VITAMINS & SUPPLEMENTS · GUIDE
What Are Fatty Acids? Saturated, Unsaturated, Omega Numbering and Essential Fats
What fatty acids are, how saturated, monounsaturated and polyunsaturated fats differ, what the omega numbers mean, and why only a few fats are called essential.
Content last updated . This page has not been medically reviewed.
Back to Omega Fatty AcidsThe question in one paragraph
Fats in food are built from fatty acids, which are chains of carbon atoms. They differ in length and in how many "double bonds" (places where the chain is bent) they contain. The NHS says a small amount of fat is an essential part of a healthy diet and that fat is a source of essential fatty acids, which the body cannot make itself. [1] All fats, saturated or unsaturated, provide the same energy per gram, 9 kcal. [1]How the terms are defined
A 2026 review in Frontiers in Nutrition groups fatty acids by double bonds: none means saturated, one means monounsaturated, and two or more means polyunsaturated. [2] The NHS adds that most fats and oils contain a mix of saturated and unsaturated fats in different proportions. [1]| Type | Double bonds | Typical foods named by the sources | Source |
|---|---|---|---|
| Saturated | None | Butter, palm and coconut oils, cheese, red meat; often solid at room temperature | [3][2] |
| Monounsaturated | One | Olive oil, rapeseed oil, avocados, some nuts | [1][2] |
| Polyunsaturated | Two or more | Salmon, vegetable oils, some nuts and seeds | [4][2] |
MedlinePlus lists trans fat with saturated fat among the less healthy fats. [3]
What the omega numbers mean
Polyunsaturated fats are split by where the first double bond sits: at the third carbon (omega-3) or the sixth carbon (omega-6) counted from the methyl end of the chain. [2] Oleic acid, the fat usually called omega-9, is written 18:1n-9 in the research we read, which means one double bond, so it is monounsaturated. [5] Writers also use "n-3" and "n-6" for the same thing, as in EFSA's opinions. [6]Which fats are essential
"Essential" means the body cannot make enough, so it must come from food. [4]- Linoleic acid (LA, omega-6) and alpha-linolenic acid (ALA, omega-3) are the shortest omega-6 and omega-3 fatty acids and are not made by the human body. [2]
- EFSA wrote that, strictly speaking, only LA is essential among the omega-6 fats, and that ALA is considered nutritionally essential because it is the precursor of EPA and DHA. [6]
- The body can convert some LA to arachidonic acid (ARA) and some ALA to EPA and DHA, but the 2026 review says these conversions occur in low quantities in humans. [2]
- Omega-9 oleic acid is not described as essential in the sources we read. See omega-9 and monounsaturated fats.
Evidence labels used in this library
Established nutritional role (the body needs it) applies to LA and ALA. Claims that go further, such as treating a disease, need their own evidence and are covered one by one in omega-3 evidence by use. Where research is thin we say "insufficient evidence"; where we found nothing usable we say "Proper evidence not found".Frequently asked questions
What is the difference between saturated and unsaturated fat?
Saturated fatty acids have no double bonds; monounsaturated have one; polyunsaturated have two or more. [2] Saturated fats are often solid at room temperature. [3]
What does "omega-3" mean?
It names where the first double bond sits: the third carbon from the methyl end of the chain. [2]
Are all polyunsaturated fats omega-3 or omega-6?
The NHS describes two main types of polyunsaturated fat, omega-3 and omega-6. [1] The sources we read do not discuss other polyunsaturated fat families, so we do not cover them here.
Why are some fats called "essential"?
Because the body does not make enough of them. MedlinePlus says you must get essential fatty acids from food. [4]
Can the body make EPA and DHA?
Partly. ALA can be converted to EPA and DHA, but only in low quantities in humans. [2] See ALA, EPA and DHA.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Limits and uncertainty
- Foods contain mixtures, so research on "a fat" is often research on a diet pattern.
- MedlinePlus says eating moderate amounts of polyunsaturated and monounsaturated fat in place of saturated and trans fat can benefit health. [4] The benefit comes from the swap, not from adding fat on top of an unchanged diet.
- Official intake numbers for essential fats differ by country and year. We tried to read US National Institutes of Health fact sheets and received an access error, so we do not quote US figures. For EU and UK figures see the table in Omega Fatty Acids Explained.
Safety and who should ask a clinician
Ordinary dietary fat is not a hazard for most people. Concentrated supplements are a different matter: ask a clinician or pharmacist before starting one if you take blood thinners or other medicines (medication interactions), are pregnant or breastfeeding, have a fish or shellfish allergy, have kidney or liver disease, or have surgery planned. A child or adult who has swallowed a large amount of any supplement should get help from Poison Help at 1-800-222-1222 (US), or 911 in an emergency.
References
- NHS, Fat: the facts. Read 2026-10-22.
- Berkowitz L et al. The controversial role of linoleic acid in cardiometabolic health: from molecular pathways to human studies. Front Nutr 2026. Read 2026-10-22.
- MedlinePlus, Facts about saturated fats. Read 2026-10-22.
- MedlinePlus, Facts about polyunsaturated fats. Read 2026-10-22.
- Hepatic oleate regulates liver stress response partially through PGC-1alpha during high-carbohydrate feeding (mouse study). J Hepatol 2016. PubMed. Read 2026-10-22.
- EFSA NDA Panel, Labelling reference intake values for n-3 and n-6 polyunsaturated fatty acids, EFSA Journal 2009;1176 (copy of the opinion hosted by the EFSA Focal Point Bulgaria; efsa.europa.eu returned an access error). Read 2026-10-22.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.