There is lots of conflicting information in the media about which are the healthiest oils to cook with. In this article I explain about the smoke point, what happens when you heat oils, the role of antioxidants, which increase your risk of cardiovascular disease and the science behind the claims to guide you through which are the healthiest oils to cook with.
What is the smoke point?
This is the temperature at which when heated, the oil or fat starts to produce a visible smoke.
Heat degrades oils to aldehyde compounds.
One study heated extra virgin olive oil, sunflower and linseed oils to 190 degrees Celsius for a minimum of 20 hours, and found that while all three oils produced alpha, beta-saturated aldehydes, which are known to be toxic, sunflower oil had the highest concentration1. Additionally, olive oil was the slowest to form both aldehydes and also other toxic compounds called alkyl benzenes1. The most aldehyde compounds were produced with increasing temperatures. Whilst it is clear that aldehyde compounds are formed during normal stove top cooking, this study heated the oil for substantially longer that would be routinely done in the home1, which makes it potentially less relevant for drawing domestic conclusions. Compared with polyunsaturated and mono-unsaturated oils such as olive oil, saturated fats such as coconut oil are more heat resistant.
There is evidence that re-heating oils causes them to change from a ‘cis’ formation into a ‘trans’ formation, which is a structural rearrangement of the fatty acid molecules that make up the oil. Every time the oil is re-heated, more of the oil changes from cis to trans. These are the trans or hydrogenated fats which are often in the news. There is evidence that oils deteriorate with repeated heating2and are linked to an increase risk of disease3.
Aldehyde compounds and health
The evidence that aldehyde compounds damage health are mixed. There are some reports that aldehyde compounds are not toxic, but there are an increasing number of studies that demonstrate toxicity and are difficult to ignore. A number of these aldehyde compounds such as formaldehyde, are naturally made by our bodies and are thought to have numerous roles as chemical signals, intracellular signalling, and the immune system4. There is evidence that generation of some aldehydes are produced because of oxidative stress within cancers themselves and also that they might contribute to cancer formation4. We do know that these proteins are capable of damaging DNA and proteins by forming cross-links in cellular models4.
These aldehyde compounds are highly reactive, and have also been associated with an increased risk of neurological diseases such as Alzheimer’s, liver disease, and contact dermatitis1,4.
There is limited evidence that diets containing higher anti-oxidants are thought to be protect against the production of damaging aldehydes and lipid oxidation products3,4. In some animal models, consumption of vitamin E, an antioxidant reduced the concentration of aldehyde compounds4. However, trials in humans that have investigated the effects of supplementary antioxidants have been inconclusive3.
Does what you cook matter?
Additionally, what you are cooking might be as important as the oil you choose, since aldehyde compounds are formed in the digestion of meat and fish but not fruit and vegetables. There is evidence that eating foods rich in antioxidants, helps divert the break-down of meat and fish from harmful compounds, into harmless ones instead3.
Interestingly there is also some limited evidence that phenols in red wine, can prevent absorption of the lipotoxin malondialdehyde, and therefore suggestion that consumption might be protective when eating fried meat or fish3. However, this has yet to be tested conclusively in humans.
Reduction of saturated fats is associated with a reduced risk of cardiovascular disease5. Coconut oil is a saturated fat, and ingestion has been found to increase LDL levels, which is associated with an increased risk of cardiovascular disease5. Conversely, the PREDIMED study found that people randomised to include additional extra virgin olive oil or nuts to their diet had a lower risk of cardiovascular diet6.
Extra virgin olive oil contains antioxidants, including vitamin E and polyphenol compounds. In comparison, most commercially available coconut oil is heavily processed which decreases levels of naturally occurring antioxidants and polyphenols.
Even if you use extra virgin coconut oil which has a higher concentration of antioxidant compounds, since it is saturated fat, it is linked with increasing LDL concentration, and an increased risk of cardiovascular disease.
Summary – which oil should I use?
Overall, although coconut oil is more stable at higher heat, extra virgin olive oil is full of antioxidant and phenol compounds which are thought to be protective. Additionally, since coconut oil is a saturated fat, it is linked with increased LDL concentration, and an increased risk of cardiovascular disease. Therefore, as a general rule, extra virgin olive oil is better for your health. Whichever oil or fat you cook with, make sure to only ever to cook with it once.
1. Guillén, M. D. & Uriarte, P. S. Aldehydes contained in edible oils of a very different nature after prolonged heating at frying temperature: Presence of toxic oxygenated α,βunsaturated aldehydes.Food Chemistry131,915–926 (2012).
2. Guallar-Castillón, P., Bmj, F. R.-A.2012. Consumption of fried foods and risk of coronary heart disease: Spanish cohort of the European Prospective Investigation into Cancer and Nutrition study. bmj.com
3. Kanner, J. Dietary advanced lipid oxidation end products are risk factors to human health. Mol Nutr Food Res51,1094–1101 (2007).
4. Grootveld, M., Ruiz-Rodado, V. & Silwood, C. Detection, monitoring and deleterious health effects of lipid oxidation products generated in culinary oils during thermal stressing episodes. (2014).
5. Khaw, K.-T. et al. Randomised trial of coconut oil, olive oil or butter on blood lipids and other cardiovascular risk factors in healthy men and women. BMJ Open8,e020167 (2018).
6. Estruch, R. et al.Primary Prevention of Cardiovascular Disease with a Mediterranean Diet Supplemented with Extra-Virgin Olive Oil or Nuts. N Engl J Med378,e34 (2018).