Good Fats & Bad Fats

Fat is a word that is viewed negatively and is often associated with poor nutrition. Some people believe that eating fat means getting fat, that all saturated fats are bad, and polyunsaturated fats are good. The real picture is a lot different. Here, we’ve unravelled some of the issues that have been made unnecessarily over-complicated.

We need good sources of healthy fats for a number of body functions. Put simply, without fat, we die. Some fatty acids are also essential as our bodies can’t make them, so we must eat them instead.

Fats are triglycerides (TGs). Once you’ve eaten or drank, any calories your body doesn’t need in the short-term get converted to triglycerides. They’re made up of three fatty acids with a glycerol backbone. Each type of fatty acid differs by the number and position of double bonds in their structure, and it’s the type of fatty acid present in the TG that denotes the type of fat.

What are the main types of fats?

There are six main groups of fats:

  • Saturated fats
  • Monounsaturated fats
  • Omega-3 polyunsaturated fats
  • Omega-6 polyunsaturated fats
  • Trans fats
  • Medium-chain triglycerides (MCTs)

Saturated Fats

Saturated fatty acids (SFAs) have no double bonds in their structure. They’re most commonly found in meat and dairy products, but are also in some plant fats.

Monounsaturated Fats

Monounsaturated fatty acids (MUFAs), or omega-9 fats as they’re sometimes known, have one double bond in their structure. You can find high amounts in olive, rapeseed and flaxseed oils. Having more of these fats as a proportion of our total fat intake may help to reduce our blood LDL levels (bad cholesterol), whilst keeping our HDL levels (good cholesterol) high[1, 2], reducing the risk of heart disease[3, 4].

Polyunsaturated Fats

Polyunsaturated fatty acids (PUFAs) have more than one double bond in their structure. There are two types; omega-6 and omega-3 PUFAs. As a general rule, increasing levels of PUFAs in our diet has been shown to help reduce total blood cholesterol[5, 6]. However, this also means the good HDLs may be reduced.


Omega-6 fatty acids are found in a wide range of nut and seed oils, and in foods of plant origin. Due to their cheap cost of production, they can also be found in significant amounts in highly processed food products.


Omega-3 fatty acids are found in oily fish such as salmon, mackerel, sardines, pilchards, and trout. They can also be found in flaxseed and flaxseed oil, which is a great source for vegetarians and vegans. Read our Guide to EPA and DHA for more information.

Trans Fats

Fatty acids can also differ in structure at the point of the double bond, making them a trans fat. This changes the molecule’s appearance and behaviour. Trans fats are formed when liquid oils are converted into a semi-solid fat (hydrogenation). Often, they’re used in confectionery, margarines, and some heavily processed foods. These are the bad fats and we should limit their consumption.


The fats listed above are known as long chain triglycerides (LCTs). There’s another group known as medium chain triglycerides (MCTs). These are a lot rarer in nature, but are present in a few popular fats, such as coconut oil. MCTs are absorbed and metabolised differently to LCTs and are treated like an energy-dense carbohydrate source rather than a fat. MCTs are a type of saturated fat and you’ll find them under saturated on food labels.

Essential fat

There are two essential fatty acids (EFAs) that we need to include in our diet. These are linoleic acid (LA), an omega-6, and alpha-linolenic acid (ALA), an omega-3. There are also four conditionally EFAs. These are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), both are omega-3s, and arachidonic acid (AA) and gamma-linolenic acid (GLA), omega-6s. If we don't consume enough LA and ALA, they also become essential.

Fatty Acid Type of PUFA Rich dietary source
Linoleic acid (LA) Omega-6 Corn, safflower, sunflower, soyabean, peanut oils
α-linolenic acid (ALA) Omega-3 Flaxseed, soya, rapeseed oils
Arachidonic acid (AA) Omega-6 Small amounts in animal fats
γ-linolenic acid (GLA) Omega-6 Hemp seed, borage seed, evening primrose oils
Eicosapentaenoic acid (EPA) Omega-3 Oily fish
Docosahexaenoic acid (DHA) Omega-3 Oily fish

Oxidation and free radicals

So we can understand both good and bad fats, we need to understand oxidation. Oxidation is a naturally occurring process where a molecule reacts with oxygen and forms free radicals - this is where the molecule becomes potentially damaging to tissues.

Antioxidants are substances which help stop oxidation. These include vitamins C and E, some types of vitamin A, selenium, and phytonutrients like carotenoids and flavonoids. Eating a healthy diet rich in these substances will go a long way to help slow down oxidation.

Processed foods

The modern diet is high in overly processed junk food that are full of saturated fats, and contain large amounts of cheap vegetable oils. Some of these vegetable oils have been hydrogenated and are high in trans fats that result in poorer health outcomes. These vegetable oils are also high in processed omega-6s which have been damaged by oxidation. By consuming these, you’re consuming oxidised fats with a high risk of them damaging tissues. However, not all processed foods are bad, read our article to find out more.

How much fat should I consume?

The Dietary Reference Intakes for Japanese suggest that 20-30% of our energy intake should be from fats[7]. From this, no more than 7% should be from saturated fats[7]. None of the saturated fatty acids are essential, so we don't need them but they’re an excellent source of energy and shouldn’t be shunned.

The optimal ratio of omega-6:omega-3 is thought to be around 1-2:1[8]. However, Western diets have been shown to be as high as 6-20:1[8] and Japanese diets around 4:1[9]. Areas where the ratio is high, CVD rates tend to also be high, and in regions where the ratio is nearer to 1-2:1 the incidence of CVD tends to be significantly lower[8]. An omega-6:omega-3 ratio of 1-2:1 is suggested with total polyunsaturates being 10-12% of total energy[8]. Monounsaturates should make up the remainder of total energy.

Omega-3 Supplements

It’s quite rare to be consuming insufficient omega-6s, but it’s not uncommon for low intakes of omega-3s. If you don’t eat oily fish or flaxseeds, then supplementing with flaxseed oil or a specifically formulated oil blend might be a good idea. Cod liver oil supplements have been available for decades but offer little benefit in respect of fatty acid supplementation, as the doses of EFA are low. If you prefer to take a capsule fish oil supplement, check that it’s from a reputable brand first, as some cheaper, low quality brands haven't been treated properly and they may even be rancid (a term for an oxidised fat). Oils are very prone to oxidation and must be stored in cool, dark conditions. If they taste bad, they’re rancid.

Marine algae supplements also provide EPA and DHA, and ALA supplements are available.

Key Points

  • It’s fine to consume moderate amounts of saturated fat
  • Avoid trans fats
  • Include foods rich in monounsaturated fats
  • Include omega-6 polyunsaturates from natural sources; avoid them in junk foods
  • Increase your intake of omega-3 fats by eating oily fish, flaxseed oil or ground flaxseeds
  • MCTs are an efficient source of energy


  1. Grundy SM. Comparison of monounsaturated fatty acids and carbohydrates for lowering plasma cholesterol. N Engl J Med. 1986; 314(12):745-8.
  2. Ginsberg HN, et al. Reduction of plasma cholesterol levels in normal men on an American Heart Association Step 1 diet or a Step 1 diet with added monounsaturated fat. N Engl J Med. 1990; 322(9):574-9.
  3. Kris-Etherton PM, et al. High-monounsaturated fatty acid diets lower both plasma cholesterol and triacylglycerol concentrations. Am J Clin Nutr. 1999; 70(6):1009-15.
  4. Katan MB, et al. Should a low-fat, high-carbohydrate diet be recommended for everyone? Beyond low-fat diets. N Engl J Med. 1997; 337(8):563-6; discussion 6-7.
  5. Simopoulos AP. Omega-3 fatty acids in health and disease and in growth and development. Am J Clin Nutr. 1991; 54(3):438-63.
  6. Clarke R, et al. Dietary lipids and blood cholesterol: quantitative meta-analysis of metabolic ward studies. BMJ. 1997; 314(7074):112-7.
  7. MHLW. Dietary Reference Intakes for Japanese (2015). Tokyo, Japan 2018.
  8. Simopoulos AP. The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomed Pharmacother. 2002; 56(8):365-79.
  9. Hamazaki T, et al. The Japan Society for Lipid Nutrition recommends to reduce the intake of linoleic acid. A review and critique of the scientific evidence. World Rev Nutr Diet. 2003; 92:109-32.

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