A similar search on olive oil reveals about 10,000 matches. There is a significant amount of research to dive into here, and it will take some careful reading.

Does Fish Oil Improve Cardiovascular Disease?

The most popular claim about fish oil supplements might be that they can protect your heart. Manufacturers claim that fish oil supplements can help support your cardiovascular health and reduce the risk of things like heart attacks, strokes, and other problems.

Unfortunately, and despite some early supporting evidence, the most up-to-date and thorough investigation reported that fish oil supplements probably don’t do any of those things.

According to a Cochrane review from 2018 that looked at 79 different studies, there is moderate- to high-quality evidence that fish oil supplements do not prevent you from having heart attacks, strokes, or developing heart disease. They found that fish oil provided no protective benefits against dying from a cardiac event either.

However, there was some evidence that they can help reduce your triglycerides and raise your HDLs (good cholesterol) (1). These results are in line with multiple high-quality reviews that have come out since 2012 (9, 33, 84, 85, 98, 106),

Bottom Line

Fish oil supplements do not protect you from heart attacks, strokes, or heart disease, despite any manufacturer’s claim, but they might help improve your triglycerides and HDL cholesterol.

Does Fish Oil Lower Symptoms of ADHD?

Attention deficit hyperactivity disorder (ADHD) is associated with many symptoms including inattention, hyperactivity, impulsivity, and other health problems (39). While the exact cause of ADHD has not yet been identified, there are many potential causal factors that scientists have identified.

There is good evidence to suggest those with ADHD have lower levels of omega-3s in their body, and naturally fish oil supplementation has been suggested as a potential treatment (48).

A robust and thorough review from 2012 looked at 13 studies on PUFA supplementation for ADHD symptoms. In the studies, omega-6s were tested against placebo and dexamphetamine (a common ADHD medication), omega-3s were tested against omega-6s, and a combination of omega-3s and omega-6s were tested against placebo.

The researchers found small benefits in three studies for the omega-3 and omega-6 combination against placebo, but nothing else. They concluded the evidence up until then was insufficient to show that omega-3s specifically help with ADHD, but future evidence may change that (39).

An informal review from 2017 found that out of the 25 studies they identified, only half of them showed omega-3 supplements had benefits, and the other half did not. Variations in study design, study length, the number of participants, dosage, and type of supplements have made it difficult to draw any conclusions (2).

A pair of reviews from the same authors published in 2015 and 2016 failed to find any significant effects of omega-3 supplementation on those with ADHD. However, the 2016 study showed very small benefits on emotional lability and oppositional behavior (24, 25).

Three reviews from 2011 and 2014 reported finding small benefits, but no strong evidence of a meaningful effect (11, 48, 78).

Bottom Line

The current scientific evidence on fish oil supplementation for ADHD is mixed. Some studies report small benefits, while others fail to find anything worthwhile. We have no studies to point to that say fish oil actually works, but it may be worth a try.

Does Fish Oil Reduce Anxiety?

anxietySimilar to those with ADHD, people with anxiety and other psychiatric disorders have been shown to have decreased levels of omega-3s in their blood (59, 62, 93, 99). Naturally, fish oil supplementation was proposed as a potential treatment.

The initial animal studies looking at it showed promise (34, 76, 77, 75), as many reported the animal studies demonstrated decreased anxiety on a few specialized anxiety tests.

The human studies, however, were less clear. Four studies reported that supplementing with fish oil resulted in decreased anxiety compared to placebo in substance abusers (18), medical students (58), people who just suffered a heart attack (45), and undergraduate college students (102).

Another study from 2004 looked at the effects of six weeks of EPA (one type of omega-3 fatty acid) followed by placebo, or vice-versa for patients with obsessive-compulsive disorder. They found no effects on anxiety (38).

One smaller study on six patients with post-traumatic stress disorder found there were no beneficial effects of EPA at all, and some subjects reported their symptoms getting worse (103).

Bottom Line

There is a reasonable, plausible connection between fish oil supplements and anxiety, as well as promising animal and human trials. However, we need more robust trials, especially in those diagnosed with generalized anxiety disorder, to make any definitive conclusions. Fish oil may help with anxiety but we can’t be confident yet.

Does Fish Oil Reduce Symptoms of Alzheimer’s Disease?

Omega-3 fatty acids play a critical role in brain function and were thus a natural target for investigation of potential treatments for Alzheimer’s disease. Multiple reviews have found that those with Alzheimer’s disease and other causes of dementia have lower levels of fatty acids in their blood plasma and their brain (13, 17, 87).

However, experimental placebo-controlled trials where patients were actually given fish oil supplements in attempt to either prevent Alzheimer’s, slow its progression, or treat it, have generally failed to show any convincing benefits (22, 36, 75, 81, 91).

A few studies performed on patients with mild Alzheimer’s reported beneficial effects (22, 61, 70), although these studies had smaller numbers of participants.

A thorough meta-analysis was performed in 2016 to try and see if they could detect any beneficial effects by pooling the results of many different experiments. They found three high-quality placebo-controlled studies that found supplementation provided no effects cognitive function (17).

An earlier meta-analysis from 2012 reported that the three studies they analyzed did not show any significant benefits for preventing cognitive decline in healthy subjects (92). Two more recent reviews reinforced these general conclusions, but reported they were able to detect some small benefits for patients with mild Alzheimer’s disease (19, 87).

Bottom Line

The research on fish oil and Alzheimer’s disease is mixed overall, but the strongest studies we have right now suggest supplemental omega-3s do not have any preventive or curative effects.

Does Fish Oil Help Reduce Symptoms of Arthritis?

Due to their potential anti-inflammatory properties, fish oil supplements have been investigated for their effects on arthritis. The research in this area has focused on rheumatoid arthritis, which is an autoimmune disease associated with inflammation, joint damage, and pain, as well as osteoarthritis, a more common form that is also associated with joint damage and pain.

The research on rheumatoid arthritis has shown that fish oil supplements, in addition to standard medication, can help decrease joint pain and stiffness better than a placebo or control treatment. However, the results of this study were questionable because of the low quality of the available studies (40).

We have much less information on osteoarthritis. Despite some laboratory research, we just don’t have that many clinical trials available to determine if fish oil can help (20).

In a meta-analysis from 2017, the authors were only able to find five trials that looked at osteoarthritis, and were unable to come up with any solid conclusions because the quality of the research was so poor (90).

Bottom Line

Fish oil supplements might help with managing the symptoms of rheumatoid arthritis, but we need higher quality science to be sure. We don’t have enough trials on osteoarthritis to come to any conclusions at this point.

Does Fish Oil Help Prevent And Treat Various Cancers?

Most claims regarding the intake of a specific nutrient and the prevention or treatment of cancer are exaggerated, and the claims about fish oil are no different.

Let’s start here: there are no studies that definitively show that taking fish oil supplements can prevent or treat any type of cancer. However, many different types of studies have been performed and looked at many different aspects of cancer care.

Multiple reviews of large-scale observational studies have suggested it is unlikely that fish oil consumption is associated with a decreased risk of prostate cancer, (4, 7, 15) pancreatic cancer, (79), or cancer of any type (49, 105). However, two reviews found that fish oil supplementation was associated with decreased risk of breast cancer (15) and colorectal cancer (101).

Fish oil has also been studied for its potential effects in combating cachexia, the term for weight loss, loss of appetite, and general physical wasting associated with cancer treatments. One review from 2011 found that in the highest quality studies, fish oil supplementation provided no significant benefits (83).

Another article reported various other potential benefits for patients with gastrointestinal cancer (32). Laboratory studies on cells, as well as animal studies, have seen anti-cancer effects, and studies on humans have showed improved immune responses, lower complications rates, improved survival rates, and improved quality of life.

Another study on pancreatic cancer found that increased fish oil intake may be associated with better survival rates (65).

The literature on fish oil and cancer prevention and treatment is dense. The questions we want answers to are simply hard to study. Many different types of omega-3s, varying levels of exposure and dosages of omega-3s, numerous types of cancers, and different genetic predispositions and environmental factors make answering these questions very difficult. The science will continue to trickle in on the subject.

Bottom Line

We have no strong evidence to show fish oil supplements can prevent or treat cancer, but they may provide some nutritional benefits which need to be studied further.

Does Fish Oil Reduce Depression?

The literature on depression and fish oil continues to follow the broad trend: the results are mixed overall with some studies reporting no benefits above and beyond a placebo pill, with others hinting at small to modest benefits.

There is a plausible connection by which fish oil could help: subjects with depression had lower concentrations of EPA and DHA, as well as a higher ratio of omega-6 to omega-3s, when compared to people without depression (28). In addition, it has been reported that a lower intake of omega-3s from fish or supplements is associated with increased risk of depression (43).

An in-depth review from Cochrane, who regularly put out the best quality systematic reviews and analyses of studies, was performed in 2015. They found 26 total relevant studies, and after synthesizing and analyzing the results, reported that fish oil supplements did in fact have small benefits compared to placebo.

However, while the effects were present statistically, the effect size was so small it was of doubtful real-life relevance. Unfortunately, their estimates of the effects were imprecise due to poor research quality (6). Putting it plainly, fish oil supplements might work a little, but we just can’t be confident yet. Some reviews showed benefits (8), while others failed to find any (10).

A few reviews looked at depression associated with pregnancy and childbirth, termed perinatal depression. Again, the results were mixed. Two reviews found there were no benefits beyond placebo (12, 53), while another loosely-performed and less-stringent review found there were benefits for pregnant women or new mothers (51).

Complicating matters further, some researchers have suggested that some omega-3s may be better than others. Multiple analyses have reported that fish oil formulas that have higher concentrations of EPA work better (46, 59, 66, 67)

Bottom Line

We can’t confidently say fish oil supplements can help people with depression. We need more high-quality research here.

Does Fish Oil Reduce Risk Of Diabetics?

Those with type 2 diabetes who are looking for a supplement to help them manage their condition may be disappointed. The literature on the effects of fish oil supplementation on various aspects of diabetes management, like fasting glucose levels, HbA1c (a three month average of blood glucose), insulin levels, and the sugar spikes after eating, is clearly negative.

Multiple systematic reviews and meta-analyses have found that fish oil supplementation does not make any significant impact on these diabetic parameters (21, 52, 54, 69). Two reviews did report some small benefits (23, 73), but the majority of the literature points in the other direction.

Bottom Line

Fish oil supplements won’t help you control your diabetes.

Does Fish Oil Improve Eye Disorders?

Healthy EyesThe omega-3s in fish oil have also been proposed as a treatment for eye-related disorders and visual problems. Since DHA is a critical component of your eyes, supplementing with it could hypothetically provide health benefits.

Fish oil supplements have been proposed as a treatment for age-related macular degeneration (AMD), a condition associated with the wearing away of parts of the retina and subsequent vision loss. One thorough review of two high-quality placebo-controlled randomized trials found no evidence suggesting supplementation can prevent or slow the progression of AMD (60).

Despite these negative results,  an observational study found that those with higher intakes of omega-3s from seafood or supplements were at a lower risk for developing AMD (100).

Dry eye syndrome has also been investigated and the results have been mixed. In two collections of studies from 2014 and 2017, investigators found omega-3 supplementation was able to improve some symptoms of dry eye but not others (55, 68). Both reviews were unable to make any definitive recommendations.

Lastly, another major focus has been on visual development and visual acuity in infants and young children. Once more, the results were somewhat mixed. Some studies report mothers won’t see any benefits for their developing child (41, 64, 80), while some suggest benefits for supplementation during the first year (31, 89).

Bottom Line

The current evidence on fish oil supplementation and eye disorders is not strong enough to make any definitive recommendations. Some studies found benefits, while others reported none.

Does Fish Oil Boost Immune System?

Some preliminary studies on the effects of fish oil supplementation on the immune system have been performed. In the majority of the studies available on this topic, researchers measured the activity of different cells involved with the immune response.

Some studies have reported positive changes on these immune markers in various populations over various time periods (5, 26, 30, 42, 44, 63, 82, 86, 96), while one reported no changes (27)

Despite these interesting effects, few studies reported on actual clinical outcomes, i.e. if fish oil supplementation resulted in less people getting sick or developing infections. One study reported that in infants, there was no difference in the frequency in the development of eczema after supplementation (30).

Two studies found that there were no differences in infection rates between the group taking fish oil and the control group (82, 96).

Bottom Line

While fish oil supplements may have an effect on measures of immune function, there is no convincing evidence yet that they can help prevent you from actually getting sick.

Does Fish Oil Benefit The Skin?

Only a handful of studies have investigated the benefits of fish oil supplements on skin conditions. I was able to find two trials that investigated the effects on acne. One small trial with only 13 people and no control group reported that after 12 weeks, some people got better and some people got worse (57).

This study is too small to conclude anything, but another more in-depth study was performed in 2014. This one looked at 45 people with acne, and there was a comparison to a control group. The authors reported the omega-3 group beat out the control group and had less acne and inflammation (56).

Fish oil has also been studied for its potential anti-inflammatory effects on psoriasis, an autoimmune disorder resulting in inflammatory skin lesions. Unfortunately, there are no conclusive studies that definitively show beneficial effects on disease symptoms and severity (94).

Bottom Line

We don’t have enough positive research to confidently say fish oil can be helpful for skin conditions.

Does Fish Oil  Improve Fertility?

According to one 2018 study, in a sample of 501 couples trying to have a baby, about 13 percent of women and 9 percent of men were taking fish oil (74). Now, let’s imagine a study that could tell us if fish oil supplements can improve fertility. We would need to take a large group of infertile couples, give them either fish oil or a placebo pill, and see how successful each group ends up.

Unfortunately, I wasn’t able to find any study like this. The effects of fish oil on fertility simply hasn’t been studied that much. However, a few studies suggest that fish oil supplementation may have some positive effects. One study from 2012 found that asthenozoospermic men, or men who have sperm that cannot move as well as normal sperm, had lower levels of DHA.

In addition, they reported that fertile men had higher omega-3 content in their blood and sperm, as well as higher ratios of omega-3 to omega-6 (88). One analysis of three studies found omega-3 supplementation improved sperm motility, but not sperm concentration (50).

There is even less research on females. One study reported supplementing with omega-3s improved embryo morphology (47). Another study reported omega-3 supplementation may help to improve women’s reproductive lifespan and slow ovarian aging, although the authors note that this is very early research and must be investigated further (3).

Bottom Line

We have no direct evidence fish oil supplements can improve fertility, although there is some preliminary research that hints at some benefits.

Does Fish Oil Help in Weight Loss?

Like most supplements, fish oil has been suggested to help with weight loss. According to one review paper, the omega-3s in fish oil are supposed to help you burn more fat, expend more energy, improve circulation, and alter your metabolism (104).

In addition, a few animal studies have reported the addition of omega-3s has helped prevent excessive fat accumulation when the animals were on a diet designed to purposely increase their weight (16).

However, human trials have failed to generate anything exciting. Two meta-analyses looking at available placebo-controlled trials reported fish oil supplementation does not result in any weight loss benefits. Both trials reported other effects on some markers of health (29, 104).

If you are looking to lose weight, fish oil probably won’t help you. Losing weight requires addressing many different factors including your overall diet, exercise and activity levels, psychosocial factors, and behavioral change factors. Taking some fish oil capsules simply isn’t enough.

Bottom Line

The current evidence we have suggests fish oil supplements will not help you lose weight.

Is Fish Oil Safe?

Fish oil supplements are generally safe and typically well-tolerated. One review reported that in all of the studies they assessed, there were no serious or adverse events from taking them. Commonly reported side effects include a fishy aftertaste, gastrointestinal discomfort, nausea, diarrhea, and prolonged bleeding (98). 

You should consult a doctor prior to using fish oil if you have any questions.

Conclusion

Like many supplements, the claims surrounding fish oil are not really supported by the actual scientific evidence and are merely speculative, non-inclusive, or outright wrong. Fish oil supplements are unlikely to protect you from heart disease, diabetes, arthritis, or cancer, and they won’t help you lose weight.

Some of the evidence on depression, ADHD, anxiety, and Alzheimer’s disease is mixed but generally suggests there is not much to be excited about. Lastly, some claims just haven’t been studied enough. No one can confidently say fish oil supplements improve the health of your eyes, immune system, skin, or reproductive system.

Unless some better evidence emerges, you probably shouldn’t spend your money on fish oil.