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Whether you are roasting a chicken in the oven, browning onions in a frying pan or choosing a spread for your toast, oils are at the heart of our culinary activity.
We have a dizzying array of choice. From sunflower to flaxseed, avocado to coconut, around 30 varieties of oil are now used for cooking. Your decision on which to use could have a profound effect on your health, with consequences for your cholesterol, blood pressure and risk of cardiovascular disease.
If you believe the headlines, then palm oil is out, sunflower oil is on shaky ground and there seems no end to the benefits that extra virgin olive oil brings to the table. But are these claims backed up by solid science? And how do the health effects of these products weigh up against their environmental costs?
Saturated or unsaturated?
First, some chemistry. Cooking oils contain fats, which are made from long chains of carbon atoms linked together. Saturated fats, found in red meat and dairy, are so named because each carbon atom is linked to the next by a single bond. The remaining electrons of each carbon atom are then available to form bonds with hydrogen atoms – making the molecule fully “saturated” with this element. This structure makes these fats very rigid and stable, which is why butter and lard are solid at room temperature.
Unsaturated fats, commonly present in plants and oily fish, have at least one double bond between neighbouring carbon atoms, which reduces the number of bonds that can be formed with hydrogen atoms. If there is only one double bond, it is monounsaturated; if there are more, it is polyunsaturated. The double bonds make these fats more fluid, which is why they are usually liquid oils. Most cooking oils are made up of a blend of different fats, including both saturated and unsaturated varieties. This is a good thing, since both have important biological functions. “We need fats to help us absorb certain vitamins in the gut,” says , a nutritional biochemist at the University of Hertfordshire, UK. “They also form an essential part of the lipid bilayer that makes up the membrane of each of our body’s cells.”
But as with so much in life, it is a question of balance. While we need some saturated fat in our diet, most people consume too much. “Tropical oils, such as palm oil and coconut oil, are very high in saturated fatty acids, and the types of saturated fatty acids they contain are known to raise cholesterol,” says , a nutritional scientist at King’s College London. This “bad” or low-density lipoprotein (LDL) cholesterol can kick-start the process of atherosclerosis – the buildup of plaques in the arteries – which can raise the risk of heart disease and stroke. For that reason, Berry believes they should be avoided.
Omega-3 or omega-6?
Polyunsaturated fats are generally considered to be healthier. There are two types commonly found in cooking oil: an omega-3 fatty acid called alpha-linolenic acid, found in flaxseed and rapeseed oil, and an omega-6 fatty acid called linoleic acid – which is particularly prevalent in sunflower and cornflower oil. Both types of fats are packed with antioxidants and are linked to health benefits such as improved insulin sensitivity.
There is some controversy and disagreement about their relative merits, however. Some nutritionists believe that omega-6-rich oils could cause chronic inflammation – the driving force behind many conditions affecting Western societies, such as heart disease, type 2 diabetes and even Alzheimer’s. These concerns arise from the fact that omega-6 fats can be broken down by the body into arachidonic acid, a pro-inflammatory molecule.
Omega-6 fats also compete with omega-3s for their place in the cell membrane. This is a problem because cell membranes – particularly those in your retina, brain and heart muscle – rely on omega-3s to function optimally. The high number of double bonds in omega-3s, coupled with how these double bonds are positioned, makes them more fluid than other fats, and so their presence also increases the fluidity of the cell membranes. This helps neurotransmitters and hormones bind to membrane receptors more effectively, allowing messages to be quickly sent around the body.
For these reasons, it might seem sensible to prioritise oils rich in omega-3. In , participants were asked to take a teaspoonful of flaxseed oil – which contains up to 45 per cent alpha-linolenic acid – with each meal. After 12 weeks, the participants had lower LDL cholesterol and blood pressure compared to a control group taking the omega-6 linoleic acid.
It is worth noting, however, that the omega-3 fats with the greatest benefits for health aren’t present in cooking oils. They are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), found in oily fish such as mackerel, salmon and sardines. Both of these are longer and more unsaturated than alpha-linolenic acid, so they have much more of a fluidising effect on cell membranes. While our bodies can convert alpha-linolenic acid into EPA and DHA, it isn’t an efficient process.
“Fish omega-3s lower blood pressure, lower inflammation and they’re good for heart and brain health,” says , a nutritional immunologist at the University of Southampton, UK. “But you would never cook with fish oil. The plant omega-3s behave a little bit like fish oil and have some of those effects, but they are less potent.”
Diets rich in extra virgin olive oil may protect us from high blood pressure Jeffrey Greenberg/Universal Images Group/Getty Images
In any case, the idea that omega-6 vegetable oils are inherently harmful has been contested. For example, concluded that higher levels of omega-6s, particularly linoleic acid, are associated with a reduced risk of cardiovascular disease and type 2 diabetes. So there may be no need to throw out your sunflower or cornflower oil just yet. “I think it’s perfectly healthy to cook with these oils,” says Berry. “They are packed with lots of vitamins and have antioxidant properties. Plus, we know that the polyunsaturated fatty acids in some of these seed oils can help reduce cholesterol.”
Cooking away the health benefits?
Adding further complications are the effects of cooking. Fats at room temperature behave very differently to fats heated up in a frying pan. As unsaturated oils are less stable, they break down at lower temperatures. The point at which the oil breaks down is called its “smoke point”.
The between carbon atoms means that polyunsaturated fats are more readily oxidised when heated. This can create a rancid taste – which means that omega-3-rich products like flaxseed oil are generally unsuitable for cooking. Even more problematically, oxidation releases extremely reactive chemicals called free radicals, which attack living tissues.
One particular , which is thought to contribute to type 2 diabetes, atherosclerosis and cognitive degeneration. The longer you heat oil, and the more you reuse it, the . “If you go around the back of a not-particularly-good pub restaurant and smell the kitchen, and it smells rancid, then you know that they’re reusing their oils and cooking their oils at too high a temperature,” says Hoffman.
We can minimise the risks in our home cooking, however. Provided you avoid heating above an oil’s smoke point and don’t repeatedly use the same oil for frying, then vegetable oils should be considered safe, says Berry.
The advantages of extra virgin olive oil
Missing from all these analyses, however, is a surprising outsider, extra virgin olive oil, which is neither a saturated nor polyunsaturated oil. Instead, extra virgin olive oil consists almost entirely of oleic acid – a monounsaturated fat – and there is a growing consensus that it may be the healthiest oil of all.
Extra virgin olive oil is high in polyphenols, which may protect the body from illness Tom Stoddart/Getty Images
In the early 2000s, over 7000 people in Spain aged 55 to 80 were enrolled onto the . One group followed a standard Mediterranean diet supplemented with large amounts of extra virgin olive oil. These participants were essentially given a 1-litre bottle of extra virgin olive oil per week with the recommendation to consume at least 4 tablespoons per day per person. Importantly, they didn’t just drizzle the oil on salads – they were encouraged to cook with it too.
Consuming extra virgin olive oil reduced a person’s risk of heart attack or stroke by 30 per cent
A second group ate a Mediterranean diet supplemented with nuts, while the third was told to reduce their dietary fat intake. All participants also received group and personalised dietary advice sessions. The results showed that, over a period of five years, consuming high amounts of extra virgin olive oil reduced a person’s risk of having a heart attack or stroke .
A came to similar conclusions, finding extra virgin olive oil consumption is linked to lower blood pressure, higher levels of “good” cholesterol and lower levels of “bad” cholesterol – all important indicators of cardiovascular health. “The evidence is clear – extra virgin olive oil is by far the healthiest oil to cook with,” says nutrition scientist at the University of Navarra in Spain, who led the PREDIMED study.
So why is extra virgin olive oil so good for you? Its monounsaturated fat may just be one small factor in its benefits. Unlike other cooking oils, which are extracted using chemicals and high temperatures, extra virgin olive oil is made by squeezing, grinding and pressing raw olives. This means that the product is high in polyphenols – molecules made by plants to protect themselves against stress. Polyphenols are antioxidants, which means they mop up free radicals, reducing inflammation. “They prevent fats oxidising in the body, particularly the bad form of cholesterol, LDL. That protects the body against atherosclerosis,” says Hoffman.
Extra virgin olive oil is different to virgin olive oil and standard olive oil, which are manufactured using heat and industrial chemicals. This processing can remove the majority of polyphenols, which is why many experts recommend that we opt for extra virgin over the lower grades. Although cooking may destroy some of the polyphenols, the PREDIMED trial suggests that it is still superior to other fats, provided that you avoid the smoke point.
Manufacturing may reduce some of the health benefits of olive oil Angel Garcia/Bloomberg/Getty Images
Any dietary change will face some barriers to uptake. The four tablespoons of extra virgin olive oil a day that were consumed in the PREDIMED trial, for instance, would be considered quite normal in a country like Greece, where a family might get through a bottle every two weeks, but it may seem excessive for the typical consumer in the US or UK. The polyphenols in extra virgin olive oil can also give it a fragrant flavour, which may be undesirable for some palettes.
Price may be an additional issue. “A lot of people probably won’t buy extra virgin olive oil just because it’s too expensive, so I would say rapeseed oil is probably the second-best to use,” says Hoffman. “It contains high levels of monounsaturated fat, which is a good thing, but unfortunately it doesn’t contain the high level of antioxidants found in extra virgin olive oil, so it doesn’t score quite as highly.”
Avoiding palm oil
Almost all liquid oils will also face some limitations to their culinary uses. “Solid fats give structure and texture to a cake,” says at Queen Margaret University, Edinburgh. “If you make biscuits with butter, you either rub it into the flour, or you can cream it. But you obviously can’t do that with a liquid oil.”
That’s why, if you walk down a supermarket aisle selling ready-made cakes and pastries, you will notice one ubiquitous ingredient: palm oil. Palm oil is packed with saturated fats, so it has a high melting point and can be used as a solid fat. It is also cheaper than butter and has a much longer shelf life. It can be blended, behaves well when heated and helps preserve packaged foods. But it is also , contributing to global warming and threatening the habitats of endangered species like the orangutan. It is also unhealthy due to its high saturated fat content. Several researchers are trying to come up with alternatives.
For example, Liddle and her colleagues have developed a new ingredient, PALM-ALT, to replace palm shortening, an ingredient used in the baking industry that is derived from palm oil. PALM-ALT is 100 per cent plant-based and contains a mixture of rapeseed oil, fibres and a byproduct from the linseed industry. It contains 88 per cent less saturated fat than palm shortening, but as it is semi-solid, it still helps maintain the texture, flavour and colour of baked goods. So far, the team has used the ingredient to make cakes, biscuits, bread, doughnuts, cookies and flapjacks.
A panel of taste testers was given either PALM-ALT baked goods or the same goods made with palm shortening. “People couldn’t identify a significant difference; it tastes the same,” says Liddle, who along with her team is looking at ways to commercialise the ingredient.
Elsewhere, C16 Biosciences, a spin-out company from the Massachusetts Institute of Technology, has developed a palm oil alternative for the cosmetics industry, called Torula oil, that has the same properties as palm oil but without the environmental impact. The oil is made by yeast through a fermentation process. The company is now looking to for the food industry, which they hope to use in ice creams, chocolate and pastries.
Such developments will only add to the already burgeoning range of products available to us. The scientific research can give us some guidelines, but there is still room for plenty of flexibility based on our culinary aims and gustatory preferences. “When you’re looking at an oil, I think it’s a case of thinking what am I using it for and what taste do I like?” says Berry. “As long as it’s not a highly saturated fat or oil, I think any oil is going to confer some health benefits. But if you want the added boost of polyphenols, then extra virgin olive oil is the top choice.”
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