For about thirty years the advice was to eat less fat. It was wrong — or more precisely, it was too blunt to be useful, and the version that replaced it is more demanding but far more accurate: what matters is which fat.
Why your body needs fat at all
Fat is not merely stored energy. Every cell membrane you have is built largely from fat. Your body needs fat to absorb vitamins A, D, E and K, which do not dissolve in water. Fats are the raw material for hormones, and they insulate nerve fibres so signals travel properly.
Two fats are called essential in the strict nutritional sense: linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3). Your body cannot manufacture them, so they have to come from food. A diet with no fat is not a healthy diet.
The four types, and what each does
The difference comes down to chemistry — how many double bonds sit in the fatty acid chain — but the practical consequences are what matter.
| Type | Mainly found in | General effect on blood lipids |
|---|---|---|
| Saturated | Fatty meat, butter, cheese, cream, coconut and palm oil | Raises LDL |
| Monounsaturated | Olive oil, rapeseed oil, avocados, most nuts | Tends to lower LDL when it replaces saturated fat |
| Polyunsaturated | Oily fish, walnuts, flaxseed, sunflower and soybean oil | Tends to lower LDL when it replaces saturated fat; includes the essential fats |
| Trans (industrial) | Partially hydrogenated oils — now largely removed from the food supply | Raises LDL and lowers HDL |
Trans fat is the clearest case in all of nutrition science, which is why it was dealt with by regulation rather than advice: it moved both numbers the wrong way at once and offered nothing in return. The FDA determined that partially hydrogenated oils are no longer generally recognised as safe, and they have been phased out of the US food supply. Small amounts occur naturally in some meat and dairy; those are not the ones the concern was about.
The word that does the work: replacing
Notice that the middle two rows say “when it replaces saturated fat.” That is not padding. It is the finding that overturned the low-fat era.
When people cut saturated fat and replaced it with refined carbohydrate — the white bread, low-fat biscuits and sugary “diet” products of the 1980s and 90s — the benefit largely disappeared. When they replaced it with unsaturated fat instead, cardiovascular outcomes improved. The variable that mattered was not how much fat came out but what went in its place.
This is why current American Heart Association guidance is framed as substitution rather than subtraction, and why olive oil is not treated as something to ration.
Omega-3 and omega-6
Both are polyunsaturated and both are necessary. Omega-3s — particularly EPA and DHA from oily fish such as salmon, mackerel and sardines — are associated with lower triglycerides and are the reason fish appears in nearly every heart-health recommendation. ALA, the plant form in walnuts and flaxseed, converts to EPA and DHA in the body but inefficiently, so it is not a straightforward substitute.
Omega-6 is abundant in common cooking oils. You may have read that omega-6 is inflammatory and should be minimised; that claim is more contested than it is usually presented, and major reviews have not supported restricting it. Where there is broad agreement is on the value of getting enough omega-3, which most people do not.
Reading a label without the arithmetic
A nutrition label gives total fat and then breaks out saturated and trans fat. It usually does not list mono- and polyunsaturated separately — so the practical method is subtraction: a product with a high total fat figure but a low saturated figure is getting most of its fat from unsaturated sources, which is the direction you want.
The ingredients list is the other half. “Partially hydrogenated” is the phrase that identifies industrial trans fat, and a product can round its trans fat figure to 0 g per serving while still listing it. Since the FDA phase-out this is rare, but the ingredients list remains the reliable check.
Where to go next
- What cholesterol is and how your body uses it — the numbers these fats move.
- What sodium does in your body.
- Heart-healthy foods — the shopping end of all this.
- Reading nutrition labels.
Sources for this page: American Heart Association, FDA, National Institutes of Health, Harvard T.H. Chan School of Public Health and Mayo Clinic. General educational information, not medical advice — see our editorial policy.