Anyone interested in omega-3 will come across fish oil within minutes. Salmon, mackerel, capsules with a fish symbol on the bottle — the answer seems so obvious that the question behind it is hardly ever asked.
But it is worth asking. Fish don't make the fatty acids we eat them for themselves. They take them in. And they take them in along with everything else that drifts through the sea.
This article explains which three fatty acids are covered by the umbrella term omega-3, how much the body needs each day, what actually happens to plant-based omega-3 in the body — and why, in the end, the question of the source is more interesting than the number of milligrams.
Das Wichtigste in Kürze
- Omega-3 comes in two forms: one from plants such as flaxseed and walnuts, and one from the sea. The body mainly needs the one from the sea.
- From the plant-based form, the body can produce only a tiny fraction of the marine form on its own. Flaxseed oil alone therefore does not meet the requirement.
- Fish do not produce omega-3 themselves — they eat algae. Algae oil is therefore the original source, while fish oil is only the detour to get there.
- On the package, what matters is not how much oil is in the capsule, but how much omega-3 it contains. The guideline is 250 milligrams per day.
First, the essentials in simple terms
"Omega-3" is not a single substance, but a collective term for three fatty acids. They differ in where they come from and what the body does with them. That's all you need to know to start:
| Which one | Found in | What the body does with it |
|---|---|---|
| The plant-based one | Flaxseed oil, flaxseeds, chia seeds, walnuts, rapeseed oil | It's the raw material. The body can use it to make the other two — but not very efficiently. |
| The one for heart and circulation | Microalgae. It also ends up in fish through the food chain. | The body uses it to make a range of its own messenger substances. |
| The one for the brain and eyes | Microalgae. It also ends up in fish through the food chain. | It's a building block and is firmly embedded in the cell walls of the brain and retina. |
That leads to the two points this article is about: The two important ones originally come from algae, not fish. And: The body can barely make them from the plant-based form. Let's take a closer look at both.
The three abbreviations you'll see on every package
The nutrition label lists abbreviations, not descriptions. There are exactly three, and they correspond to the three rows above:
| Abbreviation | Stands for | This is |
|---|---|---|
| ALA | Alpha-linolenic acid | the plant-based one |
| EPA | Eicosapentaenoic acid | the one for heart and circulation |
| DHA | Docosahexaenoic acid | the one for the brain and eyes |
From here on, the article uses the abbreviations — just like the bottle on the shelf. Once you know what they mean, you can read any nutrition label without help.
What the body really does with plant-based omega-3
The conversion of ALA to EPA and then to DHA happens through a chain of enzyme steps. The first and slowest step is carried out by delta-6 desaturase — the same enzyme that also processes omega-6 fatty acids. So the two fatty acid families compete for the same bottleneck, and in a typical Western diet, omega-6 far outweighs omega-3. The final step to DHA also takes a detour through another part of the cell — making it considerably less efficient still.
In numbers, it looks like this:
| Conversion | Approximate rate |
|---|---|
| ALA → EPA | about 5 to 8 percent, tending to be higher in women of childbearing age |
| ALA → DHA | usually under 1 percent, barely measurable in some studies |
So the bottleneck isn't EPA, but DHA. What this means in everyday life becomes clear when you translate the numbers into tablespoons.
What that means in tablespoons
The figures for EPA still seem manageable: to get around 250 milligrams of EPA through conversion, you need roughly three to five grams of ALA a day. That's about a teaspoon of flaxseed oil — realistic if you remember to take it.
For DHA, the math doesn't work. With a conversion rate of around half a percent, you'd need about 50 grams of ALA to get 250 milligrams of DHA — every day. Flaxseed oil contains about 53 percent ALA, which works out to around 95 grams of oil, or about seven tablespoons. Every day. That would also add about 850 kilocalories from the oil alone.
This isn't a dietary recommendation, but a comparison of proportions. It explains why flaxseed oil, chia, and walnuts are excellent foods — and why they still don't meet your DHA needs. If you want DHA, you need to take it in, not rely on your body to make it.
Where fish get their omega-3
This is where things get interesting. Because the same is true of fish. Salmon, herring, and mackerel don't make significant amounts of EPA and DHA themselves either. They get them from their food.
At the start of this chain are microalgae. They are the actual producers of long-chain omega-3 fatty acids in the sea. Tiny plankton eat the algae, small fish eat the plankton, and bigger fish eat the small fish. With each step, DHA moves further up the chain — and eventually ends up on your plate.
Fish oil is therefore not the source of omega-3. It's an intermediate step. The fatty acid in salmon is the same fatty acid an alga produced weeks earlier.
What else comes along on the journey through the food chain
A food chain transports more than nutrients. Fat-soluble contaminants build up along the same route, increasing at every step — a process known as biomagnification. Three groups are particularly relevant:
- Methylmercury enters seawater through industrial activity and natural processes and binds to muscle tissue. Large, long-lived predatory fish at the top of the chain carry the highest levels.
- Dioxins and dioxin-like PCBs are fat-soluble and therefore accumulate in fatty tissue — exactly where the oil is found.
- Microplastics and the additives bound to them are the most recent addition to this list and have not yet been fully assessed.
Good fish oil is purified, usually by molecular distillation, and reputable manufacturers have it tested to ensure it meets the limits. The point isn't that fish oil is dangerous. The point is that it needs to be purified in the first place — because the oil comes from the middle of a food chain rather than its beginning.
Algal oil: the direct route
Algal oil skips the chain. It's obtained from microalgae, usually from the genus Schizochytrium, which are cultivated in closed tanks on a nutrient medium — much like yeast or yogurt cultures. The algae grow under controlled conditions and never come into contact with the open sea.
This has three straightforward benefits:
- No buildup of contaminants from the marine food chain. What has never been in the sea can't absorb anything from it.
- No fishing. Most of the world's fish oil comes from wild-caught small schooling fish such as anchoveta, sardines, and anchovies — species that play an important role as forage fish in the ecosystem.
- No fishy taste. The characteristic taste and fishy burps after taking fish oil capsules are mainly caused by oxidation products. Algal oil is made to be odorless.
The remaining question is whether the body handles one oil differently from the other. It doesn't: DHA molecules are identical, whether they come from a cultivated alga or from one that has passed through a fish. Studies of absorption likewise show that DHA from algal oil reaches the blood just as well as DHA from fish oil.
Algal oil and fish oil compared directly
| Algal oil | Fish oil | |
|---|---|---|
| Position in the chain | The source — the algae produce EPA and DHA themselves | Intermediate step — the fish take in both |
| Source | Cultivated in closed tanks | Wild-caught, mainly small schooling fish |
| Contaminants from the sea | No contact with the marine food chain | Purification required, usually by molecular distillation |
| Fatty acid profile | Usually higher in DHA | Usually higher in EPA |
| Taste | Neutral | Fishy once oxidation begins |
| For vegans | Suitable | Not suitable |
| Fish allergy | Contains no fish protein | Unsuitable for those affected |
For the sake of fairness, one point should be added: the two differ in their fatty acid profiles. Traditional fish oils usually provide more EPA than DHA, while many algal oils do the opposite. So if you're specifically looking for a high amount of EPA, check the label rather than the source — and if you want both, you can find algal oils that list both fatty acids.
What EPA and DHA do in the body
Unlike most plant compounds, omega-3 has reliable, verified health claims. They're tied to a daily amount, and that amount is the key measure when comparing products:
| Fatty acid | Claim | Minimum daily amount |
|---|---|---|
| EPA and DHA | contribute to normal heart function | 250 mg EPA and DHA |
| DHA | contributes to the maintenance of normal brain function | 250 mg DHA |
| DHA | contributes to the maintenance of normal vision | 250 mg DHA |
| EPA and DHA | contribute to the maintenance of normal blood triglyceride levels | 2 g EPA and DHA |
| EPA and DHA | contribute to the maintenance of normal blood pressure | 3 g EPA and DHA |
| ALA | contributes to the maintenance of normal blood cholesterol levels | 2 g ALA |
There is an additional claim for pregnant and breastfeeding women: the mother's intake of DHA contributes to the normal development of the brain and eyes in the fetus and breastfed infant. The amount is an additional 200 milligrams of DHA on top of the general recommendation.
The table shows two things. First, the threshold for most claims is 250 milligrams a day — that's the number that matters. Second, the table refers to EPA and DHA, not fish oil or algal oil. The claim applies to the fatty acid, not its source.
How much a day — and how to tell if you're deficient
As a general guide for adults, aim for a combined 250 milligrams of EPA and DHA every day. Pregnant and breastfeeding women should get an additional 200 milligrams of DHA. These amounts can be obtained through food: two servings of fish a week, one of them oily, will cover them on average.
The word "average" carries a lot of weight here. If you don't eat fish — because of the taste, your beliefs, or an allergy — you will practically never get this amount from food, because plant foods provide ALA and, as the calculation above shows, conversion to DHA isn't enough.
There's no simple self-test for deficiency. Symptoms such as dry skin, dry eyes, or difficulty concentrating are often linked to it, but they can have many causes and aren't proof. If you want to know for sure, you can have your Omega-3 Index measured — it measures the proportion of EPA and DHA in red blood cell membranes, giving a long-term reading over about three months. If symptoms persist, seek medical advice first, not a capsule.
Foods that contain omega-3
| Food | Fatty acid | Approximate amount per 100 g |
|---|---|---|
| Salmon, herring, mackerel | EPA and DHA | about 1.5 to 3 g |
| Sardines, anchovies | EPA and DHA | about 1.5 to 2 g |
| Flaxseed oil | ALA | about 53 g |
| Ground flaxseeds | ALA | about 20 g |
| Chia seeds | ALA | about 18 g |
| Walnuts | ALA | about 9 g |
| Rapeseed oil | ALA | about 9 g |
Two practical tips: flaxseeds need to be ground, otherwise they pass through the gut largely unchanged. And flaxseed oil should be used cold and stored in the fridge — it oxidizes quickly, and oxidized oil not only tastes rancid, it also loses its purpose.
How to recognize a good supplement
The options are overwhelming, but the criteria for choosing one are straightforward:
1. EPA and DHA, not the amount of oil
Many packages prominently display the amount of oil — for example, "1000 mg." That's the capsule's fill weight, not its omega-3 content. What matters is how much EPA and DHA it actually contains. This figure is on the nutrition label, and it's the only one you can compare with the 250 milligram target.
2. Protection against oxidation
Long-chain fatty acids are sensitive. A good supplement contains antioxidants such as rosemary extract, mixed tocopherols, or ascorbyl palmitate, and comes in light-protective glass rather than clear plastic. The TOTOX value is an established measure of freshness; a value of up to 26 is generally considered acceptable in the industry.
3. Traceable source
For algal oil, this means specifying which algae and whether they were cultivated or harvested. For fish oil, it means specifying the species, fishing area, and purification method. If neither is stated, there's usually a reason.
4. What else is in the capsule
Some supplements combine omega-3 with vitamin D — a sensible combination, since both are fat-soluble and can be taken together with a meal. Vitamin D contributes, among other things, to normal immune system function and the maintenance of normal bones.
How and when to take it
Omega-3 is fat-soluble, so take it with a meal that contains some fat. There's no evidence that taking it in the morning or evening makes a difference — what matters is taking it regularly, because levels in the cell membranes build up over several weeks. If you're expecting a change, think in months, not days.
Follow the serving size stated on the package. If you take blood-thinning medication or are due to have surgery, discuss taking higher amounts with your doctor beforehand: EPA and DHA can affect blood clotting. The same applies if you have an existing medical condition or are pregnant — omega-3 itself isn't the problem, but it's important to coordinate its use with your doctor.
How well omega-3 is tolerated
Omega-3 is well tolerated at typical amounts. Possible side effects include:
- Burping and aftertaste — common with fish oil and usually a sign that oxidation is beginning. Algal oil doesn't have the fishy taste.
- Digestive issues such as loose stools or a feeling of fullness, especially when taking larger amounts on an empty stomach.
- Longer bleeding time at high amounts — the reason to check with your doctor before surgery.
The European Food Safety Authority has assessed an additional daily intake of up to 5 grams of EPA and DHA as safe. Typical supplements contain far less.
In brief
- Omega-3 includes three fatty acids: ALA from plants, and EPA and DHA, which originally come from microalgae.
- The body can convert ALA, but not very well: about 5 to 8 percent becomes EPA, and less than 1 percent becomes DHA. Flaxseed oil alone cannot meet your DHA needs.
- Fish don't make EPA and DHA themselves — they take both in through microalgae. Fish oil is therefore in the middle of the food chain, while algal oil is at the beginning.
- Methylmercury, dioxins, and PCBs build up along the same route. That's why fish oil needs to be purified; cultivated microalgae have no contact with the open sea.
- The fatty acids themselves are identical. DHA from algal oil reaches the body just as well as DHA from fish oil.
- 250 milligrams of EPA and DHA a day is the benchmark for comparing supplements — not the amount of oil in the capsule.
