We’ve all got some cooking oil handily sitting next to the hob, but it’s usage in most dishes belies the hidden health implications, while contrasting nutritional information and a broad range and selection — olive, coconut, vegetable — don’t exactly ease the confusion. The question then is why might some cooking oils be considered unhealthy, and which ones should you look out for, or avoid? After all, getting the right cooking oil might just be the simplest health switch you can make in the kitchen.
What To Look For In Your Cooking Oil
The smoke point is the temperature at which an oil starts to burn and smoke. Each oil has its own smoke point, mainly down to the varying ratios of fatty acids such as unsaturated and saturated which differ between them. It’s advised to not consume meals cooked with an oil that’s heated past its smoke point because there is evidence to suggest that carcinogens like acrolein are produced with oils that are heated past a certain point.
- Refined oils generally have a higher smoke point (but have other negative effects which we’ll discuss below).
- Oils high in polyunsaturated fats such as sunflower and flaxseed tend to have lower smoke points.
- Oils high in monounsaturated fats such as avocado and olive have medium smoke points.
- Oils high in saturated fats such as coconut oil have higher smoke points.
- When oils are exposed to light, heat and air, it will lower their smoke point.
Dark, Glass Containers And Use By Date
Oxidised oils can threaten physiological and biochemical functions of the body. Oils are fragile, and prone to oxidation and becoming rancid which forms free radicals and destroys antioxidants.
Oils are light sensitive and air sensitive, so make sure you’re buying oils in dark containers and preferably in glass. Store them fully out of light then, in a cool dark place, and don’t buy oils in plastic, as toxic compounds such as PVCs can leach into the oil.
Make sure you’re keeping the cap on tightly when you’re not using it and cap it as soon as you’ve poured out what you need. Don’t buy a huge bottle of olive, avocado or macadamia oil if you won’t use it up within a few months.
Trans fat is found in hydrogenated oils, meat and dairy, and is only found in one place in nature — animal fats. With technology, food industries are making these toxic fats synthetically by hardening vegetable oils in a process called hydrogenation, which rearranges their atoms to make them behave more like animal fats and improve their shelf life.
Trans fat makes bad cholesterol go up and the good go down. All trans fats cause negative effects, irrespective of their origin, with studies finding that a high intake of trans fats may increase the risk of heart disease by 50 percent (1), while removing all trans fat from the diet would prevent tens of thousands of heart attacks. (2)
Form Recommends: The 3 Healthiest Cooking Oils
Saturated fats are generally known to be unhealthy, but not all saturated fat is created equal, especially coconut oil, which is about 90 percent saturated fat, a higher percentage than butter.
However, it is also composed of around 65 percent medium chain triglycerides (MCTs) which are absorbed rapidly into the bloodstream and burned as fuel. Studies have found that consuming MCTs can help reduce body weight, fat and triglyceride levels (3).
Coconut oil also contains lauric acid which is antimicrobial and antibacterial, as well as oleic acid, linoleic acid and vitamin E (4).
It is a stable fat, which means it’s slow to oxidise, thus resistant to rancidity. It also has a higher smoke point. Coconut oil is generally considered a sustainable choice, as it is regarded as a renewable resource that can be grown again, and again. In terms of taste, coconut oil will of course have a hint of coconut flavour, and works best with South Asian dishes.
Olive Oil & Avocado Oil
There’s a general consensus that monounsaturated fats (MUFA) found in avocados, nuts, seeds, olives, and omega-3 polyunsaturated fats (PUFA) found in hemp, chia seeds, flaxseed and algae, are healthy. This is validated in microbiome studies that show oleic acid – a monounsaturated fat found in olive oil and omega-3 PUFAs promote the growth of beneficial microbes and reduce bacterial endotoxin release. They can also enhance microbial diversity to protect the microbiome (5).
Extra virgin olive oil is ok for low-temperature cooking, but is best for drizzling after cooking or over salads. Extra virgin or cold-pressed means the oil was mechanically processed rather than chemically processed, so they don’t contain potentially harmful chemicals and additives.
Refined olive oil is ok to use for higher temperature cooking like stir-frying or baking. Even though it’s refined, it’s safer when it comes to cooking at high temperatures compared to the oils below.
Avocado oil has the benefit of a higher smoke point than most (275°C), especially compared to olive oil (194°C). It also has a fairly neutral flavour which means it can work in most dishes.
The Cooking Oils To Avoid
Refined Vegetable And Seed Oils
Most refined vegetable and seed oils such as corn oil, soybean oil, cottonseed oil, peanut oil, safflower oil, canola oil and sunflower oil contain trans fats (6,7), and in many countries, due to regulations, brands are allowed to declare that their products contain 0g of trans fat even if they contain a small amount of it (8).
Most of these oils will also have a very high omega 6 to 3 ratio, which affects our body’s ability to manufacture omega-3s.
Now, omega-3s and omega-6s are polyunsaturated fats and they’re considered essential because our body isn’t capable of making them so we are required to get them from our diet. Essential fatty acids (EFAs) perform many different functions in the body. They are critical for our skin health, they are the components that help keep our skin cells healthy and the skin’s membrane functioning optimally.
Omega-3 is converted into EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) – which are important for our heart and brain health, while omega-6s are a precursor for a hormone-like substance called prostaglandins, which have an impact on many functions of the body, such as cell growth and the constriction and dilation of the smooth muscle cells in your veins.
The focus should be about obtaining omega-3s though, with the ideal ratio of omega 6s to 3s between 1:1 and 4:1. Due to an abundance of omega-6s in the fast food, animal products, vegetable oils and fried food we eat as a society, our ratio is said to be about 20:1.
This ratio is a problem because excessive amounts of omega-6s promote carcinogenesis and inflammation and can promote diseases such as cardiovascular disease, cancer and osteoporosis. It has also been associated with conditions like acne.
Consuming a high intake of omega-6s eventually leads to metabolic byproducts that are toxic, which can lead to artery build-up and blood clots that cause heart attacks, allergies as well as other inflammatory disorders. Our body simply isn’t adapted to having that many omega-6s in the system.
Canola or Rapeseed Oil
Most of the canola oil found in products are partially hydrogenated which means that it contains trans fat, and some can contain up to 40 percent trans fat. Trans fat raises LDL cholesterol (9) and has the ability to clog arteries (10). Even if it’s not partially hydrogenated, you still get trans fat from normal canola oil.
This trans fat is formed during processing where they bleach the oil and it can be around 2-4 percent trans fat (8). From this policy brief published by the World Health Organisation (12), many European countries are keeping trans fat levels under 2g per 100g of oil so any oil that is over 2 percent trans fat would be over the limit.
Palm oil is one of the most used oils in the world (13) due to being one of the most efficient vegetable oil crops there is — the same piece of land can produce three times more oil than if it were to be used to produce soybean oil or sunflower oil. This means palm oil is the one of the least expensive vegetable oils, but it is also known to create issues like deforestation and animal extinctions, and various health issues.
Palm oil is known to significantly raise your LDL-C, also known as the ‘bad’ cholesterol, and your risk of developing atherosclerosis, a potentially serious condition where arteries become clogged with fatty substances called plaques, or atheroma (14).
This is mainly because palm oil is high in saturated fatty acids — it contains 44% saturated fatty acid palmitic acid. There’s evidence that consumption of palmitic acid increases your risk of developing cardiovascular diseases (15), while studies have also found that palm oil and other refined vegetable oils contain a potentially toxic chemical contaminant 3-MCPD (3-monochloropropane-1, 2 diol) (16).
There’s a number of studies on the carcinogenicity and genotoxicity of 3-MCPD which were carried out on animals, and while there are no clinical studies on humans reported so far, the concern from these studies show that it could have detrimental effects on your kidneys and fertility (17), as well as neurological scars in monkeys (18).
Palm oil has the highest levels of 3-MCPD out of all other refined vegetable oils. Manufactured processed foods will generally contain oxidised palm oil. Oxidised oil is unhealthy — it can cause an increase in free fatty acids, phospholipids and cerebrosides, and has been found to suppress the body’s natural appetite-suppressing signals from leptin and insulin. If you’re going to go out of your way to avoid unhealthy oils, make steering clear of palm oil a particular priority.
1: Willett WC, Stampfer MJ, Manson JE, Colditz GA, Speizer FE, Rosner BA, Sampson LA, Hennekens CH. Intake of trans fatty acids and risk of coronary heart disease among women. Lancet. 1993 (https://pubmed.ncbi.nlm.nih.gov/8094827/)
2: Brouwer IA, Wanders AJ, Katan MB. Effect of animal and industrial trans fatty acids on HDL and LDL cholesterol levels in humans–a quantitative review. PLoS One. 2010 Mar
3: Xue C, Liu Y, Wang J, Zhang R, Zhang Y, Zhang J, Zhang Y, Zheng Z, Yu X, Jing H, Nosaka N, Arai C, Kasai M, Aoyama T, Wu J. Consumption of medium- and long-chain triacylglycerols decreases body fat and blood triglyceride in Chinese hypertriglyceridemic subjects. Eur J Clin Nutr. 2009 Jul (https://pubmed.ncbi.nlm.nih.gov/19156155/)
4: Bergsson G, Arnfinnsson J, Steingrímsson O, Thormar H. In vitro killing of Candida albicans by fatty acids and monoglycerides. Antimicrob Agents Chemother. 2001 Nov (https://pubmed.ncbi.nlm.nih.gov/11600381/)
5: Costantini L, Molinari R, Farinon B, Merendino N Impact of Omega-3 Fatty Acids on the Gut Microbiota 2017 Dec (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5751248/)
6: Azizian, H. and Kramer, J. K. A rapid method for the quantification of fatty acids in fats and oils with emphasis on trans fatty acids using Fourier transform near infrared spectroscopy (FT‐NIR). Lipids, 40(8), 855–67. 2005 (aocs.onlinelibrary.wiley.com/doi/abs/10.1007/s11745-005-1448-3)
7: O’Keefe, S., Gaskins‐Wright, S., Wiley, V. and Chen, I.‐C. Levels of trans geometrical isomers of essential fatty acids in some unhydrogenated U.S. vegetable oils. Journal of Food Lipids, 1, 165–76. 1994
8: Crosby. G Ask the Expert: Concerns about canola oil 2015
9: Brouwer, I. A., Wanders, A. J., & Katan, M. B. Effect of animal and industrial trans fatty acids on HDL and LDL cholesterol levels in humans–a quantitative review. 2010 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2830458/)
10: Chen, C. L., Tetri, L. H., Neuschwander-Tetri, B. A., Huang, S. S., & Huang, J. S. (2011). A mechanism by which dietary trans fats cause atherosclerosis. The Journal of nutritional biochemistry, 2011 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3125015/)
11: WHO Eliminating trans fats in Europe A policy brief 2015 (https://www.euro.who.int/__data/assets/pdf_file/0010/288442/Eliminating-trans-fats-in-Europe-A-policy-brief.pdf)
12: Gesteiro E, Guijarro L, Sánchez-Muniz FJ, et al. Palm Oil on the Edge. Nutrients. 2019;11(9):2008 (https://pubmed.ncbi.nlm.nih.gov/31454938/).
13: Sun, Y., Neelakantan, N., Wu, Y., Lote-Oke, R., Pan, A. and van Dam, R. M. Palm oil consumption increases LDL cholesterol compared with vegetable oils low in saturated fat in a meta-analysis of clinical trials. Journal of Nutrition, 145(7), 1549–58. 2015
14: Mancini A, Imperlini E, Nigro E, et al. Biological and Nutritional Properties of Palm Oil and Palmitic Acid: Effects on Health. Molecules. 2015 (https://pubmed.ncbi.nlm.nih.gov/26393565/)
15: Gesteiro E, Guijarro L, Sánchez-Muniz FJ, et al. Palm Oil on the Edge. Nutrients. 2019;11(9):2008. (https://pubmed.ncbi.nlm.nih.gov/31454938/)
16: Jędrkiewicz R, Kupska M, Głowacz A, Gromadzka J, Namieśnik J. 3-MCPD: A Worldwide Problem of Food Chemistry. Crit Rev Food Sci Nutr. 2016 (https://pubmed.ncbi.nlm.nih.gov/25830907/)
17: Heywood R, Sortwell RJ, Prentice DE. The toxicity of 1-amino-3-chloro-2-propanol hydrochloride (CL88,236) in the rhesus monkey. Toxicology. 1978 (https://pubmed.ncbi.nlm.nih.gov/418534/)
18: Larsen JC. 3-MCPD esters in food products. Brussels: ILSI Europe; 2009. (https://ilsi.eu/publication/3-mcpd-esters-in-food-products-summary-report/)