The rest of this page covers what each form actually is, how they have been compared with each other, how each one tends to sit with people day to day, how to read a label that is trying to look better than it is, and what a blood level can and cannot tell you. This page is educational and not medical advice.
Fish oil is oil taken from the flesh of oily fish such as anchovy, sardine, mackerel and salmon, and it supplies EPA and DHA attached to a glycerol backbone as triglycerides, or as ethyl esters once the oil has been concentrated.
That second part is where most of the confusion lives. Oil pressed straight from fish is a natural triglyceride, the same chemical arrangement fat takes in food. To raise the share of EPA and DHA in a capsule, manufacturers strip the fatty acids off the glycerol backbone and bind them to ethanol instead, which produces an ethyl ester. Some products stop there. Others put the fatty acids back onto a glycerol backbone, which is why you see the term re-esterified triglyceride, often shortened to rTG on a label.
None of these are fake or inferior by definition. They are different chemistry, and the label is where you find out which one you are holding. A product that says only fish oil concentrate, with no mention of triglyceride, re-esterified triglyceride or ethyl ester, has not told you.
Fish oil is also the form most people encounter first, partly because it is the most widely produced and partly because it is the form used in most of the published comparisons between omega-3 products. It is a food-derived oil, so it carries the usual food-oil issues. It can oxidize with heat, light and time, and an oil that has gone off smells and tastes like it has gone off.
Krill oil is extracted from small shrimp-like crustaceans, and its distinguishing feature is that a large share of the EPA and DHA is bound to phospholipids rather than triglycerides.
Phospholipids are the same class of molecule your cell membranes are built from. They have a water-friendly head and fat-friendly tails, which means krill oil mixes into watery environments in a way that pure triglyceride oil does not. Krill oil also contains astaxanthin, the carotenoid pigment that gives the oil its deep red colour, and a portion of its omega-3 still sits in triglyceride form.
Two practical consequences follow. First, krill is a crustacean, so krill oil is not an option for people avoiding shellfish. Second, krill oil products are generally not concentrated the way fish oil concentrates are, so the amount of EPA and DHA stated on the label per capsule is the single most useful number on the package. This page does not state amounts, and it should not. The label states the amount, and a clinician decides what is appropriate for you.
Krill oil has been studied against fish oil more than any other alternative form. Research suggests that the phospholipid carrier influences how EPA and DHA appear in the blood compared with fish oil, and a published comparison of the two oils examined both bioavailability and health effects 1. That does not make krill oil a different nutrient. It is the same EPA and DHA arriving in a different package.
Algae oil is EPA and DHA grown directly from marine microalgae, which is where the omega-3 in fish comes from in the first place.
Fish do not manufacture EPA and DHA. They accumulate it by eating microalgae, or by eating smaller fish that ate microalgae. Growing the algae in controlled culture and extracting the oil skips the fish entirely, which is why algae oil (also called algal oil) is the option people on plant-based diets use, and why it has no fish in it to taste like fish.
The practical questions people ask about algae oil are whether it is really the same molecule and whether the body handles it the same way. On the first point, EPA and DHA are defined molecules, and algal EPA and DHA are those molecules. On the second, research suggests that DHA and EPA from microalgal oil reach the blood at levels comparable to fish oil in adults 3.
Algae oil products differ from each other more than fish oils do. Some supply mainly DHA, some supply both EPA and DHA, and the ratio is not standard across brands. If you are moving from a fish oil to an algae oil, the EPA and DHA lines on the two labels are the comparison that matters, not the total oil weight on the front of the bottle.
Direct comparisons between these oils have focused on how much EPA and DHA reaches the blood and on what happens to blood lipid markers, with a smaller number looking at clinical symptoms.
A published comparison examined the bioavailability of krill oil versus fish oil along with health effects 1. A network meta-analysis compared the lipid-modifying effects of krill oil and fish oil 2. A randomized, double-blind crossover study compared plasma EPA and DHA after a single administration of a phospholipid-enhanced fish oil product and a krill oil product 4. On the plant side, a 2025 study compared the bioavailability of DHA and EPA from microalgal oil and fish oil in adults 3.
Beyond blood levels, a randomized placebo-controlled double-blind trial compared fish oil and krill oil on clinical symptoms and cardiometabolic risk factors in people with major depressive disorder 5.
What comes out of that set is not a winner. It is a reasonable picture that all three forms deliver EPA and DHA into circulation, that the carrier molecule shifts how quickly and efficiently that happens, and that the differences between specific commercial products can be as large as the differences between categories. The product in front of you, the amount on its label and your own starting blood level matter more than the category name.
All three forms are absorbed, and the carrier molecule mainly affects how efficiently and how fast EPA and DHA show up in the blood rather than whether they show up at all.
Here is the mechanics in plain terms. Dietary fat is broken down in the small intestine with the help of bile and pancreatic enzymes, then packaged for transport. Triglycerides go through that process the way food fat does. Ethyl esters need the ethanol bond cleaved first, which is one reason ethyl ester products are commonly taken with a meal that contains fat. Phospholipids, because they have that water-friendly head, disperse differently in the gut before absorption even begins.
Research suggests that the phospholipid form in krill oil changes EPA and DHA bioavailability relative to fish oil 1, and a crossover study measured plasma EPA and DHA after acute administration of a phospholipid-enhanced fish oil product compared with a krill oil product 4. For algae oil, research suggests that microalgal DHA and EPA are delivered to the blood comparably to fish oil in adults 3.
The habit that helps every form is taking it with food that contains fat. That is not a schedule or a dose, it is how fat-soluble nutrients get absorbed. If you want to know what your absorption actually looks like over months rather than hours, that is a blood test question, covered further down this page.
Tolerability is the reason most people switch forms, and the complaints cluster around aftertaste, reflux and capsule size rather than anything clinical.
Fish oil is the one people most often describe as repeating on them, particularly on an empty stomach. Freshness matters here more than form, because an oxidized oil tastes rancid and is harsher on the stomach. Capsules stored in a hot car or a sunny windowsill do worse than capsules kept cool and dark. A fish oil that smells sharp or sour when you open the bottle has aged.
Krill oil capsules are usually smaller, the oil carries less of a fishy note for many people, and the capsules are often taken without a meal for that reason, although taking any omega-3 with food still supports absorption. The trade-off is the shellfish source and, often, a lower stated amount of EPA and DHA per capsule.
Algae oil sidesteps the fish taste entirely because there is no fish in it. Some algae oils come as liquids with flavouring, which suits people who struggle with capsules.
Flavoured liquid fish oils, enteric-coated capsules and freezing capsules before swallowing are all common ways people reduce aftertaste. Mild digestive upset, loose stools or a lingering taste are the usual reasons people stop. Anything more than mild, or anything that persists, is worth raising with a clinician rather than working around.
The goals studied split roughly into blood lipid markers, omega-3 blood status itself, and mood symptoms, with overlap between the three forms.
For blood lipids, studies suggest that krill oil and fish oil both have lipid-modifying effects, and the two were compared against each other in a network meta-analysis 2. For mood, fish oil and krill oil were compared on clinical symptoms and cardiometabolic risk factors in people with major depressive disorder in a randomized placebo-controlled trial 5.
For blood status, the measure most often used is the omega-3 index, the proportion of EPA and DHA in red blood cell membranes, and a higher omega-3 index is associated with cardiovascular risk differences 6. Blood levels of EPA and DHA are also associated with measures of brain structure and function 7.
Two cautions belong here. An association between a blood level and an outcome is not the same as a supplement producing that outcome in you, and no omega-3 supplement treats, cures or prevents a disease. The honest framing is that these oils raise EPA and DHA in the body, that EPA and DHA levels relate to several things researchers measure, and that a licensed provider is the person to decide whether a supplement fits alongside your other care.
The table below lines up the three forms on chemistry, what they have been compared on, and the practical points that decide most purchases. No amounts appear here, because the label and a clinician set those.
| Form | What it is | What research has looked at | Practical notes |
|---|---|---|---|
| Fish oil (triglyceride or re-esterified triglyceride) | EPA and DHA on a glycerol backbone, from oily fish | Bioavailability versus krill oil 1; lipid-modifying effects 2; symptoms and cardiometabolic risk factors in major depressive disorder 5 | Most widely available form. Fishy aftertaste is the common complaint. Freshness and storage matter. Not suitable for plant-based diets |
| Krill oil (phospholipid) | EPA and DHA largely bound to phospholipids, plus astaxanthin | Bioavailability versus fish oil 1; plasma EPA and DHA after acute administration versus a phospholipid-enhanced fish oil 4; lipid markers 2 | Crustacean source, so not for shellfish avoidance. Smaller capsules, less fishy for many people. Read the EPA and DHA lines, not the oil weight |
| Algae oil (algal) | EPA and DHA grown from marine microalgae | Bioavailability of DHA and EPA versus fish oil in adults 3 | Plant-based and fish-free. EPA to DHA ratio varies a lot between products. Often available as a flavoured liquid |
Read the back panel, find EPA and DHA listed as separate lines, and check that the form is named.
The front of a bottle usually states the weight of oil per capsule. That is not the omega-3 content. A capsule of fish oil contains EPA, DHA, other fatty acids and the oil they sit in, and only the supplement facts panel tells you how the first two break down. If EPA and DHA are not listed individually, or are rolled into a single omega-3 total alongside ALA, you cannot compare that product with anything else.
Four more things to look for:
Price per capsule tells you almost nothing. Price against the EPA and DHA on the panel is the comparison that holds up.
Anyone taking prescription medication, anyone with a bleeding or clotting condition, anyone scheduled for surgery or a procedure, people who are pregnant or breastfeeding, children, and anyone with a seafood or shellfish allergy should get a clinician's input before starting an omega-3 supplement.
Shellfish allergy is the clearest line, because krill is a crustacean. Fish allergy rules out fish oil. Algae oil is the form people turn to in both cases, though the finished product still needs checking for cross-contamination warnings.
The other situations are less about the oil being unsuitable and more about it not existing in isolation. Omega-3 fats influence how blood clots, which is why a planned procedure or an existing clotting condition is a conversation, not a guess. If you are already under care for anything, the person managing that care should know what you are adding.
This is also where the whole-picture question comes up. A supplement is one input. Diet, body composition, sleep, alcohol, training load and existing conditions all sit alongside it, and a licensed provider is the one who weighs them together. Discuss any supplement you are considering, and any lab result you are trying to interpret, with a clinician or licensed provider before acting on it.
Yes. The omega-3 index measures the combined share of EPA and DHA in red blood cell membranes, expressed as a percentage of total fatty acids, and it reflects intake over months rather than what you ate yesterday.
That is the point of using red blood cells. They turn over slowly, so their membrane composition averages your omega-3 intake across a long window and does not swing with a single fish dinner or a missed capsule. A higher omega-3 index is associated with differences in cardiovascular risk 6, and EPA and DHA blood levels are associated with measures of brain structure and function 7.
The Omega-3 Index is the marker to ask for, and it is a different test from a standard lipid panel. A cholesterol panel says nothing about your EPA and DHA status.
| Marker | Typical lab reference range | FitScript optimal range |
|---|---|---|
| Omega-3 Index (EPA + DHA as % of red blood cell fatty acids) | 6 to 12 % | 8 to 12 % |
Reference ranges vary by lab, age and sex, so read your own report against the range printed on it. FitScript's optimal range is an optimization target, not a diagnostic cut-off.
FitScript's membership includes 150+ biomarkers tested per year, with results interpreted by Atlas, FitScript's AI health advisor, and any treatment decision made by a licensed provider after reviewing the member.
A result is an observation about your current status, not a diagnosis, and the useful move is to read it in three steps.
What the value shows. The percentage tells you how much of the fatty acid content of your red blood cell membranes is EPA and DHA right now. It is a status marker, the same way ferritin is a status marker for iron stores.
What it may reflect. A result sitting below the optimal range may reflect low intake of oily fish, an omega-3 supplement that contains less EPA and DHA than the front of the bottle implied, inconsistent use, absorption that is not what you assumed, or a diet high in other fats that crowd the membrane. It may also simply reflect that you started supplementing recently and the membranes have not caught up.
What is worth discussing. A suboptimal result is a reason to look at intake, at the actual EPA and DHA on your label, and at whether anything is interfering with absorption. A result outside the standard lab reference range in either direction, or a result that does not move at all despite consistent intake, is worth discussing with a clinician. Never ignore a result that falls outside the range your lab prints.
Because red blood cells turn over slowly, a retest shows a change only after enough time has passed for membranes to remodel. Your clinician decides when to retest, and pairs the index with the rest of your panel rather than reading it alone.
Oily fish is the main dietary source of EPA and DHA, and it is the reason supplement questions often come down to how often you actually eat it.
The highest-supplying foods are salmon, sardines, anchovies, mackerel, herring and trout. Shellfish contribute smaller amounts. Canned sardines and anchovies are the cheapest route and keep in a cupboard, which matters more than most nutrition advice admits. White fish such as cod and haddock are lean and contribute far less.
Seaweed and marine algae supply omega-3 directly from the original source, although the amounts in culinary seaweed are modest compared with an algae oil product.
The common mix-up is flaxseed, chia and walnuts. These supply ALA, a shorter-chain omega-3, which the body must convert into EPA and then DHA. That conversion is limited in humans, which is why a spoonful of flaxseed is not interchangeable with oily fish or an algae oil, even though all of them are labelled omega-3.
One more thing worth knowing. Omega-3 content in farmed fish depends on what the fish was fed, so it varies between producers rather than being a fixed property of the species. Eating fish regularly is still the most straightforward way to raise intake, and a blood test is how you find out whether what you are eating is enough for you.
The catalog carries all three forms, so the choice is made on form, source and label rather than on what happens to be stocked.
No ranking is implied by that order, and no product here is positioned as better than another. Each is described by form and brand only. The EPA and DHA amounts are on each product's own label, and a clinician decides what is appropriate for an individual.
FitScript publishes this comparison of supplement forms. Brand names are the property of their respective owners.
Most people choose on constraints first, then on tolerability, then on what their blood level does.
Constraints narrow things fast. A plant-based diet or a fish or shellfish allergy points toward algae oil. Shellfish avoidance rules out krill. A swallowing difficulty points toward a liquid. Someone who has already tried a standard fish oil and could not get past the aftertaste often moves to a krill oil or an algae oil for that reason alone.
After constraints, the comparison is between labels, not between categories. Two fish oils can differ more from each other in EPA and DHA content than a fish oil differs from a krill oil. Find the EPA and DHA lines, check whether the form is named, and check whether anyone outside the company has tested the batch.
Then let the blood level settle the argument. The Omega-3 Index is the marker that reports what reached your cell membranes, which is the only part of this that is actually about you. People often choose a form on taste and convenience, stay consistent with it because it is tolerable, and retest later to see where their status sits. Consistency with a form you can live with usually matters more than the chemistry argument that brought you here.
If you want to know what checking your status involves before booking anything, the omega-3 index test cost page covers what the test is and how pricing works.
Neither is better for everyone. They supply the same two omega-3 fats, EPA and DHA, on different carrier molecules, and the choice usually comes down to allergies, diet, how the oil sits with you, and what the supplement facts panel actually lists. Krill is a crustacean, so it is not an option for anyone avoiding shellfish. An omega-3 index blood test is the part that tells you something about you rather than about the product.
It is the combined amount of EPA and DHA in your red blood cell membranes, reported as a percentage of total fatty acids. Because red blood cells turn over slowly, the result reflects your intake across months rather than what you ate in the last few days. It is a separate test from a standard cholesterol panel, which says nothing about omega-3 status.
Typical lab reference ranges run from 6 to 12 %, and FitScript's optimal range is 8 to 12 %. Ranges vary by lab, age and sex, so read your own result against the range printed on your report. The optimal range is an optimization target, not a diagnostic cut-off, and a clinician is the person to interpret your number alongside the rest of your panel.
Yes. Algae oil is EPA and DHA grown directly from marine microalgae, which is the original source that fish accumulate it from, so it contains no fish. The EPA to DHA ratio varies widely between algae oil products, so compare the EPA and DHA lines on the label rather than the total oil weight on the front of the bottle.
The amount is set by the product label and by a clinician who knows your situation, not by an article. What you can do yourself is read the supplement facts panel so you know how much EPA and DHA a serving actually contains, since that is often far less than the oil weight printed on the front. Bring the label and your blood result to the person managing your care.
The aftertaste usually comes from the oil itself or from oil that has started to oxidize. Taking it with a meal that contains fat helps, as does keeping the bottle cool and dark rather than on a sunny shelf. Flavoured liquids, enteric-coated capsules and freezing capsules before swallowing are common workarounds. An oil that smells sharp or sour when you open it has aged and is not worth finishing.
Many people can, if oily fish such as salmon, sardines, anchovies, mackerel or herring appears regularly in their week. Flaxseed, chia and walnuts supply ALA, a different omega-3 the body converts into EPA and DHA only in limited amounts, so they are not interchangeable with fish or algae oil. A blood test is the straightforward way to find out whether what you currently eat is enough for you.
That is a decision for the clinician handling your procedure, not something to work out on your own. Omega-3 fats influence how blood clots, so tell your surgical team and anyone managing your care what you are taking, including the brand and the amounts printed on the label, and follow the instructions they give you.
Treat it as an observation rather than a diagnosis. Look at how often you eat oily fish, check how much EPA and DHA your current supplement actually contains per serving, and consider whether you have been taking it consistently and with food. Then discuss the result with a clinician or licensed provider, who can read it alongside the rest of your panel and decide what, if anything, changes and when to retest.