Heart health
On fish oil? What your cholesterol results mean
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed October 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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Fish oil usually lowers triglycerides. The DHA in it can nudge LDL up a little, by about 0.19 mmol/L on average in trials. Doses above 1 gram of omega-3 a day are linked to a higher chance of an irregular heartbeat. Check LDL, ApoB and triglycerides before you start or stop, then again 8 to 12 weeks later.
Not sure which markers you need? Build your test →You took fish oil because it was meant to be good for your heart. Then you stopped, and you feel better. Or a cholesterol result came back higher than you expected. Either way, you want to know what is going on.
As of October 2026, this question is everywhere. A September 2026 briefing from Natural Medicines warned that doses over 1 gram a day are linked to a 25% to 37% higher risk of an irregular heartbeat. It also flagged LDL rises. This guide shows you how to read your own numbers.
1. What does fish oil do to my cholesterol?
It moves three numbers, and not all in the same direction. Fish oil carries two omega-3 fats, EPA and DHA. They behave a little differently.
| Marker | DHA | EPA |
|---|---|---|
| LDL cholesterol | Up, about 0.19 mmol/L | No clear change |
| HDL cholesterol | Up, about 0.12 mmol/L | No clear change |
| Triglycerides | Down | Down |
The figures come from a 2011 review of randomised trials. Triglycerides are the fats in your blood that rise with sugar, alcohol and refined carbs. LDL is the cholesterol carried in particles that can lodge in artery walls.
The triglyceride drop can be large. The American Heart Association says prescription doses of 4 grams a day cut triglycerides by 20% to 30% in people whose levels run high. Most shop capsules give far less than that.
2. How do I read my results on fish oil?
Look at the whole set, not LDL alone. A falling triglyceride with a small LDL rise is the usual picture.
- ApoB is a protein with one copy on each harmful particle, so it counts them.
- Non-HDL cholesterol is your total minus HDL, a wider view than LDL.
- HDL is the cholesterol carried back to your liver.
| Your pattern | What your data suggests |
|---|---|
| Triglycerides down, LDL up a little, ApoB steady | The common pattern. Fewer fats in the blood, particle count unchanged. |
| LDL and ApoB both up | More particles, not just fuller ones. Worth a talk with a clinician. |
| Triglycerides still high | Your dose may be low, or the cause lies elsewhere. Look at diet and alcohol first. |
ApoB settles the question when LDL moves. If ApoB holds steady, the particles carry more cholesterol but there are not more of them. Our guide to normal ApoB levels shows the UK ranges. Our cholesterol blood test guide covers the rest of the lipid profile.
3. Can fish oil affect my heart rhythm?
At higher doses, it seems so. Atrial fibrillation, or AF, is an irregular and often fast heartbeat. The UK medicines regulator updated its advice on prescription omega-3 in January 2024.
“Systematic reviews and meta-analyses of randomised controlled trials have highlighted a dose-dependent increased risk of atrial fibrillation.”
MHRA Drug Safety Update, January 2024
The risk was highest at 4 grams a day. A 2021 review of seven trials in 81,210 people found a 25% higher chance of AF overall. Above 1 gram a day, it was 49%. At 1 gram or less, the rise was small.
These trials mostly enrolled people with heart disease or its risk factors. The MHRA says it cannot judge the risk for healthy people taking shop supplements. So read this as a reason to watch your dose, not a reason to fear omega-3.
AF does not show up on a blood test. It needs an ECG, a recording of your heart’s rhythm. If your heart races, flutters or skips, see your GP, as the NHS advises. Our heart palpitations guide covers the blood tests that rule out other causes.
4. I stopped fish oil and feel better. Why?
The likeliest reason is your gut. The US Office of Dietary Supplements lists the usual side effects as mild ones. They include a fishy taste, heartburn, nausea, an upset stomach, loose stools and headaches.
Those fade fast once you stop. Taking capsules with a meal often helps if you want to carry on.
Check your real dose too. The 1,000 mg on the front of the tub is the oil, not the omega-3. Add up the EPA and DHA lines on the back, then multiply by the capsules you take each day. That total is the number the AF studies use.
5. When should I test?
Twice. Once before you start or stop, so you have a baseline. Then again 8 to 12 weeks later, once your levels have settled.
Triglycerides and LDL read most accurately after a 9 to 12 hour overnight fast. Our fasting blood test guide explains how to prepare.
Food is still the simplest source. The NHS suggests at least 2 portions of fish a week, including 1 of oily fish. NICE tells clinicians not to offer omega-3 capsules to lower heart risk, with one prescription exception. If your triglycerides are high, our guide to lowering triglycerides starts with diet and alcohol.
6. Which test reads all of this?
A standard NHS cholesterol check gives you the lipid profile. It usually leaves out ApoB. Our Advanced Heart Health panel (£159) reads LDL, HDL, triglycerides and non-HDL with ApoB, Lp(a), hs-CRP and HbA1c. That is the full set to compare before and after.
A blood test measures biomarkers. It offers wellness insight, not a diagnosis. The heart health hub has the rest of our heart guides.
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Helvy’s home blood tests report your heart markers with plain-English context, so you can see what a supplement is doing. Samples are processed by UKAS-accredited UK labs.
Frequently asked questions
Does fish oil raise LDL cholesterol?
It can, a little. In a 2011 review of trials, DHA raised LDL by about 0.19 mmol/L against placebo, while EPA showed no clear change. Triglycerides fell with both.
Does fish oil cause atrial fibrillation?
Higher doses are linked to it. A 2021 review of seven trials found a 25% higher chance of AF overall, and 49% at doses above 1 gram of omega-3 a day. Most people in those trials already had heart disease or its risk factors.
How much fish oil is too much?
The AF signal is strongest above 1 gram of EPA plus DHA a day, and highest at 4 grams. Check the EPA and DHA lines on your label, not the total oil figure on the front.
Why do I feel better after stopping fish oil?
Usually because the mild side effects have gone. These include a fishy taste, heartburn, nausea, an upset stomach and loose stools. They tend to settle quickly after you stop.
Should I check my cholesterol before taking fish oil?
It helps. A baseline of LDL, ApoB and triglycerides, then a repeat 8 to 12 weeks later, shows what fish oil is doing for you. A 9 to 12 hour fast gives the most accurate readings.
Can a blood test show atrial fibrillation?
No. AF is a heart rhythm problem and needs an ECG. Blood tests can rule out other causes of palpitations, such as thyroid or salt imbalances. See your GP if your heart races or skips.