Omega-3 in the Body

You have swallowed a high-quality omega-3 capsule — but do you know what actually happens in your body afterwards? The answer is surprisingly complex and largely determines how much of what you take actually reaches where it is needed. Bioavailability is the key word: not every form of omega-3 is absorbed equally well, and even with identical EPA and DHA contents on the label, the actual amount of active ingredient that reaches your cells can vary considerably. In this article you will learn how omega-3 fatty acids are digested, transported, incorporated into cell membranes, and ultimately used in biochemical processes — and why this is decisive for your choice of supplement.

Step 1: Digestion and absorption in the small intestine

The first contact with the digestive system begins in the stomach, where omega-3 fatty acids must be in a fat-containing environment to be optimally digested. The actual absorption then takes place in the small intestine — a multi-step process that is precisely coordinated.

As soon as fats enter the small intestine, the pancreas releases lipases. These enzymes cleave triglycerides — the main form in which fish oil is present — at positions sn-1 and sn-3, producing free fatty acids and a 2-monoglyceride. The released omega-3 fatty acids EPA and DHA are now in soluble form and can proceed to the next step.

At the same time, the liver secretes bile acids, which are released into the small intestine via the gallbladder. These bile acids are amphiphilic molecules — they have both water-soluble and fat-soluble components. They envelop the hydrophobic fatty acids and thereby form tiny structures known as micelles. These micelles transport the omega-3 fatty acids to the surface of the intestinal villi, where they are taken up by enterocytes (intestinal mucosal cells) via passive diffusion and specific fatty acid transporters.

This is also the reason why you should never take omega-3 on an empty stomach: without fat in the meal, almost no bile is secreted — too few micelles form, and absorption drops drastically. A study in the European Journal of Clinical Nutrition showed that the bioavailability of omega-3 is up to 50% higher when taken with a fat-containing meal than when taken without food (PubMed PMID 22665172).

Step 2: Transport in the blood

Within the enterocytes, the absorbed fatty acids and monoglycerides are reassembled into triglycerides. These are then packaged together with cholesterol, fat-soluble vitamins, and special proteins (apolipoproteins) into large transport particles — the so-called chylomicrons.

Chylomicrons are too large to enter blood capillaries directly. Instead, they first enter the lymphatic system and flow via the thoracic duct into the superior vena cava, where they empty into the main circulation. This explains why blood plasma can temporarily become milky and cloudy after a high-fat meal — a phenomenon called lipaemia.

In the bloodstream, the triglycerides of the chylomicrons are cleaved again by the enzyme lipoprotein lipase (LPL) at the vessel wall. The released fatty acids — including EPA and DHA — are taken up by surrounding tissues or repackaged into further lipoproteins. Omega-3 fatty acids then circulate in plasma embedded in lipoprotein fractions such as VLDL, LDL, and HDL, as well as in phospholipids, which serve as efficient carriers. The phospholipid-bound fraction in plasma is considered a particularly stable and well-utilisable transport pool.

Step 3: Incorporation into cell membranes

The actual destination of EPA and DHA is cell membranes — more specifically: the phospholipids of the lipid bilayer that surrounds every body cell. Phospholipids consist of a hydrophilic head group and two fatty acid tails. EPA and DHA are preferentially incorporated into the sn-2 position of these phospholipids, often replacing the omega-6 fatty acid arachidonic acid (AA).

This exchange is biochemically significant: DHA has six double bonds, which makes its carbon chain extremely flexible. Membranes rich in DHA exhibit greater fluidity — they are more dynamic, more permeable to ions, and enable more efficient signal transmission. This is particularly relevant in tissues that depend on rapid responses.

The highest omega-3 concentrations are found in the following tissues:

• Brain: DHA accounts for approximately 40% of polyunsaturated fatty acids in grey matter — learn more in our article on DHA and brain function
• Retina: DHA proportion in photoreceptors up to 60% — decisive for phototransduction
• Cardiac muscle cells: high concentrations of EPA and DHA stabilise the electrical potential
• Blood platelets (thrombocytes): EPA directly influences aggregation and inflammatory response
• Sperm cells: very high DHA concentration in the tail structure, essential for motility

Saturating cell membranes with EPA and DHA is not a rapid process. It takes weeks to months of regular intake before a measurable change in membrane fatty acid profiles can be detected.

Step 4: Omega-3 as a precursor of eicosanoids and resolvins

Omega-3 fatty acids are not merely passive structural components of membranes — they are also precursors of highly potent endogenous signalling molecules. These signalling molecules, collectively known as eicosanoids, regulate inflammatory processes, blood coagulation, vascular tone, and many other physiological processes.

EPA is converted by the enzyme cyclooxygenase (COX) into series-3 prostaglandins and by lipoxygenase (LOX) into series-5 leukotrienes. These compounds are considered less pro-inflammatory — or even anti-inflammatory — compared to the series-2 prostaglandins and series-4 leukotrienes derived from arachidonic acid (omega-6).

DHA is the precursor of another class of signalling molecules that has been intensively researched recently: resolvins (specifically the D-resolvin series) and protectins (also known as neuroprotectin D1). These substances are active mediators of inflammation resolution — they actively terminate inflammatory processes rather than merely suppressing them. This is a conceptually different mechanism of action from that of many anti-inflammatory drugs. In addition, both EPA and DHA inhibit the release of arachidonic acid from membrane phospholipids, which directly reduces the production of pro-inflammatory eicosanoids.

More on the background of this biochemical balance can be found in our article on the omega-3 to omega-6 ratio. For people who do not eat fish and use ALA as their primary omega-3 source, the low conversion rate to EPA and DHA is particularly relevant: why algae oil as a direct source is the better alternative is explained in the article Omega-3 for Vegans.

Triglyceride form vs. ethyl esters: which is more bioavailable?

Not all omega-3 products are alike — and this is not only due to the EPA/DHA content, but also to the chemical form in which the fatty acids are present. This difference has a considerable influence on actual bioavailability in the body.

Natural triglyceride form (TG)

In natural fish flesh and unprocessed fish oil, EPA and DHA are present as components of triglycerides. Lipases in the small intestine can cleave triglycerides very efficiently, since the enzymes have been evolutionarily designed precisely for this structure. The bioavailability of the natural TG form is good and serves as the reference. Non-concentrated fish oil (e.g. simple cod liver oil) is often in this form.

Ethyl esters (EE)

To achieve high EPA/DHA concentrations, fish oil is subjected to a concentration process industrially. The fatty acids are separated from glycerol and esterified with ethanol — ethyl esters are formed. This form is cheap to produce and allows very high concentrations in small capsules. The disadvantage: lipases cleave ethyl esters significantly less well than natural triglycerides, since the ethanol backbone is foreign to the digestive system. Studies show a 25–50% lower bioavailability compared to the TG form. Many affordable high-dose fish oil products use this form.

Re-esterified triglyceride (rTG)

The premium solution: after the concentration phase, the ethyl esters are enzymatically rebuilt onto a glycerol framework — a re-esterified triglyceride (rTG) is formed. The result is a highly concentrated formulation that mimics the biological structure of natural fish oil. A landmark study by Dyerberg et al. (2010) showed that rTG form has up to 73% better bioavailability compared to ethyl esters (PubMed PMID 20638827). rTG is the highest-quality and most bioavailable form — the price is correspondingly higher.

Phospholipid form (krill oil)

Krill oil contains EPA and DHA not as triglycerides, but directly as phospholipids — that is, in the same carrier form in which they are found in cell membranes. Phospholipids are naturally more water-soluble than triglycerides and can diffuse through the intestinal wall without the need for complex micelle formation. The bioavailability of krill oil is comparable to rTG and in some studies even slightly higher. Disadvantage: krill oil contains significantly less EPA and DHA per gram than concentrated fish oil, and the price is high.

Form	Bioavailability	Typical price	Found in	Special feature
Natural TG	Good (reference)	Medium	Unprocessed fish oil, cod liver oil	Low EPA/DHA content per ml
Ethyl ester (EE)	25–50% worse than TG	Affordable	Many high-dose capsules	Affordable, high concentration
rTG	Up to 73% better than EE	High	Premium products	Best bioavailability among fish oils
Phospholipids (krill)	Very good	Very high	Krill oil	Little EPA/DHA per capsule, contains astaxanthin

What this means for your purchasing decision is explained in detail in our buyer's guide: what makes good omega-3?

The omega-3 index: how is omega-3 status measured?

Whether you are actually getting sufficient omega-3 into your body can be measured concretely. The best-validated instrument for this is the so-called omega-3 index — a biomarker developed by William Harris and Clemens von Schacky and used in numerous epidemiological studies.

The omega-3 index indicates the proportion of EPA and DHA in the total fatty acid content of the membranes of red blood cells (erythrocytes) — expressed as a percentage. Erythrocytes are chosen because their fatty acid membrane remains relatively stable over the life of the cell (approx. 120 days) and thus reflects an average of the supply over the preceding 3–4 months — similar to how HbA1c reflects long-term blood glucose.

The reference values are:

• Below 4%: Low omega-3 status — risk zone, typical for a Western diet without fish consumption
• 4–8%: Middle range — supply is improvable
• Above 8%: Optimal range — associated with better cardiovascular and neurological markers in observational studies

Tests to determine the omega-3 index are available as home test kits (blood drop via finger prick) or can be requested from a laboratory through a doctor. Once you know where you stand, you can adjust your supplementation in a targeted way — and measure success again after 2–3 months.

How long until omega-3 takes effect? Timeline of accumulation

A common mistake: omega-3 is taken for a few weeks, no subjective effect is felt — and the supplement ends up back in the cupboard. This misunderstands the biochemical reality: omega-3 does not act acutely like a painkiller, but gradually changes the fatty acid composition of your cells.

The timeline divides roughly into three phases:

• Blood plasma (days): EPA and DHA are measurably elevated in plasma even after a single dose. The fasting level rises with regular intake within a few days, but is unstable and depends strongly on the timing of the last dose.
• Erythrocyte membranes (4–8 weeks): The omega-3 index rises with a latency of approximately one month and stabilises at a new level after 2–3 months of regular intake. This is the clinically relevant biomarker.
• Brain and deeper tissues (months to years): The DHA concentration in brain tissue changes very slowly. Animal experimental data and human studies show that complete tissue saturation of the central nervous system can take months to up to two years — which is why long-term studies are needed particularly for neurological endpoints.

This timeline explains why short-term clinical trials with omega-3 often show weaker effects than long-term studies. And it underlines: omega-3 supplementation is a marathon, not a sprint.

Omega-3 breakdown: what happens to excess EPA and DHA?

Not everything that is absorbed ends up in cell membranes or is processed into signalling molecules. A proportion of the circulating omega-3 fatty acids is subjected to so-called beta-oxidation in the liver and peripheral tissues — the central process of fatty acid combustion for energy production.

EPA and DHA provide considerably more energy per mole than saturated fatty acids, as they can be completely oxidised. However, the energy value is rarely the reason for their supplementation — unless you are in a caloric surplus in which the liver uses fatty acids for VLDL production.

Here lies a central mechanism of the triglyceride-lowering effect of high-dose omega-3: EPA and DHA inhibit the enzyme DGAT (diacylglycerol acyltransferase), which is responsible for triglyceride synthesis in the liver, and simultaneously increase beta-oxidation. As a result, the liver produces fewer triglyceride-rich VLDL particles and secretes fewer of them into the bloodstream. This mechanism is so well documented pharmacologically that high-dose EPA (e.g. icosapent ethyl) has been approved as a medication for triglyceride reduction in some countries — in Germany and the EU, on medical indication.

Intake tips for maximum bioavailability

Anyone who understands the biochemistry of omega-3 metabolism can significantly optimise their own supplementation. Here are the most important practical conclusions:

• Always take with a main meal — ideally with the fattiest meal of the day (lunch or dinner). Those who take it at breakfast without fat lose up to half of the potential bioavailability.
• Pay attention to the formulation — rTG form or phospholipid form (krill oil) offer significantly better absorption rates than ethyl esters. This should be stated on the packaging.
• Do not combine capsules with too many other fatty acids — very high fat quantities from other sources at the same meal can dilute the relative absorption of omega-3, since lipases are limited.
• Pay attention to oxidation protection — omega-3 oils with natural vitamin E (tocopherol) are more stable. Rancid fish oil (recognisable by an unpleasantly strong fishy smell) should not be taken.
• Regularity before high dosing — daily 1–2 g EPA+DHA over months is more effective for tissue saturation than sporadic high-dose intake.
• Measure the omega-3 index — only then will you know whether your supplementation is actually working.

More on the question of the optimal daily dose and who needs which amount can be found in the dosage overview.

Frequently asked questions

Further sources:

• Dyerberg et al. (2010): Bioavailability of marine n-3 fatty acid formulations — PubMed PMID 20638827
• Schuchardt et al. (2011): Importance of EPA and DHA — Food intake vs. supplements — PubMed PMID 22665172
• EFSA: Scientific Opinion on Dietary Reference Values for fats (2012)