Cooking oil: does heating cause health problems?

By: Glenn Cardwell - Accredited Practising Dietitian

The thought comes up regularly - if you cook with oil does it lose all its healthy properties? Do unsaturated fats become saturated once they are heated? In other words should oil only be used for salad dressings? These are common questions. Let me try and make sense of this topic.

Frying oil at home

I have done a search of all the literature on cooking oils. OK, that's a lie. I have actually read five extensive review papers on the effect of heating on oils. And they certainly tested out my chemistry. And none came to a simple solution to make my life easier. Let's start with the one we can sort out quickly.

You may have heard that frying any oil will convert it to a saturated fat, making it bad for your heart. Let's assume that you are referring to unsaturated cooking oils like olive oil or peanut oil (mainly monounsaturated), or safflower oil (mainly polyunsaturated). Let's also assume you only use the oil once in frying, because you are using just enough to get the food cooked. What will happen during cooking? Nothing. The oil will remain pretty much as it was originally. Nothing I've read says any different.

Deep frying oil

Nevertheless, there is a potential problem when an oil is repeatedly used for deep-frying. I don't know anyone who has a deep fryer anymore. We had one back in the late 60s for cooking chips (fries), fish and potato fritters. Don't think it made it past 1975. So, I suspect the only source of a deep-fried food for any reader is from a fast-food venue.

Cooking oil at high temperatures for hours over many days does cause the fats in the oil to begin to oxidise (go rancid), which spoils the smell, taste and colour of the oil. When consumed, this rancidity can cause inflammation and damage to the artery linings and a rise in the risk of heart disease, including a rise in blood pressure. Long term heating also causes a loss of vitamin A and antioxidant compounds naturally present in oils like extra virgin olive oil.

How much cooking before an oil goes bad?

I would love to give you a specific cooking time that oils "turn bad", but there are so many variables. Simply put, most studies show a decline in the oil quality (from a health perspective) after 5-10 hours of frying. The shorter time frame is for the polyunsaturated oils, according to the Heart Foundation. That means the once-off use of oil in cooking shouldn't pose a problem, while repeatedly heated oil could be aging your blood vessels rather quickly.

The one thing nobody considers

When you buy your Extra Virgin Olive Oil do you ever give it a taste test (licking some off your finger)? If your olive oil was cheap-ish it is probably also old-ish, as in starting to go rancid. Now taste a good local olive oil. Probably full of fruit flavours. Dietitian Rosemary Stanton has written a lot on fats and has often said that European olive oil sold in Australia has been around too long and has rancid tones to its flavour. And she's an official olive oil taster, so I'm going to believe her.

This is not just a problem in Australia and New Zealand. The Americans are being sold rancid olive oil and no-one seems to care.

Buying local olive oil is a good argument on many fronts - environmental, economic, and nutrition. There is plenty of great tasting oil in Australia, New Zealand and the US. No doubt the folk in Europe can get wonderful fresh Spanish, Greek and Italian olive oil.

What does it all mean?

Simple message - eat few deep-fried takeaways; enjoy more home cooking. (Note: some big take-away franchises use better quality oil, filter it regularly and top up with fresh oil, therefore lowering any health risk). When simply frying food, then purchase a cheaper unsaturated oil (eg Canola, Grapeseed). If you want some flavour in an Asian dish, then maybe some sesame oil. Save the top-end olive oil for the salad dressing or for drizzling on bread. Keep your oil cool, sealed and away from light, to slow down the oxidising process.

Santos CSP et al (2013). Effect of cooking on olive oil quality attributes. Food Research International 54: 2016-2024

Choe & Min (2007) Chemistry of deep-fat frying oils. J of Food Science 72: R77-R86

Stier RF (2013) Ensuring the health and safety of fried foods. Eur J Lipid Science & Technol 115: 956-964

Ng CY et al (2014) Heated vegetable oils and cardiovascular disease risk factors. Vascular Pharmacology 61: 1-9

Glenn Cardwell is an Accredited Practising Dietitian with 32 years in clinical and public health nutrition. To find out more about Glenn you can visit his website


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