You might have heard that you should avoid eating food containing vegetable or seed oils and wondered why? In this blog I want to explain some of the science and the thinking around this, it isn’t all completely understood so some of it is still a hypothesis (but so are many things in the nutrition world!). It is easy to say it’s simply because they are an ultra-processed food, which is true, but what is it precisely that makes vegetable oils a problem in our diet?
Let’s first be clear that vegetable oil or seed oils are the same thing! I’m not sure why the term vegetable oil was ever used as these oils are made from seeds. Seeds like sunflower, rapeseed, cotton seed, peanut, corn and soya bean (beans are a type of seed). Maybe the word vegetable was used just to make it sound like it might be good for you, that’s my theory anyway!
I’m not suggesting that the seeds themselves are necessarily bad for you, so it is fine to sprinkle them on your salad, although it you have an autoimmune issue you might want to think twice (see this blog). The problem isn’t eating a few that are sprinkled on the top of a salad... it is about the consumption if the oil when it is industrially processed in massive refineries. It is the end product of the processing plus the sheer amount of it that we are consuming that is problematic and is linked to health problems.
Crude Oil & Its Refining
So how are the seeds are processed? This image shows an oil refinery, of the food kind, not the petroleum kind, but if you didn’t know better it would be easy to mix them up. You perhaps didn’t expect oil that you cook with to be made in such vast and complicated refineries when you might think it is simply pressed seeds. But as I have already stated, this is an ultra-processed food.
Millions of seeds are mechanically pressed or ground to release their oils, then a chemical solvent called hexane is used to continue the extraction process. Next the hexane is separated from the oil by evaporation and distillation and the oil then goes through several other processes including filtering, degumming, dewaxing, bleaching, deodorising and polishing (another filtration process). These processes remove some of the fatty acids, pigments such as carotenoids and chlorophyll, they take out the natural waxes and get rid of all the fibre. The refining removes many of the essential nutrients and also generates undesirable compounds such as 3-MCPD-esters and trans-fatty acids which have been linked to genotoxicity and are carcinogenic, i.e. they can damage DNA and cause cancer.
Ultra-Processed & Devoid of Nutrients
As an aside, Wikipaedia defines an ultra-processed food (UPF) as, "an industrially formulated edible substance derived from natural food or synthesized from other organic compounds. The resulting products are designed to be highly profitable, convenient, and hyperpalatable, often through food additives such as preservatives, colourings, and flavourings". Vegetable oils are found in all kinds of ultra-processed food from margarine, crisps, chocolate, ice cream, cakes and pastries, ready meals, chips and even things like bread and chicken nuggets.
The processing described above leaves an oil which is literally pure fat, it doesn’t contain many micro-nutrients, it is just calories. If we look up the US FDA site on the nutrient profile of foods, we see that the only micronutrients it contains are vitamin E at 68.5mg per 100g and vitamin K1 at 6.9ug (and what we really need in our diets is K2 and not K1). So, vitamin E is the only potential micronutrient benefit.
Polyunsaturated Fats
All fat containing foods come with all 3 types of fat, which means it will be a combination of saturated fat, monounsaturated fat and polyunsaturated fat (PUFA). You never get a food that is 100% of any of them. If we look at sunflower oil it in 100g we find 9g of saturated fat, 63g of monounsaturated fat and 21g of polyunsaturated fat.
We can’t make polyunsaturated fats so we need to get them from our diet, they are therefore essential fatty acids. This includes both omega-3 and omega-6 as well as other lesser-known omegas. Both omega-3 and Omega-6 fats are processed by the same set of enzymes that break them down into different lipid mediators (bioactive fat molecules) but which perform opposing functions in our bodies. Most of the omega-6 mediators promote a more inflammatory state, contribute towards platelet clumping and constriction of blood vessels, while omega-3 mediators work to prevent and limit inflammation, and platelet clumping and enhance dilation (expansion) of blood vessels. While we do need a little of the omega-6s, they play an essential role in helping the inflammatory response when we need it to help heal our bodies, too much is not a good thing. You would be right in thinking that we want more of the anti-inflammatory omega-3s in our diet.
How Our Dietary Intake Has Changed
As humans we have been consuming plenty of omega-3 fats for millions of years, from marine life like fish, as well as ruminant grass-fed animals (similar to cows and sheep). Anthropologists believe during the Paleolithic era that the intake of marine omega-3s was approximately 660–14,250 mg/day. Today we are eating on average only 100–200 mg/day today. Plus the ratio of omega-6/3s that we consume has increased from around 4:1 during Paleolithic times to 20:1 today, a significant change. One of the key omega-6 fats is linoleic acid and over the past 100 years we have doubled our intake. The ratio of omega-6 to omega-3 fat in our diets has increased because of the recent addition of highly processed omega-6 rich seed (vegetable) oils. To provide you with some examples of the omega-6/3 ratio of different oils, sunflower oil has a 40:1 ratio which is better than many others such as corn at 60:1 and safflower oil at 77:1. Linoleic acid has become the most abundant PUFA in the modern western diet. Since 1960 our average intake has steadily risen from 2.7 g/day to around 4.9 g/day to 21.0 g/day (providing between 4 and 10% of our total calorie intake). The per capita consumption in the US of soybean oil increased >1000-fold between 1909 to 1999 and is even higher now, this is one of the major sources of linoleic acid in our diets, in the UK a lot of it comes from sunflower oil. These oils are used to cook nearly all of the food you buy in restaurants or from take-aways and are incorporated into nearly every ready meal and many packaged foods.
An Inflammatory Problem
Linoleic acid is an essential fat needed for our skin to form an effective vapor barrier. It was once an exceptionally rare fatty acid in the diet during human evolution which is why we developed the ability to store it. This explains why along with our increasing intake there is a significant increase in the amount of linoleic acid stored in our human fat tissue with studies finding a 2.5 fold increase since the 1950s. This increase parallels the increase in inflammatory, metabolic, autoimmune and allergic diseases in our modern society. Although this is an association (not necessarily causal) one theory growing in popularity is that the increasing high ratio of omega-6 to omega-3s in our bodies is driving chronic low-grade inflammation leading to poor health and the rise of chronic disease. The effect of increasing linoleic acid in the human body leads to higher oxidation of low-density lipoprotein (one of the molecules that carries cholesterol around the body), increased platelet aggregation, and problems with the structure of cell membranes in addition to a general increased level of inflammation.
If we take oxidised lipids, oxidation occurs when there is a reaction between high energy free radical molecules, oxygen, and in this case a fat (or lipid) which results in the oxidation and degradation (or breakdown) of the molecule. Often known better as a fat going rancid. This is what is happening in our bodies when we have a high level of inflammation. Inflammation is tissue damage which is caused by these highly unstable molecules which have electrons available to react with other substances and oxidise them.
The Impact of A High Omega-6 Intake
Animal studies including those in mice and partridges have found that increasing the amount of omega-6 in the diet does lead to changes in the composition of body fat. In partridges the saturated fat decreased by 17.9% and the unsaturated fats increased by 10.6% (in a linear fashion) as sunflower oil level increased in the diet. Of the unsaturated fats, monounsaturated fatty acids decreased linearly by 27.3%, whereas polyunsaturated fatty acids increased linearly by 51% and linoleic acid itself by 83.4%. Another study fed mice higher levels of omega-6 fats and bred them on the same diet for four generations. They found a gradual increase in fat mass, with no change in overall food intake, as well as hyperinsulinemia (raised insulin levels – want to know more check out this post). Other researchers looking again at mice found that adult mice which received a lower ratio of omega-6/-3 in their diet after birth were more resistant to becoming obese, they had 50% less body fat and were better at removing glucose from the blood stream, suggesting that they were insulin sensitive. Finally, another study of mice supplied with diets enhanced with soybean oil (rich in linoleic acid) found that a higher intake of soybean oil led to excessive weight gain, higher levels of fasting blood glucose and serum insulin, increased levels of inflammatory markers such as TNF-α (tumor necrosis factor), IL6 (an inflammatory cytokine), HsCRP (high sensitivity C-reactive protein another marker of inflammation) and changes in gene expression. The authors concluded that soybean oil (which is 55% linoleic acid) was more likely to cause obesity and diabetes than fructose or coconut oil in mice.
Other studies have found higher levels of fat accumulation in the livers of mice are related to the higher level of dietary PUFAs which is dose related, meaning the more PUFA consumed the more fat was found in the liver in particular in relation to linoleic acid. The mice also had an acutely increased content of arachidonic acid, which is the main substance produced by the breakdown of linoleic acid, an inflammatory signalling molecule. There was a dose dependent relationship meaning that the more linoleic acid consumed the more arachidonic acid was found in the liver. There are some human studies which found that lowering the amount of linoleic acid in the diet reduced the amount of oxidised lipids in blood plasma and could also prevent and reverse fatty liver disease. As we age the levels of oxidised LDL become greater and greater so more and more reason to avoid too much linoleic acid!
I don’t particularly like to reference to animal studies but one of the difficulties in studying humans is that we are long lived that you would have to wait a long time to see lifetime results. But a couple of studies have looked at the ratio of omega-6 to omega-3 fatty acids in expectant human mothers and what happened to their children. One measured maternal fat consumption, the levels of fatty acids in the blood of the mothers and in the umbilical cords and then charted body fat composition in their children. There was no difference found at age one year old but at three years old there was a difference with children born to mothers who had a higher ratio of umbilical cord blood omega-6, they had higher skin fold measurements (a measure of subcutaneous fat) and a higher chance of being obese. Another study that investigated the ratio of omega-6 to omega-3 in maternal placentas found an association with visceral obesity (fat around the organs in the body) and obesity at age 6 with the authors suggesting this was linked to the excessive consumption of omega-6 PUFAs in the modern Western diet.
Other human studies have looked at insulin resistance in relation to linoleic acid. One study gave 16 young healthy adults 100ml of soybean oil and found it reduced whole-body insulin sensitivity compared to the control over a 6 to 8-hour period. Whilst the evidence on the effect of omega-6 PUFAs in relation to Type2 Diabetes and insulin resistance is mixed many researchers are warning that increased consumption of linoleic acid could accelerate the progress of Type2 diabetes by decreasing insulin sensitivity and compromising insulin secretion from beta-cells in the pancreas. They suggest that elevated linoleic levels in blood could disrupt the regulation and expression of key genes controlling blood glucose control leading to insulin resistance and that increased oxidation of lipids that increases inflammation and may induce pancreatic dysfunction. If you don't already know about insulin resistance then have a read of this blog.
It is important to point out that there are many epidemiological studies and some animal trials that have found opposing results to those already discussed. We know that we can’t trust epidemiological studies to prove causation, it could well be that there are several confounders, other considerations that affected the results of studies. So for example, if we found that rising divorce rates were associated with people who drive BMW cars we would not say that owning a BMW is a cause of divorce. It is possible to plot all kinds of factors and find trends between them that are completely unrelated. Many of the animal trials that have been done are with poor trial design making their results difficult to interpret in a meaningful way. What we do need is better quality trials investigating this further. There is still more for us to learn!
Why Less Is Better
There are a host of other interesting intervention studies out there looking at how reducing linoleic acid or the ratio of omega-6 to omega-3 in the diet has anti-inflammatory effects as well as some population studies looking to draw correlations. One 12-week trial in humans looked at this in relation to chronic headache patients and found that lowering dietary linoleic acid significantly reduced the abundance of oxidised lipids in the blood and the amount of linoleic acid in the blood lipids. Another 16-week trial in migraine patients found that a diet lower in linoleic acid was associated with significant improvements in stress perception, sleep quality and perceived health compared to the control group, migraine disability scores also improved substantially and there were also improvements in pain interference and pain intensity. A cross-sectional study of people with lower back pain and temporomandibular disorder (a jaw disorder) found that higher levels of the omega-6 arachidonic acid (AA) were associated with a greater probability of both pain conditions for women, but not men. Higher levels of the omega-3s were associated with a lower probability of both pain conditions for men, but not women. This has been replicated in other studies of pain conditions with a higher ratio of omega-6/omega-3s being associated with greater pain intensity for orofacial pain, headache, low back pain, and bodily pain. Another animal intervention study fed mice different high fat diets differing only in the omega-6/3 ratio, fed the lowest omega-6/3 ratio had the lowest non-HDL (atherogenic lipoproteins) and inflammation (IL-6) as well as less aortic atherosclerotic lesions and the severity of atherosclerosis increased as the omega-6/3 ratio increased.
Real Food Like Nature Intended
And so to get back to what we should be eating... for health.
There is a lot more research still needing to be done, it is not yet proven that the ratio of omega-6 to omega-3 or the high level of linoleic acid in our diets is a causal factor in our poor metabolic health and chronic disease, but there are a lot of signs pointing in that direction. It is always worth coming full circle back to our evolution and what our species-specific diet should be. As a Paleolithic hunter gatherer (which is still the environment we are genetically adapted for) we should be eating a diet high in animal-based foods including meat, fish and eggs and that will provide a low omega-6 to omega-3 ratio. In addition this will be a diet that is higher in saturated fat, if you want to learn more about why I don't think saturated fat is the problem it was made out to be, take a read of this two part blog, part 1 and part 2. Of course we also want to avoid all the highly processed food that is contaminated by vegetable oils. Perhaps that will give you some food for thought!
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Disclaimer: Before changing your diet or lifestyle and taking any supplements always seek the advice of your doctor or another suitably qualified professional such as a nutritional therapist. This is particularly important if you are on medication for which you must seek medical advice. The content of this blog is for informational purposes only and is not a substitute for professional medical advice. Always seek the advice of your doctor with regards to any questions you have about a medical condition.
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