Not everyone gains the same heart benefits from fish oil

Fish oil has always had a good reputation. People take it hoping to keep their hearts strong, but the real story behind those omega-3s is more complicated.

A new Finnish study shows that one key fatty acid, eicosapentaenoic acid or EPA, behaves differently in every person. It’s a reminder that nutrition isn’t a single formula – it’s a personal equation written by metabolism.

Different fish oils reactions

Researchers at the University of Helsinki and the Wihuri Research Institute gave 38 healthy adults large doses of EPA for four weeks. The goal was simple: see how EPA changes blood fats in people without heart disease. The results, however, were anything but simple.

“The samples taken during supplementation clearly show the effect of EPA on all participants. Having said that, all samples were different among the group. In other words, each individual has a unique lipoprotein lipidome in their circulation, a ‘lipid fingerprint’, if you will, that persisted despite EPA supplementation,” said Professor Katariina Öörni.

Everyone’s body reacted, but no two reactions matched. That unique “lipid fingerprint” stayed stable even as EPA flooded the bloodstream.

Fast effects that are short lasting

EPA levels shot up quickly. Within one week, blood concentrations multiplied several times. Then, just as fast, they dropped once the supplements stopped. People who started with the lowest EPA levels saw the biggest jumps.

Even short-term use changed how fats behaved in the blood. EPA improved lipid balance and reduced the stickiness of lipoproteins – the fat-carrying particles that can latch onto artery walls. That’s important because once lipoproteins cling to arteries, they start forming dangerous plaques.

The study found that EPA didn’t just float through the system. It reshaped it. Lipid profiles shifted toward fewer saturated and omega-6 fats and more omega-3s. That balance is linked with better heart health and lower inflammation.

Fish oils change energy

The changes went deeper than blood tests. Inside the body, metabolism began to pivot. Glucose levels rose slightly, while lactate and ketone bodies fell – clear signs that cells were relying more on oxygen-driven energy rather than short bursts of anaerobic power.

This switch fits what scientists know about EPA’s influence on fat burning. The fatty acid activates certain genes that favor oxidation of fats and suppress fat creation. That shift helps the body clear triglycerides faster and lowers circulating fat levels.

EPA also triggered a fall in two key inhibitors of fat metabolism – apoC-III and ANGPTL3. When these molecules drop, fat breakdown speeds up. The result is cleaner, less congested blood chemistry.

Changes in blood fats

High-dose EPA didn’t just change how much fat was in the blood – it changed what kind of fat. Lipoproteins carrying harmful fats became less dense and less sticky.

LDL particles, often labeled “bad cholesterol,” contained fewer saturated lipids and more of EPA’s unsaturated type. HDL particles, the “good” ones, grew larger and richer in cholesterol, a feature linked with stronger artery protection.

EPA molecules also replaced inflammatory lipids in cell membranes. This swap affected how cells responded to stress and immune triggers. “It’s interesting to see how, for instance, dietary changes affect lipoprotein quality, or the individual lipid fingerprint,” Öörni notes.

EPA’s influence didn’t stop at metabolism. It disrupted inflammatory signals inside immune and vascular cells.

By competing with arachidonic acid, EPA pushed the body to make more calming compounds like resolvin E1. These molecules help tissues recover after inflammation and protect blood vessels from long-term damage.

Fish oils affect everyone differently

Not everyone saw the same benefits. Some participants’ lipid profiles improved dramatically, while others changed only slightly. That variation points to the role of genetics, metabolism, and baseline health in shaping nutritional outcomes.

“The findings highlight the importance of metabolism in the prevention of cardiovascular diseases. EPA’s effects varied more between individuals than we expected. We also demonstrated that these effects dissipate quickly, which is good to know in case EPA were to have adverse effects,” said doctoral researcher Lauri Äikäs.

The quick rise and fall of EPA levels mean its effects are strong but temporary. If supplements stop, the benefits fade. Yet this fast turnover may also protect users from unwanted reactions.

One diet doesn’t fit all

This study reinforces a simple truth: one diet does not fit all. EPA works, but how well it works depends on each person’s unique metabolism.

The researchers now plan to test how EPA affects inflammatory cells and the molecules that control immune balance.

Their work shows that the body doesn’t just absorb nutrients – it negotiates with them. Each person’s chemistry writes its own version of health. And in that story, metabolism, not medicine, may be the quiet guardian of the heart.

The study is published in the journal JCI Insight.

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