Dietary Fats: Good vs Bad for Your Heart
- , by SANUSq Research team
- 9 min reading time
For decades we were told to fear fat. The science has moved on — and it turns out the type of fat matters far more than the amount.
Rethinking dietary fat: what the science actually says
High levels of LDL cholesterol, triglycerides, or both are among the leading risk factors for cardiovascular disease. The standard advice — lose weight and cut saturated fat — still holds. But how you cut saturated fat turns out to matter enormously, and it's central to the debate over dietary fats good vs bad.
For years the default was a low-fat, high-carbohydrate diet. Yet the evidence increasingly favours replacing saturated fat with unsaturated fat rather than with carbohydrate — the pattern embodied by the Mediterranean diet.
In people at high cardiovascular risk, a Mediterranean diet supplemented with extra-virgin olive oil or nuts reduced the incidence of major cardiovascular events compared with a reduced-fat diet (Estruch et al., 2018).
The four families of dietary fat
Talk of good and bad fats is shorthand for four broad families, each behaving differently once eaten.
- Saturated fats — solid at room temperature, and found mainly in butter, fatty meat, full-fat dairy, coconut and palm oil. They tend to raise LDL cholesterol, which is why mainstream guidance still asks most people to moderate them.
- Monounsaturated fats — the dominant fat in olive oil, rapeseed oil, avocados and most nuts, and the backbone of the Mediterranean pattern.
- Polyunsaturated fats — the family containing the omega-3 and omega-6 fatty acids the body cannot make for itself. Oily fish, walnuts, flaxseed and sunflower seeds are the usual sources.
- Trans fats — largely industrial, created by partially hydrogenating liquid oils to make them solid and stable. This is the one group with no redeeming case: they raise LDL and lower HDL at the same time, and their use in food has been restricted in many countries.
Only the last of those is unambiguously bad. The rest sit on a spectrum, which is why the question is rarely settled by a single number on a nutrition panel.
Why what replaces the fat matters
The reason the old low-fat advice disappointed is that it said nothing about what filled the gap. Take butter out of a diet and those calories come from somewhere — and for many households they came from refined starch and sugar. Swapping saturated fat for refined carbohydrate does little for LDL cholesterol and can push triglycerides up and HDL down, which is close to the opposite of the intention. Swapping the same saturated fat for olive oil, nuts or oily fish is a different proposition altogether, and it is the version of the advice the Mediterranean evidence above actually tested.
What a high-fat, walnut-rich diet did to cholesterol
One striking illustration of "not all fat is equal" comes from a one-year weight-loss study of overweight and obese women, who followed one of three eating patterns: low-fat and high-carbohydrate; low-carbohydrate and high-fat; or a walnut-rich, high-fat, low-carbohydrate diet. The headline findings:
- Similar weight loss across all three plans.
- The low-fat diet produced the most weight loss in insulin-resistant women but did little for their blood lipids.
- The walnut-rich diet — high in polyunsaturated fat — improved cholesterol the most, with a fall in LDL ("bad") cholesterol and a greater rise in HDL ("good") cholesterol than the other diets.
In other words, a diet high in the right fats lowered lipids while still achieving weight loss comparable to the low-fat approach. It's a neat rebuttal to the idea that a high fat diet effects are automatically bad for the heart — the composition is what counts.
What makes walnuts so heart-healthy?
Walnuts stand out among nuts. Where most are rich in monounsaturated fat, walnuts are packed with polyunsaturated fat — and they're the only nut with a high level of alpha-linolenic acid (ALA), a plant-based omega-3 known for heart and anti-inflammatory benefits. ALA is linked to reduced clotting, lower inflammation and better brain health.
Acute consumption of walnuts and walnut oil favourably affected post-meal blood fats, endothelial function and cholesterol handling in adults with mild hypercholesterolaemia (Berryman et al., 2013).
Beyond lowering LDL, walnuts appear to protect the endothelium — the lining of the blood vessels — and to help HDL do its job of clearing excess cholesterol. Researchers credit the combination of ALA, gamma-tocopherol and plant sterols. Taken together, the healthy fats and bioactive compounds in walnuts lower LDL, improve blood-vessel function, ease blood pressure and reduce inflammatory markers — chipping away at heart-disease risk from several directions at once. Reason enough, perhaps, to add a daily handful.
Not keen on walnuts — or after a more concentrated plant source of the same omega-3? Sacha inchi oil is one of the richest plant sources of alpha-linolenic acid (ALA), the very fatty acid that makes walnuts stand out.
How to put this on your plate
None of this requires a special diet. A handful of ordinary habits covers most of it:
- Cook with unsaturated oils. Extra-virgin olive oil for dressings and gentle cooking, and a more heat-stable oil where the pan gets hot.
- Eat fish that swims in cold water. Salmon, mackerel, sardines and herring carry the long-chain omega-3s EPA and DHA.
- Keep nuts and seeds within reach. A small handful in place of a biscuit changes the fat profile of the day without changing the calories much.
- Read the ingredients, not just the panel. "Partially hydrogenated" oil anywhere in an ingredients list is a trans-fat flag.
- Moderate rather than banish. Butter and cheese need not disappear; the aim is that they stop being the default source of fat in the diet.
What this does not mean
A high-fat diet is not automatically a healthy one, and "unsaturated" is not a licence for unlimited quantity. Fat is the most calorie-dense nutrient we eat, so a diet generous in oil and nuts still has to fit the energy needs of the person eating it. The walnut study was a weight-loss trial in overweight and obese women: it tells us that the composition of a reducing diet changes its effect on blood lipids, not that piling fat on top of an unchanged diet would do the same.
Two further caveats are worth keeping in view. ALA, the plant omega-3 in walnuts, is converted to the longer-chain EPA and DHA only inefficiently in the body, so plant sources and oily fish work best as complements rather than substitutes. And blood lipids are one risk marker among several — blood pressure, blood sugar, smoking, activity and weight sit alongside them. If your cholesterol is being managed medically, dietary changes belong inside that conversation rather than instead of it.
Frequently asked questions
Which dietary fats are good vs bad?
Broadly, unsaturated fats — found in olive oil, nuts, seeds and oily fish — support heart health, while trans fats are harmful and saturated fats are best kept moderate. The most useful shift isn't simply "less fat" but replacing saturated fat with unsaturated fat rather than refined carbohydrate.
Are high-fat diets bad for you?
Not inherently. The high fat diet effects depend heavily on the type of fat: a diet high in healthy unsaturated fats, as in the Mediterranean pattern, is associated with better cardiovascular outcomes, whereas one high in saturated and trans fats is not.
Is saturated fat really that bad for your health?
The picture around saturated fat health is more nuanced than the old "all fat is bad" message, but mainstream evidence still supports moderating it — and crucially, replacing it with unsaturated fat rather than sugar or refined carbohydrate for the best effect on blood lipids.
What are trans fats, and are they still in food?
Trans fats are mostly industrial, produced by partially hydrogenating liquid oils so they behave like solid fat. They raise LDL cholesterol and lower HDL at the same time, which is why their use has been restricted in many countries; small amounts also occur naturally in dairy and meat. Scanning an ingredients list for "partially hydrogenated" oils is the simplest way to spot the industrial kind.
How many walnuts should I eat a day?
Studies often use around 30–56g a day — roughly a small handful. Because they're calorie-dense, they're best worked into your overall intake rather than added on top, ideally in place of less healthy snacks.
A plant-based source of omega-3 ALA
Sacha inchi is one of the richest plant sources of alpha-linolenic acid (ALA) — the same omega-3 that makes walnuts so heart-friendly.
References
- Estruch R, Ros E, Salas-Salvadó J, et al. Primary Prevention of Cardiovascular Disease with a Mediterranean Diet Supplemented with Extra-Virgin Olive Oil or Nuts. N Engl J Med. 2018;378(25):e34. PMID 29897866
- Berryman CE, Grieger JA, West SG, et al. Acute consumption of walnuts and walnut components differentially affect postprandial lipemia, endothelial function, oxidative stress, and cholesterol efflux in humans with mild hypercholesterolemia. J Nutr. 2013;143(6):788–94. PMID 23616506
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