Choosing which type of oil or fat to use when cooking can feel pretty confusing as there are a lot of mixed messages in the media about this. To answer this question fully we need to look at the overall evidence base and weigh up a few different things, from oxidation and smoke points to the type of fats and antioxidants found in different oils.

How Heat Affects Oils

Have you ever had a bottle of oil in your kitchen which has turned dark or cloudy, smelled stronger than usual and tasted funny? This happens when light, heat and air causes oil to oxidise or breakdown which turns the oil rancid, this can also produce aldehydes and destroy vitamin E and vitamin K¹. Aldehydes are compounds which have been linked with diseases such as: heart disease, cancer and dementia but there is little solid evidence that they cause harm in the human diet^2-4.

Some people say that you shouldn’t use vegetable oils for cooking as heating these oils produces trans fat, a harmful type of fat which is associated with an increased risk of heart disease. This is only half true; if vegetable oil are heated to a very high temperature this can form small amounts of trans fat, but it is not possible to form significant amounts of trans fat in home kitchens as hydrogen also needs to be present. So unless you own an industrial kitchen where you can heat a vegetable oil so much that it begins to harden, then there is little chance of producing much trans fat when cooking at home⁴.

Some vegetable oils such as olive oil, rapeseed oil and sunflower oil contain vitamin E and polyphenols which can protect the oil from oxidation; this makes these oils more stable to heat and light until these antioxidants are used up over time¹.

Saturated and monounsaturated fat are slightly more stable to the effects of heat than polyunsaturated fat; so reusing oils high in polyunsaturated fats or using these for deep fat frying is more likely to cause oxidation¹.

But one of the most important things to consider about heating oils is the smoke point. This is the temperature where the fat begins to break down and become rancid, so a higher smoke point means that a fat is more stable when it is heated and so it is better to use for cooking at higher temperatures⁴.

Table 1: The Smoke Point of Various Oils^5-6

As well as choosing the most heat stable oils, it’s important to store and use oils correctly to protect them from becoming rancid.

The Health Benefits of Different Oils

There is a lot of controversy about whether saturated fat is good or bad for us^7-8. Although the current evidence base shows that replacing saturated fats with refined carbohydrates (like sugar, white bread and white pasta) does not have health benefits, replacing saturated fat with wholegrains or unsaturated fat does seem to reduce the risk of heart disease; with more evidence starting to show that polyunsaturated fat is particularly heart healthy^9-10. It is also important to mention that in many Western societies we eat more saturated fat than is recommended^10-11. So for our heart health it is best to replace fats or oils high in saturated fat (like butter, palm oil or coconut oil) with those high in unsaturated fat (like olive oil, rapeseed oil, sunflower oil, safflower oil and soybean oil). Check out the comparison of oils chart below for a full breakdown of the types of fats found in different types of oils.

Comparison of the Fats in Various Oils

(from The BNF (2009) Culinary Oils and Their Effects)¹

And as I’ve already mentioned, some oils also contain vitamins and antioxidants which add further health benefits. For example vitamin E is an antioxidant which protects our cells from damage, and vitamin K is essential for bone health and blood clotting¹.

Vitamin E and K Levels in Various Oils

(from The BNF (2009) Culinary Oils and Their Effects)¹

It is also important to look at the effects of whole-foods rather than just the specific fats or vitamins they contain. For example, olive oil has a lot of good quality evidence to support its health benefits, especially in terms of reducing the risk of heart disease and stroke^12-13, and some research has actually found that using virgin olive oil to fry food is still associated with a reduced risk of heart disease¹⁴. There is also evidence which supports the benefits of rapeseed oil in terms of heart and metabolic health^15-16.

But remember that unless you are trying to gain weight all oils should only be consumed in small amounts as they are high in calories; for example one tablespoon of any oil contains roughly 120 calories, which is the same as half a jam donut or two digestive biscuits^17-18.

Main Points:

1. Heat and light causes oils to breakdown and become rancid, so store oils away from sunlight, avoid cooking for long periods at very high temperatures and avoid reusing oils
2. The healthiest oils to cook with have a high smoke point and are high in unsaturated fat and antioxidants; good options include: avocado oil, refined or virgin olive oil, sunflower oil, safflower oil and rapeseed oil
3. Although ghee and refined coconut oil have a high smoke point, these are not the healthiest fats to consume regularly as they are high in saturated fat
4. It’s best to use all oils and fats sparingly when cooking, unless you are aiming to gain weight

For more information about fats you can read my post: Dietary Fatty Avoids: An Overview 

Or check out my infographic series The Facts About Fats:

Part 1. The Basics

Part 2: Saturated and Trans Fat

Part 3: Unsaturated Fat

Part 4: Coconut Oil Debunked

References:

1. BNF (2009) Culinary oils & their health effects 
2. Baynes (2007) “Dietary ALEs are a risk to human health-NOT”
3. Feron et al. (1991) “Aldehydes: occurrence, carcinogenic potential, mechanism of action and risk assessment”
4. BNF (2009) “Answers to questions commonly asked about oils”
5. “Oil Well data sheet” (2014) available via: https://docs.google.com/spreadsheets/d/1TCpmlGXaLh4Pi-fzAAvpCDWlsNagGpVBLfG4ekEPtCs/edit#gid=15
6. “Characteristics of Various Oils” available via: http://www.todaysdietitian.com/newarchives/images/0215-2.pdf
7. Chowdhury et al (2014) “Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk: A Systematic Review and Meta-analysis”
8. Annals of International Medicine (2014): http://wphna.org/wp-content/uploads/2014/08/2014-03_Annals_of_Int_Med_Chowdhury_et_al_Fat_and_CHD_+_responses.pdf).
9. Hooper (2015) “Reduction in Saturated Fat Intake For Cardiovascular Disease”
10. Sacks et al. (2017) “Dietary Fats and Cardiovascular Disease: A Presidential Advisory From the American Heart Association” 
11. Public Health England (2016) National Diet and Nutrition Survey Results from Years 5 and 6 (combined) of the Rolling Programme (2012/2013 – 2013/2014)
12. Schwingshackl & Hoffmann (2014) “Monounsaturated fatty acids, olive oil and health status: a systematic review and meta-analysis of cohort studies”
13. EFSA Journal 2011;9(4):2033)
14. Sayon-Orea et al. (2015) “Does cooking with vegetable oils increase the risk of chronic diseases?: a systematic review”
15. Lin Lin et al. (2013) “Evidence of health benefits of canola oil” 
16. Jones et al. (2014) “DHA-enriched high-oleic acid canola oil improves lipid profile and lowers predicted cardiovascular disease risk in the canola oil multicenter randomized controlled trial” 
17. McCance & Widdowson’s: The Composition of Foods – Fifth Revised and Extended Edition
18. FSA (2011) “Food Portion Sizes: Third Edition”