Omega 3: what fish oil really does to your blood values

Three tubs of fish oil on the shelf, all with 1,000 mg on the label and a little heart on the box. You grab one, take it for six months, and you have no idea whether anything happens. That is the problem with an omega 3 supplement: the effect is invisible unless you measure it.

My view, and the supplement industry won't love it: fish oil is a triglyceride tool, not a heart shield. The big trials are fairly clear about that. And your triglycerides are the value that tells you whether your bottle is doing anything.

Below you'll see what fish oil moves, what it doesn't, and how strong the evidence is per value.

What does omega 3 do in your body?

Omega 3 fatty acids are building blocks your body barely makes itself, so you get them from oily fish or from a supplement. EPA and DHA build into your cell walls and help steer inflammatory processes. They also help your liver push less fat into the bloodstream, and you can see that in your blood.

Three omega 3 fatty acids matter. ALA sits in plant sources like flaxseed and walnuts. EPA and DHA sit mainly in oily fish, and those two are the ones that show up on a blood test.

Your body can convert ALA into EPA and DHA, but it does so slowly. In men that conversion often stays below 10 percent. Skip fish entirely and flaxseed alone rarely lands you at the same EPA and DHA levels.

That's why fish oil and algae oil are far more popular than a spoon of flaxseed oil.

What does an omega 3 supplement do to your blood values?

An omega 3 supplement of roughly 1 gram a day mainly lowers your triglycerides, on average by about 15 percent (Cochrane, 2020). Your LDL may rise slightly. Your HDL barely moves and your blood pressure drops a few points at most, so honestly, don't expect much more than that.

The drop in triglycerides is the one effect the evidence really supports. A Cochrane review of dozens of trials kept seeing that drop at higher intakes (PMID 32114706). The higher your starting value, the bigger the drop usually is.

Picture two men, both on 1 gram of fish oil a day. One starts at triglycerides of 2.6 mmol/l and slides towards 2.2 mmol/l. The other starts at 0.9 mmol/l and hardly changes, because there's little left to lower.

That's the core of it. Fish oil does something for men with high triglycerides, and little for men whose values are already low.

Does fish oil lower your cholesterol?

Not really, and that's awkward news for every label with a heart on it. Fish oil barely lowers your total cholesterol, and your LDL may even rise slightly at higher doses and with DHA-rich oils. Your HDL stays roughly the same in most studies.

That slight rise in your LDL cholesterol isn't a measurement quirk. It's a consistent finding you see across several studies. The likely reason is that some of your triglyceride-rich particles get remodelled into LDL particles.

Your HDL cholesterol moves so little in practice that you'll barely notice it on your result. Sell fish oil as a cholesterol-lowerer and you're selling something it isn't.

Want to know when cholesterol in men asks for attention? Read our piece on cholesterol in men.

Does omega 3 protect your heart?

Based on the largest trials: not demonstrably, at roughly 1 gram a day. In the VITAL trial, 25,871 participants took fish oil or a placebo for years, with no difference in heart attacks and strokes (PMID 30415637). ASCEND did the same in 15,480 people with diabetes and also found no benefit (PMID 30146932).

These aren't small studies with an odd design. They're two of the largest randomised trials ever run on this question. Had there been a clear effect, they would have found it.

My take: the gap between what the label suggests and what the trials show is uncomfortably wide. A supplement is allowed to work modestly. It just shouldn't pretend to shield your heart.

The Hartstichting also points to eating fish rather than taking a pill. That fits exactly what the trials show. Which blood values do say something about heart risk in men is covered in our piece on cardiovascular disease in men.

How much omega 3 do you need per day?

The Gezondheidsraad keeps it simple: eat fish about once a week and you land around 200 mg of EPA and DHA a day. That's the amount with the best-supported health gain. Supplement trials mostly use 1 gram, roughly five times as much.

That gap is large, and it's exactly where the argument sits. At that higher dose you do see your triglycerides fall. Your heart attacks don't fall with them, as VITAL and ASCEND showed.

The Voedingscentrum translates that advice into practice. One portion of fish of about 100 grams a week, preferably oily fish. No pill, just food.

If you never eat fish, a supplement may partly close that gap. But treat it as an addition to your diet, not as insurance.

Which fish contains a lot of omega 3?

Oily fish is the richest source. Herring, mackerel, salmon and sardines deliver roughly 1 to 2.5 grams of EPA and DHA combined per 100 grams. Lean fish like cod or tilapia delivers far less, often under 0.3 grams, so it barely counts towards your omega 3.

A plain Dutch herring is one of the best sources you can buy here. Two herrings a week already put you well above the 200 mg a day the Gezondheidsraad works with. That costs you less than a month of fish oil capsules.

Farmed salmon usually carries decent amounts of EPA and DHA too, though it varies with feed and origin. Tinned sardines and mackerel are cheap and work just as well.

If you can get it off your plate instead of out of a tub, that's the simpler plan.

What is the difference between EPA and DHA?

EPA and DHA are both omega 3 fatty acids, but they don't do the same job. EPA mainly steers inflammatory processes and has the clearest effect on your triglycerides. DHA is a building block of your brain and retina, and appears to push your LDL up a little more than EPA does.

The label often only says "1,000 mg fish oil". That number tells you little, because what matters is the amount of EPA and DHA combined. That can range from 300 mg to 700 mg per capsule.

So turn the tub around and find the line with EPA and DHA. Work out your dose from that, not from the big number on the front.

For most men watching their triglycerides, an oil with relatively more EPA is the logical pick. A doctor or dietitian can help you decide what suits your situation.

Do you need an omega 3 supplement?

That mostly depends on one number: your triglycerides. If they run high, an omega 3 supplement may bring them measurably down. If they're already low and you eat fish weekly, you're mainly buying an expensive feeling of health, so measuring before you swallow is the honest answer.

High triglycerides in men often travel with belly fat, alcohol and a high sugar intake. Fish oil tackles the symptom, not the cause. How to tackle that cause and what you can track alongside it is covered in our piece on losing belly fat as a man.

Unsure which supplements make sense for you and which don't? Start with the overview in supplements for men.

To find out whether your tub is doing anything, look at your triglycerides, your LDL and your HDL. Those three values sit together in the Prime Lipids panel. Some men choose to have a baseline first, so they can later see whether anything has shifted.

Always discuss an unusual result with your GP, who can help you decide whether further steps are needed.

References

• Gezondheidsraad. Dutch dietary guidelines: fish and fish fatty acids. 2015. gezondheidsraad.nl
• Voedingscentrum. Fish and omega 3 fatty acids. 2024. voedingscentrum.nl
• Manson JE, et al. Marine n-3 fatty acids and prevention of cardiovascular disease and cancer (VITAL). N Engl J Med. 2019. PMID 30415637.
• ASCEND Study Collaborative Group. Effects of n-3 fatty acid supplements in diabetes mellitus. N Engl J Med. 2018. PMID 30146932.
• Abdelhamid AS, et al. Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews. 2020. PMID 32114706.

Every blood test result at Caliber includes a professional assessment by a BIG-registered doctor. For treatment decisions, always discuss your results with your GP.