What is the difference between omega-3 and omega-6?

Omega-3 and omega-6 are both families of polyunsaturated fat. The difference is where the first double bond sits in the chain: 3 carbons from the methyl end in omega-3, 6 carbons in omega-6. The body cannot make ALA (an omega-3) or linoleic acid (an omega-6), so both have to come from food. In everyday Thai cooking, soybean, corn, sunflower and sesame oils are rich in omega-6, flaxseed and perilla seed oils are rich in ALA, and fish is the main food source of the omega-3s EPA and DHA.

This article explains how the two families differ, which oils and foods contain which, what Thai studies have found, and what is known about the ratio between them.

What are omega-3 and omega-6?

They are two families of polyunsaturated fatty acids (PUFAs), the kind of fat that has two or more double bonds in its chain.

The difference is the position of a double bond

The NIH Office of Dietary Supplements (ODS) explains that fatty acids are long chains of carbon atoms with a carboxyl group at one end and a methyl group at the other. Omega-3 fatty acids have a carbon-carbon double bond 3 carbons from the methyl end of the chain. Omega-6 fatty acids have it 6 carbons from that end. The "3" and "6" in the names refer to that position.

How the names are written

Fatty acids are often written as numbers: the number of carbons, then the number of double bonds, then the family. ODS gives the example of ALA, written C18:3n-3 because it has 18 carbons and 3 double bonds and belongs to the n-3, or omega-3, family.

Which fatty acids are omega-3 and which are omega-6?

The structures and families are from ODS, which names linoleic acid and arachidonic acid as two of the major omega-6s. Food sources are from ODS, the USDA data below and the Thai study of perilla seed oil.

Short and long chains

ALA and linoleic acid have 18 carbons. EPA, DHA and arachidonic acid are longer, with 20 or 22 carbons, and are called long-chain fatty acids. ODS explains that the body can turn ALA into EPA and then DHA, mainly in the liver, but only to a very limited extent, with reported rates of less than 15%.

Why do omega-3 and omega-6 have to come from food?

ODS explains that the human body can only form carbon-carbon double bonds after the ninth carbon from the methyl end of a fatty acid. Omega-3 and omega-6 fatty acids have their first double bond closer to the end than that, at the third or sixth carbon. The body therefore cannot make ALA or linoleic acid, and both are considered essential fatty acids that must be obtained from the diet.

England's NHS describes the same point in everyday terms: omega-6 is found in vegetable oils such as corn and sunflower oil, omega-3 in oily fish, and some of these fats cannot be made by the body, so small amounts are needed from food.

Which cooking oils contain more omega-6 and which more omega-3?

Every plant oil contains some of both, but in very different amounts. The figures below are from the US Department of Agriculture database (USDA FoodData Central), per 100 g of oil. In plant oils, the 18:2 fatty acids are mostly linoleic acid (omega-6) and the 18:3 fatty acids are mostly ALA (omega-3).

Most common cooking oils are much richer in omega-6 than in omega-3. Flaxseed oil is the exception in this table, with more ALA than linoleic acid. Palm and coconut oil have little of either, because most of their fat is saturated. For the saturated and monounsaturated fat in these oils, see How do cooking oils differ, and what fats do they contain?

Thai perilla seed oil and sesame oil

Perilla seed is known in Thai as nga khi mon. A Naresuan University study, published in 2026, cold-pressed perilla and sesame seeds from Tak and Mae Hong Son provinces and analysed the oils. Perilla seed oil was rich in polyunsaturated fat, mostly ALA (omega-3), while sesame seed oil had more monounsaturated fat and linoleic acid (omega-6). The ratio of omega-6 to omega-3 was about 0.29 to 1 in perilla seed oil, about 158.6 to 1 in sesame seed oil, and about 1.02 to 1 in a 50:50 blend of the two. The seeds came from two provinces, so oils from elsewhere may differ.

The same dish can therefore have a very different proportion of the two families depending on the oil used.

Nuts and seeds

Per 100 g in USDA data, walnuts have about 38.1 g of 18:2 and 9.1 g of 18:3 fatty acids, chia seeds about 5.8 g and 17.8 g, sesame seeds about 21.4 g and 0.4 g, and raw peanuts about 15.6 g of 18:2 with almost no 18:3. Chia seeds and walnuts are among the few foods with a large amount of ALA.

Which foods contain EPA, DHA and arachidonic acid?

The long-chain fatty acids come mostly from animal foods, and EPA and DHA mostly from fish.

Fish oils

The table shows USDA figures per 100 g of oil.

Fish oils are the reverse of most plant oils: rich in omega-3 (EPA and DHA) and low in omega-6. Salmon oil, for example, has about 13.0 g of EPA and 18.2 g of DHA per 100 g, and about 1.5 g of 18:2 fatty acids. EPA and DHA are still only part of the oil, which also contains saturated and monounsaturated fat.

Fish, eggs and chicken

Per 100 g cooked, Atlantic mackerel has about 0.50 g of EPA and 0.70 g of DHA in USDA data, while tilapia has about 0.13 g of DHA and very little EPA. A raw whole egg has about 0.19 g of arachidonic acid and 0.06 g of DHA per 100 g, and roasted chicken thigh meat about 0.09 g of arachidonic acid. For EPA and DHA in more fish per serving, see What is omega-3? How much EPA and DHA is in fish oil?

Does the fat in fish depend on what the fish eats?

Yes. ODS explains that the EPA and DHA in fish are made by microalgae at the base of the marine food chain and passed up to the fish that eat them, so the omega-3 content of a fish depends on its food.

Tilapia in Thailand

A study of tilapia in Thailand by the University of Stirling and the Asian Institute of Technology, published in 2006, compared the muscle of Nile tilapia and red tilapia from different farming systems and from natural waters. Wild fish and fish raised in the least intensive systems had higher proportions of ALA, EPA and DHA, higher ratios of omega-3 to omega-6, and lower proportions of linoleic acid. Intensively farmed fish had more fat, mainly saturated and monounsaturated fat and linoleic acid. The authors suggested replacing the vegetable oils high in linoleic acid in the feed of intensively farmed tilapia. The study looked at one species group, so it does not describe every fish sold in Thailand.

What about the ratio of omega-6 to omega-3?

The ratio is often mentioned, but there is no agreed figure.

What is known

ODS says some researchers have suggested looking at the relative intakes of omega-6s and omega-3s, the omega-6 to omega-3 ratio, but that the optimal ratio, if any, has not been defined. It adds that others have concluded such ratios are too nonspecific and do not reflect the levels of individual fatty acids. ODS also notes that the two families compete for the same enzymes in the body, so ALA and linoleic acid each slow the other's conversion.

In short, there is no recommended figure for the ratio.

Is there a daily amount for omega-3 and omega-6?

Not in Thailand. Neither omega-3 nor omega-6 is among the nutrients in the Thai Reference Daily Intakes (Thai RDIs), so there is no Thai daily figure for them. ODS also notes that the US Institute of Medicine did not set specific intake recommendations for EPA, DHA or other long-chain omega-3s.

The Thai daily reference for total fat

Omega-3 and omega-6 are types of fat. The Thai RDI for total fat is 65 g a day, based on 30% of the energy in a 2,000 kcal diet, and the value for saturated fat is 20 g. The NHS notes that every type of fat provides 9 kcal per gram.

How much fish do Thai people eat, and what do Thai studies show?

Fish and shellfish

A Ghent University study of Thai fisheries, published in 2021, put the current average Thai intake of fish and shellfish at 32 g a day, against 28 to 30 g a day in healthy and sustainable dietary recommendations. It is a national average based on supply data, not a measure of what individuals eat. For choosing and cooking fish, see I hardly ever eat fish. What can I eat instead?

DHA in breast milk differs by region

A study from Siriraj Hospital, Mahidol University, published in 2008, analysed breast milk from 40 breastfeeding women in Bangkok, Chanthaburi, Tak and Surin. DHA levels were higher in milk from mothers in Surin than in other areas, and the study found no correlation between the mothers' reported DHA intake and the DHA in their milk. The authors suggested that local diets and genetics might explain the differences. It was a small study of 40 women in four provinces.

If you are pregnant or breastfeeding, or take regular medicines, speak to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.