A lot of my patients take fish oil. When I ask why, the answer is usually some version of "for my heart." When I ask how they know it's doing anything, there's usually a pause.
That pause is the reason for this post. Taking fish oil and having enough omega-3 in your body are not always the same thing. There's a simple test that can show where your level stands. When I ask whether fish oil is working, I mean whether it's actually raising the omega-3 level in your blood into a better range, not whether it's lowering your heart risk, which is a separate question the trials below look at. And the research on what that number means is more interesting, and more honest, than most supplement labels suggest.
What the omega-3 index measures
The omega-3 index is a simple blood test, often a finger prick, that shows how much omega-3 is in your red blood cells over the past few months.
It looks at the two omega-3 fats found mainly in fish, called EPA and DHA, as a share of all the fats in your red blood cell walls. Red blood cells turn over slowly, so the result reflects what you've been eating and taking for months, not what you had for dinner last night.
That makes it different from guessing based on the label. The label tells you what's in the capsule. The index tells you what ended up in you.
Same capsule, different result
Two people can take the same capsule and end up with very different levels. Body size, your starting level, the type of fish oil, and whether you take it with food may all play a part.
This has been studied directly. In a trial of 115 healthy adults given different daily amounts for about five months, the dose explained much of the change in their levels, but body weight, starting level, age, sex and activity also mattered (Flock 2013). In a pooled analysis of 14 trials with more than 1,400 people, the dose, the starting level and the chemical form of the supplement together explained only about 60 percent of the differences in how much people's levels rose (Walker 2019).
In other words, the pill count on your counter is a rough guide at best.
What a higher level is linked with
In large studies, people with higher omega-3 levels tended to live longer and have fewer heart problems. These studies show a link, not proof that raising your level will change your risk.
The encouraging part is how consistent this link has been. In a pooled analysis of 17 studies with more than 42,000 people followed for a median of 16 years, people in the top fifth for blood omega-3 levels had about 15 to 18 percent lower risk of dying from any cause, and about 20 percent lower risk of dying from heart and blood vessel disease, than people in the bottom fifth, after accounting for other risk factors (Harris 2021). In about 2,500 adults in the US Framingham Offspring study, average age 66 and free of heart disease at the start, those with the highest levels had about 39 percent fewer heart and blood vessel events over about seven years than those with the lowest (Harris 2018). A separate pooled analysis of 19 studies from 16 countries found that higher levels were linked with modestly fewer deaths from heart disease, but not clearly with fewer nonfatal heart attacks (Del Gobbo 2016).
I want to be careful here. People with higher levels also tend to eat more fish, eat better overall, and be healthier in other ways. Researchers adjust for that, but they can't fully separate the omega-3 from everything that travels with it. It's also worth knowing that some of this research comes from scientists connected to companies that sell omega-3 blood testing, and the papers disclose this. That doesn't make the data wrong. It's just part of reading it honestly.
What about the 8 percent number?
If you look up the omega-3 index, you'll see 8 percent come up a lot. Many experts use about 8% or higher as a goal range. This range came from population studies, and it has not been tested as a treatment target in clinical trials. It was first proposed in 2004 by two researchers, based on earlier study data (Harris and von Schacky 2004).
There's another catch. Results can vary from lab to lab, so it helps to compare your numbers from the same lab over time. Small differences in how labs run the test can make a big difference in the result, and the common ranges were set using one specific method (von Schacky 2020).
So I treat the number as information, not a grade. It's a useful starting point for a conversation, not a target you have to hit.
What the fish oil trials actually found
This is where the story gets more humbling. Most large trials of everyday fish oil did not show fewer heart attacks or strokes. A prescription form of purified EPA helped some high-risk patients, but that is a medical decision, not a supplement choice.
A few examples:
- VITAL followed nearly 26,000 US adults aged 50 and older without heart disease. Fish oil didn't significantly lower major heart events compared with placebo (Manson 2019).
- ASCEND, in more than 15,000 people with diabetes but no known heart disease, found no significant difference in serious heart and blood vessel events (Bowman 2018).
- STRENGTH tested a high-dose omega-3 mix in more than 13,000 high-risk patients and was stopped early because it wasn't showing a benefit (Nicholls 2020).
- OMEMI, in adults aged 70 to 82 after a recent heart attack, also found no significant benefit (Kalstad 2021).
The trial that did show fewer heart events, REDUCE-IT, used a prescription, purified form of EPA in high-risk patients who were already on a statin and had raised triglycerides (Bhatt 2019). Researchers still debate why its results differ from the others. One open question is whether the mineral oil used as the comparison in that trial affected the results (Ridker 2022). Either way, those results came from a prescription drug in a specific high-risk group, and they can't be assumed to apply to store-bought fish oil. The American Heart Association also says nonprescription fish oil supplements aren't meant for treating high triglycerides, which is something to manage with your clinician (Skulas-Ray 2019).
Why I favor medical-grade omega-3 when a supplement makes sense
Here's the encouraging part. The trials that did show fewer heart events had one thing in common: a high daily dose of purified, prescription-strength EPA, one of the two main omega-3s.
- REDUCE-IT gave 4 grams a day of prescription purified EPA (icosapent ethyl) to more than 8,000 high-risk adults on a statin with raised triglycerides. Over about five years, they had about 25 percent fewer major heart and blood vessel events (Bhatt 2019).
- JELIS, in more than 18,000 adults in Japan with high cholesterol who were taking a statin, found about 19 percent fewer major coronary events with 1.8 grams a day of purified EPA. Everyone knew who was getting EPA, and people in Japan already eat a lot of fish, so this is supporting evidence rather than the main proof (Yokoyama 2007).
Trials using about 1 gram a day (VITAL, ASCEND), or 4 grams a day of a prescription EPA and DHA mix compared with corn oil (STRENGTH), didn't show a clear benefit (Manson 2019; Bowman 2018; Nicholls 2020). A pooled analysis of 38 trials found larger reductions with EPA alone than with EPA and DHA (Khan 2021).
To be fair, REDUCE-IT has been debated. In its mineral oil comparison group, LDL cholesterol and the inflammation marker CRP rose, which may have made EPA look somewhat better (Ridker 2022), and researchers disagree about how much that matters (Doi 2021; Olshansky 2020). The trial's investigators have also reported that the benefit tracked with how much people's blood EPA levels rose (Bazarbashi 2020), although a similar look at STRENGTH didn't find that higher levels helped with the EPA and DHA mix (Nissen 2021).
What "medical grade" means to me
"Medical grade" isn't an official FDA category, and the FDA doesn't approve or test supplements before they're sold (FDA). To me, it means:
- Concentration. Enough EPA and DHA per capsule for a meaningful daily amount.
- Label accuracy and purity. Independent testing of content and contaminants such as mercury, PCBs and dioxins.
- Freshness. Oxidation, or rancidity, testing within voluntary industry limits for peroxide value, anisidine value and TOTOX (GOED Monograph).
- Manufacturing standards. Made in a facility that follows current Good Manufacturing Practices (cGMP), ideally confirmed by an outside audit.
- Third-party certification. Programs such as NSF, IFOS or USP Verified.
I feel strongly that medical-grade products give you a dose and freshness you can count on, and lower-quality fish oil sold straight to consumers often doesn't. In a US study of 47 supplements, more than 70 percent didn't contain the amount of EPA or DHA on the label, and EPA ranged from about 80 to 450 milligrams per gram of oil (Kleiner 2015). In a US analysis of 72 products, most flavored ones and about 1 in 8 unflavored went over the freshness limit (Hands 2024). Studies in Canada and New Zealand found similar problems (Jackowski 2015; Albert 2015), though industry-funded testing disagrees (Bannenberg 2017; Nichols 2016). How much oxidation affects health is still unclear (Albert 2013), but I want your supplement to have a label you can trust.
A similar dose, not the same drug
The trials that showed a benefit used about 4 grams a day. When our practice recommends an omega-3 supplement, we favor medical-grade concentrates that can provide a similar daily dose, usually taken as several concentrated capsules, and the right amount for you is something we decide together.
I match the formula to the person. Most of the time, I recommend a medical-grade EPA and DHA product, and for some people, high-dose EPA alone. The heart benefit was seen with high-dose purified EPA, while high-dose EPA and DHA (STRENGTH) didn't show it. So with an EPA and DHA product, the goal is raising your omega-3 index and overall omega-3 status, not repeating REDUCE-IT. For people who look more like those studied in REDUCE-IT, high-dose EPA is the closer match.
A similar dose isn't the same product. Supplements aren't reviewed by the FDA for purity or dose the way the prescription drug is, and they haven't been tested in heart outcome trials (Hilleman 2020). If you qualify for the prescription, a supplement isn't a substitute.
Depending on your situation, I recommend one of two paths: a medical-grade omega-3 supplement or prescription EPA. We decide which one fits you during your consultation.
This is where your omega-3 index comes in
Since two people can respond very differently to the same capsule, we check your omega-3 index before starting and after a few months. That shows whether your level is rising into a better range, which is what I mean by "working," though a better number alone doesn't prove lower heart risk.
Higher doses have trade-offs. In a pooled analysis of large trials, omega-3 supplements were linked with more atrial fibrillation, especially above 1 gram a day (Gencer 2021), and EPA alone with more bleeding (Khan 2021). So a high dose is a decision for you and your provider.
The side effects worth knowing
In clinical trials of people who had heart disease or a high risk of it, higher doses of omega-3 supplements raised the chance of an irregular heartbeat called atrial fibrillation. It is less clear whether the same is true for healthier people. Studies of blood omega-3 levels from everyday eating have not shown this link. If you have a heart condition or a heart rhythm problem, talk with your provider before taking higher doses.
Fish oil can also upset the stomach. Fishy burps are a common complaint, and in the STRENGTH trial, stomach and gut side effects were reported in about 1 in 4 people taking the high-dose omega-3, compared with about 1 in 7 on the comparison oil (Nicholls 2020).
If you take a blood thinner, have surgery coming up, or have a fish or shellfish allergy, talk with your provider before starting. Algae-based omega-3 is a fish-free option.
Food first
For most people, eating fish such as salmon or sardines once or twice a week is a good place to start. That lines up with the American Heart Association, which recommends one to two seafood meals a week, especially when seafood replaces less healthy foods (Rimm 2018).
Food also brings more than omega-3: protein, selenium, vitamin D and more. And the evidence for eating fish is separate from the evidence for taking pills. If you eat fish often, lower-mercury choices like salmon, sardines, anchovies, herring, trout and pollock are good options, especially during pregnancy and breastfeeding and for young children (FDA and EPA advice about eating fish).
Questions to bring to your provider
If you take fish oil, or you're thinking about it, these are worth asking:
- Would an omega-3 index test be useful for me?
- How should I read my result?
- Is a supplement right for me, and which kind?
- Does it interact with any of my medicines?
The bottom line
Taking fish oil isn't the same as knowing your level, and knowing your level isn't the same as knowing it will change your risk. The honest version is simpler: know your number, and talk with your provider about what it means for you.
Where to take this next
Already a 4Ever Young patient? Talk with your local clinic team. Bring the supplements you take and ask whether an omega-3 index test makes sense for you.
Not near a 4Ever Young clinic? Email drwilcox@precisionhormoneconsulting.com to request your free virtual consultation. Virtual consultations are available in Texas, Arizona, Florida, Oregon and Colorado.
Darrell Wilcox, MD | @wellnessdoc_4everyoung on Instagram
Wellness Doc · 4Ever Young
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This content is for educational purposes only and does not constitute medical advice. An omega-3 index result is one piece of information, not a diagnosis or a treatment target, and supplement decisions deserve individualized evaluation with a licensed clinician who knows your history and medicines. Do not start, stop, or change any medication or supplement without guidance from your treating clinician. Individual results vary, and no outcome is guaranteed. Dr. Wilcox is licensed to practice in Texas, Arizona, Colorado, Oregon, and Florida.
