The Truth About Seed Oils

Nutrition science is difficult, the food matrix and the digestive and metabolic processes that engage with it are incredibly complex. Trust in science makes things even more complicated when money and politics can interfere and incentives can be questioned. Yet in the hopes of saving lives hundreds of thousands of researchers around the world dedicate their entire careers to understanding dietary mechanisms and the net effect they have on human health.

When you simultaneously demand proof while dismissing overwhelming evidence you are not a skeptic you are a denialist.

I have no skin in the game. I receive no funding from seed oil companies, nor from “Big Butter.” In fact I provide these articles for free simply because I work as a nutrition coach and I am a huge nerd. I want to help my community make the best decisions they can when it comes to their health and it makes no difference to me whether seed oils are healthy for you are not. I just want to know what the evidence points to.

Once again, this is an in depth look at the research so as always you can scroll down for the summary if you don’t want all the details. I have cited approximately 50 studies here some of which are meta analyses and so the cumulative research is much more.

Spoiler alert, the evidence is in and the overwhelming consensus among nutrition researchers and lipidoligists is that seed oils are good for you.

There is nuance to the above statement as is reflected in the studies and we will take a look at them but first a quick overview of what exactly we are diving into – seed oil and its content of polyunsaturated fatty acid.

Saturated, monounsaturated and trans fats are non essential, our body has everything it needs to make its own supply. Polyunsaturated fats on the other hand are essential. We need to consume these for optimal health and these include both Omega 6 and Omega 3 fatty acids. Something being essential is often used amongst the wellness community to manipulate people into “intuitive thinking.” The keto and carnivore camps like to point out that carbohydrates are non essential while other such enthusiasts like using the fact that cholesterol is essential to our brain’s makeup and in the formation of hormones as proof that we should not be afraid of loading up on dietary cholesterol. Something being essential to our health is not enough information. When you come across arguments that are all or nothing using nutrients as essential or not, try and ask a few more questions…How much is essential? Are all sources of an essential nutrient equal? Is dietary intake of this thing necessary or can our body make it from other compounds? If it’s non essential, does that mean it’s not good for us? Can a non essential whole food also contain other nutrients that are essential or beneficial? This is the kind of nuance that is worthwhile when searching for answers on food choices.

So if Omega 6 fatty acids are essential, how much do we need on a daily basis? The answer depends on age and sex but somewhere between 11-22g/day is sufficient. Good sources of these fatty acids include, walnuts, almonds, flax seeds, flax oil, sunflower oil, canola oil, avocado oil, soybean oil and safflower oil. I know, you have heard that these oils are toxic or inflammatory, I had heard the same things. To be honest I used to avoid many of these oils but bare with me and I will show you why I changed my mind.

Omega 3s are universally considered as anti-inflammatory and also only come from foods or supplements we consume. Good sources of Omega 3s are fatty fish and algae. Plant based options are flax seeds, walnuts and chia seeds but the version of fatty acid you get from these foods – Alpha Linolenic Acid (ALA) – needs to be converted to DHA and EPA. The conversion of ALA is not always great and so for most people this fatty acid is not as bio-available as EPA and DHA. This is not universally agreed upon but the general consensus is that ALA is the lesser form of Omega 3. I usually suggest to my vegan and vegetarian clients to supplement with fish oil or algae capsules. You can get blood Omega 3 tests done to see where you stand in the absorption spectrum if you want to avoid EPA and DHA supplementation.

The recommended daily dose for Omega 3s is approximately 1-1.5g/day but this can vary depending on age, sex and heart condition. For reference, my average recommendation is about 2g/day leaning heavier towards EPA content but again this will change depending on the client and their situation. I have some clients on 5g/day for a couple months just to sort out some inflammation issues.

I am not going to get into the nitty gritty of the Omega 3 (O3) and Omega 6 (O6) ratio because I fear it will take up too much time. But we do need both of these fatty acids and we need some kind of ratio for balance. There is some consensus on a 2:1 (O3:O6) ratio but it appears that this may be less accurate than previously assumed.

COMPARISONS TO SATURATED FATS

We know that fats are a necessary part of our diet, particularly Omega 3 and Omega 6 but the vast majority of our diets in western culture consume saturated fat as our main fat source. This is true for most people on a ketogenic or carnivore diet as well, in fact it can be much much higher. Obviously this has variability within populations and between cultural norms but globally there tends to be more calories eaten through saturated fats rather than polyunsaturated or monounsaturated fats. Saturated fat sources being; beef, pork, lamb, poultry, butter, ghee, lard, full fat dairy, coconut oil and baked goods. There has been a backlash away from the low fat diets recommended from doctors and government institutions through the last several decades. It has come under question whether saturated fats are actually problematic at all. Do they cause cholesterol increases? Is cholesterol increase an issue? Is high cholesterol (as tested by doctors) the cause of heart disease, obesity and metabolic disease? Carbohydrates and seed oils seem to be the new target in the fight against growing health issues in North America and across the planet but which is the true culprit? Or might it be a bit more complicated than just one food source?

The saturated fat enthusiasts will have you believe that you can eat butter and meat to your hearts content and not pay any price but what does the research say? When looking at the health effects of ingredients, food or food groups one of the best ways to determine their impact is through comparative study.

Studies looking at liver fat and liver inflammation markers suggest that consuming saturated fat while in a caloric surplus increases both liver fat and inflammation, while eating polyunsaturated fats in a caloric surplus decreases liver fat, inflammatory markers and oxidative stress.

One particular study shows the degree of liver fat that accumulates when overfeeding with various sources of foods including simple sugars. The researchers controlled for total calories of each food group and they saw an increase in liver fat from overfeeding of both sugar and fats but the increase is highest when overfeeding with saturated fats. The amount of liver fat accumulation went like this – 55% from group fed saturated fat, 33% from group fed sugar and 15% from group fed unsaturated fat.

Another interesting study took participants and overfed them to reach an increase of 3% bodyweight. One group was overfed using saturated fats and the other group was fed Omega 6 linoleic acid. After 7 weeks of overfeeding both groups gained the same amount of weight but the saturated fat group gained 58% liver fat and the linoleic acid group gained 5% liver fat. Interestingly there was more lean mass gains with the polyunsaturated fat group.

Some studies looking at stable caloric intake also see a tip towards liver fat with people who consume a majority of their calories through saturated fats yet we do not see this with a high intake of polyunsaturated fats. One of the reasons for this is that polyunsaturated fats are preferentially oxidized.

Oxidation is a buzz word that has a lot of negative connotations attached to it but it is a necessary and mostly regulated process. Compounds are most often oxidized so that their metabolites can be used by the body. It can become a problem when it occurs too often and the body can not neutralize excessive free radicals. But this is not an inherently bad process.

Research has shown that saturated fats on the other hand are less prone to hydrolysis and oxidation so there is a higher probability of transportation of dietary saturated fats and triglycerides to the liver. They also stimulate lipogenic pathways – the breakdown and release of stored fat into circulation which may also end up in the liver.

Saturated fats from dietary intake are absorbed in the small intestine and repackaged in lipoprotein particles called chylomicrons. These lipoproteins become the primary vehicle of triglycerides into the bloodstream. Studies show that chylomicrons enriched with polyunsaturated fats are less harmful as they tend to be broken up earlier in the digestion process and released as free fatty acids for tissues to utilize. This so called preferential oxidation ends up lowering the triglyceride count in our blood.

A fatty liver is a serious problem when it comes to diabetes and metabolic disease as the liver stores fat it loses its ability to regulate blood sugar levels. On top of the harm from extra calories ingested via saturated fats another issue can inhibit insulin sensitivity. Ceramide is a compound derived from saturated fats that inhibits glucose transport into cells and play a role in B-cell death. Increasing saturated fat intake has the potential in overloading the blood with ceramide yet studies like this one show that diets high in polyunsaturated fat lower circulating ceramide levels.

The above listed research along with many others point to insulin resistance as a downstream effect of overloading on the wrong kind of fat and storing fat in the wrong place. Professor Roy Taylor, diabetologist and professor of medicine and metabolism at the University of Newcastle has coined something called the Twin Cycle Hypothesis (diagram pictured below). A simplified description of this hypothesis goes a little like this –

High saturated fat intake, particularly with an excess of calories leads to fatty liver and a lessened sensitivity to insulin. Muscle insulin resistant individuals could not store a large part of their glucose as glycogen while healthy people could store upwards of a 3rd of what they consumed from an entire meal. When this glucose is not being stored in the muscle the liver will then turn it into fat and store it as fat in and around itself. As the liver retains more fat it increasingly loses its ability to manage insulin response. Instead of dampening blood glucose levels through the insulin response the liver will now release more glucose into the blood. Here we begin to see this liver cycle worsen as the pancreas increases insulin once again to try and deal with spiking blood glucose. As this first cycle runs on the liver will also begin to release more fat into the blood as well in the form of triglycerides. This extra fat is spread out into other tissues including as subcutaneous fat. This is not inherently harmful but once subcutaneous fat stores have reached capacity the extra fat begins to store around other organs, including the pancreas. This is where the second cycle begins. Fat storage in and around the pancreas slows the insulin releasing mechanism in response to dietary glucose and so blood sugar continues to rise, reinforcing both cycles.

This theory explains why some slim people get type 2 diabetes. Certain individuals exhibit a low threshold for fat storage below the skin but will store it around the organs. This is less visibly noticeable and so it is less likely for doctors to do any metabolic testing. The epidemiology also suggests that when large populations are overfed we see a sharp increase in type 2 diabetes. When we reduce calorie intake, we see a reversal of this disease. So genetics play a role in your capacity to store fat under the skin but overfeeding, especially with saturated fats can have a devastating effect on your liver and pancreas. Obviously if you add high caloric foods including refined carbs or sugar this will make things worse. Its not that it is just the saturated fat but it is a major catalyst.

A large epidemiological study looking at fatty acid levels and the incidence of insulin resistance amongst otherwise healthy individuals showed a protective effect from polyunsaturated fatty acids. But even more interesting than that is a meta-analysis including over 100 random control trials (RCT) looking at metabolic health when replacing calories from saturated fats with polyunsaturated fats. What they found was that replacing calories with polyunsaturated fats significantly reduced insulin resistance and improved blood glucose control. It is true that elevated blood sugar can damage artery walls making them more susceptible to plaque build up. And while this may look like just a glucose intake issue it seems as if it is more likely a downstream effect of a calorie surplus from saturated fats and storing fat in the organs. Once this process has begun, having a high intake of sugars will only exacerbate the problem.

Another RCT looking at calorie replacement, this time using rapeseed oil instead of ghee showed that liver fat was reduced as well as liver enzymes. On top of that blood glucose control was improved, fasting glucose was lowered, insulin levels diminished and LDL numbers dropped.

On a calorie per calorie basis, saturated fats tend to increase insulin resistance compared to polyunsaturated fats

Some people may argue that the majority of these studies show a relatively short period of oil consumption and that it may be a slow decline in health from these “dirty”, heavily processed foods. luckily enough we do have a bunch of long term studies exploring this exact question. This meta analysis looked at 38 studies, involving close to a million participants. The conclusion suggests that a higher intake of LA over time correlates with lower inflammatory markers but also a moderately lower risk of cardiovascular disease, cancer and mortality by all causes.

Let’s be clear, you don’t need to eat or cook with seed oils. Avoiding excessive amounts of them is not necessarily harmful for your health, especially if you are focused on eliminating the ultra processed, packaged foods that contain these oils. Avoiding these foods will be beneficial if you are replacing the calories with a balanced whole food approach but it is important to make the distinction that the seed oils in these products are probably not the problem.

Knowing the potential protective effects of polyunsaturated fatty acids is key to understanding the idea of balance. Misinformation suggesting that the compounds in seed oils are inherently bad leads to decisions about food that lack nuanced understanding. For seed oils in particular this may or may not be problematic – If you end up excluding monounsaturated and polyunsaturated fats or substitute saturated fats in for them then you might end up with some problems. But importantly, leaning into all or nothing thinking about a food or ingredient can lead to worse decisions in other areas. If we fall into habits of bad heuristics then we are susceptible to disinformation. The same influencers that demonize seed oils also chew on bricks of butter. This substitution is most probably harmful, while a nuanced decision to avoid an abundance of seed oils in certain forms is not. Key word here being abundance seeing as keeping polyunsaturated fats, including linoleic acid clearly has some health benefits.

CARDIOVASCULAR DISEASE

The above mentioned studies were mostly specific to comparing polyunsaturated fats from vegetable oils to saturated fats and the effect on the liver and insulin resistance. But I want to expand a little bit and look at a few studies examining cardiovascular disease risk that include comparisons with saturated fats as well as carbohydrate intake.

A couple clinical trials looking at this question are the Predimed and cordioprev studies. These trials looked at people eating a Mediteranean style diet with about 40% calorie intake through carbohydrates (moderate) enriched with olive oil and nuts compared to people on a low fat diet plan. The basic take away was that a reduction in refined carbohydrates and saturated fats along with added calories from unsaturated fats significantly lowered the participants risk of cardiovascular disease.

If you read my article on cardiovascular disease risk and prevention then you are familiar with the evidence showing that ApoB is the very best marker for cardiovascular disease risk that we have. It would stand to reason that if polyunsaturated fats were showing reduced mortality from cardiovascular disease then adding them to your diet should directly lower ApoB levels and they do. Replacing saturated fats or refined starches with polyunsaturated fats tend to reduce ApoB more than the reverse. Even replacing whole grain intake with polyunsaturated fats seems to reduce numbers though more moderately.

We need to keep in mind that the dietary guidelines we have been given are not being followed. Many social media health personalities trash talk the guidelines and assume conspiracy to keep people sick by promoting low fat diets and high carbohydrate diets. While it is true that the calorie intake from these guidelines did skew a bit heavy towards carbohydrates it is also true that they did not suggest the intake of added sugar or the lowering of polyunsaturated fats. Regardless, the vast majority of North Americans did not comply, eating diets very high in saturated fats and refined sugars. So demonizing the guidelines is at best incomplete if not a sad attempt to prop up an ideology around nutrition. Jordan Peterson is famous for this exact problem in what looks like an attempt to promote his carnivore diet.

Storytelling is a very effective way to sell an idea. We are much more attracted to individual anecdotes than we are widespread evidence. So be careful of anyone making sweeping claims based on their personal story.

An interesting 6 month long study used a low carb vegan protocol coined the Eco-Atkins diet to see effects on cardiovascular risk and body weight in hyperlipaedhemic adults. The diet was low in carbs and saturated fats and was enriched with fats from soy, nuts and vegetable oils. They compared results with participants who had a high carb/low fat diet. The Eco-Atkins group showed reduced LDL cholesterol and triglycerides, thus lowering heart disease risk.

Another study replacing a higher carb/lower saturated fat diet with corn oil reduced LDL cholesterol by about 10.5% while replacing it with olive oil reduced it by about 3%. Olive oil also reduced heart rate and blood pressure.

This meta analysis reviewed approximately 40 control trials looking at the cardio-metabolic response to canola oil intake. While CRP levels (inflammation marker) were no different between canola oil intake and comparators, many other cardiometabolic risk factors were reduced in the canola oil groups. More specifically, canola oil reduced total cholesterol, LDL cholesterol, LDL to HDL ratio, ApoB and ApoA-1. Even compared to olive oil, canola oil reduced total cholesterol and triglycerides from vldl (very low dense lipoproteins).

But we don’t only see benefits with liver function, insulin resistance and cardiovascular disease. We also have evidence that replacing calories from saturated fats and added sugars can reduce mortality risk from neurodegenerative disease and cancer. This study review looking at 2 studies involving almost 125,000 participants over multiple decades had participants replace 5% of calories from saturated fats to unsaturated fats and they saw reduced risk of dying from cvd, neurodegenerative disease, diabetes, cancer.

How many people can sustain a low carbohydrate diet for life? Many low carb diets are usually considered to be around 50g/day. For reference this is about 2 bananas. For those of us who are not the hyper disciplined influencers, maintaining. a very lean body mass and staying very active the high fat meal plans can slip into weight gain and insulin issues fairly quickly, especially if you re-introduce carbohydrates along the way.This is what I see a lot of. People who are used to a life of moderate to high carbohydrate intake looking to lose weight and grabbing onto ketogenic style diets in hopes of reducing caloric intake with regards to refined carbs. They may see a big change at first by reducing their caloric intake having got rid of a lot of the snack foods and sugary drinks. But social gatherings, busy schedules and a lack of balance in input and output with activity can lead to adding some carbohydrates back in the mix. Now we have high caloric fat foods added with sugar and maybe a lack of activity. The extra weight comes back fast. The added saturated fat makes blood glucose regulation harder and this can lead to weight gain but also metabolic disease, especially once the carbs come back into the picture.

When you compare food sources you have to really take a look at what is being substituted for what and when you compare that to society at large you need to see if people actually subscribe to guidelines or not. When we look at calories of saturated fat substitution for calories from whole grain carbs, polyunsaturated fats or monounsaturated fats we see a decrease in negative health outcomes. When we look at substitutions of calories from saturated fat with refined carbs we see similar outcomes So when you reduce saturated fats, what are you eating instead? In reviews that compare saturated fats to carbs they suggest that there is no harm done by saturated fats, but these reviews don’t differentiate between refined and unrefined carbs.

Another complexity in looking at the evidence is what source of saturated fat we are looking at. For example, Dairy fat seems to look favourable over fat from red or white meat when looking at cardiovascular risk. But it does not look favourable when compared to vegetable fat or whole grains. Yogurt and cheese seem to do a bit better in cardiovascular health outcomes and ldl cholesterol numbers compared to milk, meat and butter. But the benefit of vegetable oil is significantly better than those. The degree of improvement can be slight or it can be significant depending on what you are substituting. The nuance is important when you are bombarded by health gurus trying to sell products or services. Their ability to convince us to buy their products is increased if they get us to ignore the nuances of the food matrix.

Much of the vilification of seed oils comes from the idea that Linoleic acid (an omega 6 polyunsaturated fat found in different oils to varying degrees) is highly inflammatory as is its substrate arachidonic acid. Arachidonic acid plays a role in many functions, including inflammatory actions like thrombosis. So driving its synthesis upward through a high intake of Linoleic acid seems problematic and this is what the messaging is by health gurus like Paul Saladino, Dave Asprey, Ben Greenfield and Casey Means. But there are three things missing from the messaging.

• First is that arachidonic acid is crucial for many beneficial processes including as a precursor for signalling molecules, roles in brain development and cognition, blood clotting and well, inflammation – because inflammation is not all bad.
• Second is that arachidonic acid is not the only metabolite of linoleic acid and is itself broken down into more substrates that have various functions. Some of which include lipoxins and epoxyeicosatrienoic acids which are known to have anti inflammatory effects. This is why posting health information with claims related to a single mechanism is myopic.
• Third, though we may indeed want to keep arachidonic acid below some threshold to maintain good health it appears that lowering linoleic acid intake is not the best way to do so. You have to limit linoleic acid so drastically as to effect arachidonic levels that it becomes both difficult and potentially harmful to your health. Omega 3 intake seems to do a better job at limiting the negative effects of arachidonic acid than simply not ingesting it. Indeed the inflammation problem we may see in many countries that have a hight intake of processed foods may also be a low average omega 3 intake rather than a high average omega 6.

Indeed the wide variation of linoleic acid (LA) intake does not seem to affect tissue levels of arachadonic acid (AA). You can decrease your intake of LA by 90% and the AA does not change significantly. Increasing intake by 6 times also has no effect on tissue levels of AA.

Let’s remember that the ultra processed foods that have seed oils as an ingredient have been designed to over consume, they are high in sugar, low in fibre and calorie dense. Demonizing a single ingredient tends to show a lack of understanding of the complexities of human nutrition.

This 2012 meta-analysis of 15 RCTs concluded that ” virtually no evidence is available from randomized, controlled intervention studies among healthy, non infant human beings to show that addition of LA to the diet increases the concentration of inflammatory markers.”

But since inflammation seems to be the thing everyone is pointing at as an issue with these oils, let’s take a closer look.

INFLAMMATION

There are a number of studies looking at mechanisms of seed oil metabolites that suggest there is a problem with inflammation. While I do think that these studies are important and add to the conversation of overall health outcomes I don’t think that they are necessarily primary to the conversation. In the interest of saving you time I will just list many of the studies and their conclusions. I will also provide some of the mechanistic studies that point at certain metabolites being inflammatory because I want you to have as much of the information as possible. Just keep in mind that we can find mechanisms going in every direction. The complexity of everything at work within the human body is why we have to also look at outcomes to see if these mechanisms are predominant or neutralized somewhere along the way. I will present this list in sections of different kinds of seed oils.

Flax seed oil

Flax oil is high in ALA omega 3 and about 14% of its calories are from Linoleic acid .

A meta analysis of randomized control trials looking at flax seed oil and C-reactive protein (CRP – an inflammation marker) found no effect.

Another meta analysis of 12 randomized control trials looking at biomarkers of inflammation and oxidative stress found no effect on CRP, a significant reduction in Interleukin 6 (IL-6 – an inflammatory marker) and malondialdehyde (MDA – an inflammatory marker) with no effect on oxidation properties.

From all the trials in the above meta analyses only one study found elevation of the inflammatory marker tumor necrosis factor alpha (TNF-a).

Canola Oil –

Canola oil is high in monounsaturated fats with about 18% of its calories from linoleic acid.

This randomized double blind placebo trial found no increase in CRP. It cactually oncluded a potential protective effect from CRP.

This randomized control trial (RCT) had participants double their omega 6 intake and found no differences in Il-6, CRP or TNF-a

This study is already mentioned above but I wanted to also mention that both olive oil and canola oil had a reduction in CRP

This meta analysis was also mentioned above with regards to flax seed oil but no difference in inflammatory markers were found with canola oil either, even when compared to olive oil!

Sunflower seed oil

Sunflower seed oil is mainly made up of triglycerides and is about 59-65 percent linoleic acid.

This RCT using refined oil with its participants found no significant change in CRP and total cholesterol and LDL cholesterol were both lowered.

This RCT also found no changes to participant’s’ CRP levels

In this randomized control trial they compared sunflower oil to butter intake and some inflammatory markers were lower in the sunflower oil group. No oxidative stress was found and liver fat was also lowered without weight loss.

In this meta-analysis including 9 trials, no significant change was found in several inflammatory markers, including; CRP, TNF, IL-6 and intercellular adhesion molecule-1 (ICAM-1).

Soybean oil

Soybean oil has about 50% of its calories from linoleic acid

In this RCT intravenous feeding of a soybean lipid emulsion compared to olive oil intake showed that there was no significant difference between the two groups

This RCT comparing soybean oil to flaxseed oil and fish oil using capsules for two months saw no significant change in inflammatory markers in any of the groups.

In this evidence review the authors conclude that soybean oil is the leading edible oil consumed globally and that there is no evidence that it increases inflammation or oxidative stress.

Corn oil

Corn oil has approximately 52% of its calories from linoleic acid.

In this RCT participants took 4 tbsp/day and they saw no significant effect in CRP

Similarly, in this RCT researchers compared flax seed oil to corn oil feeding participants 2 TBSP/day for 3 months. They concluded that flax seed lowered IL-6 and corn oil had no effect on inflammatory markers.

In this RCT they compared corn oil to fish oil and found no difference in inflammatory markers in either group.

Safflower oil

Safflower oil has about 70-80% of its calories from linoleic acid.

This clinical trial using 1 TBSP/day for 3 months showed no significant change in CRP or IL-6.

A randomized control trial increasing the intake to 2 TBSP/day for 3 months also saw no significant change in inflammatory markers.

One inflammatory marker was decreased in this trial using 1 TBSP/day for 3 months.

A RCT using a massive dose of 8g/day saw a decrease in CRP and inflammation, glycemia and blood lipids!

What about Dose? Does an even higher daily dose show inflammation issues? Well this trial comparing olive oil, sunflower oil and rapeseed oil had participants consume 500 calories of oil per day for one month. No increase or decrease in inflammatory markers was found on any of the diets.

What about time? What if people consume these oils for a much longer period of time?

This meta-analysis as mentioned earlier showed longer intakes of linoleic acid is associated with lower cholesterol, lower levels of CRP, IL-6, Il-1B (a cytokine protein that plays a role in inflammation) and an increase in insulin sensitivity. This research is based off of survey information so we need to take the data with a grain of salt. But we can measure blood level omega 6 fatty acids to make sure we see the relationship between high blood omega 6 fatty acids and inflammatory markers and this study did just that. The data was reviewed from clinical trials and meta analyses and the results show an inverse relationship – the higher the Omega 6 levels in the blood/tissues – the lower the inflammatory markers.

THE EFFECTS OF LIGHT AND HEATING SEED OILS

Another claim is that the oils are highly unstable with regards to light and heat and that exposure to either causes chemical changes that are harmful to our health.

This study looked at light oxidation in fresh canola oil and found that these oils can become rancid under prolonged light exposure. They did this by looking for particular triacylglycerol isomers under certain wave lengths of light and between different temperatures of heating. They found higher amount of isomers with longer exposure of light and heat. Both taste and smell was affected by light oxidation. They concluded that dark containers can extend the shelf span of canola oils.

Another study looking at oxidation process of rapeseed oils concluded that red light seems to have a stronger effect in the production of oxidative radicals and that strong fragrant rapeseed oils are at a higher risk of oxidation.

This study compared the peroxide value and other reactive substances found in sesame oil, soybean oil and mahua oil. They concluded that the oil with the most oxidative stability was mahua oil. They suggest that the oxidative risk is associated with both the amount of linoleic acid as well as the amount of natural antioxidants within the oil. So the higher the linoleic acid the higher risk of oxidation but also the higher amount of antioxidants the lower the amount of oxidation.

Here are some health outcome studies…

In this RCT – researchers replaced normal cooking oil with corn oil or peanut oil for one year. No difference was found in CRP, blood lipids, blood glucose or insulin levels.

This study that was mentioned earlier was a comparison between coconut oil and sunflower oil for cooking. After 2 years no differences in CRP or other cardiovascular health risk markers were found.

In this 2 month RCT participants cooked 3 meals a day in soybean oil many of which were fried. Researchers sae a decrease in CRP as well as in oxidative stress. None of the other inflammatory markers changed.

RCT – A very unique study looking at heated oil consumption gave one group of participants capsules with oil that was heated for 2 hours at 240 degrees. The other groups were given olive oil, non heated safflower oil, and a conjugated linoleic acid mixture. Participants consumed about 4.5g/day for 4 weeks. No difference in inflammatory markers were found in any of the groups. There were also no changes in lipid profile, insulin indices, oxidized ldl, paraoxonase or other cardiovascular risk measures.

What about long term frying? Does it make a difference if we heat the oil for long periods of time or through many reheating cycles?

In this study 20 men with coronary artery disease and 20 healthy men were given a meal using sunflower oil heated for 1 hour at 180 degrees vs a meal with no heated oil. No change in inflammatory markers were found.

In this study – 20 obese people were given 1 meal with the foods cooked in olive oil, sunflower oil or a mixture of seed oils that had gone through 20 reheating cycles. The researchers did not look at blood markers for inflammation but some cellular mechanisms for inflammation were showing increases. The olive oil group did better. No major differences for the healthy men but the ones with Coronary artery disease showed that the unheated oil lowered some of the inflammatory pathways and the heated oil raised some of them.

CONCLUSIONS

Human studies on health effects of diet substitutions is our best way to discover whats happening at a practical scale. Together, randomized control trials and long term observational studies give us a more nuanced look at health outcomes. Whereas studies on Isolated molecules or metabolic pathways do not necessarily allow for leaps towards conclusions on the net effects on human health. They do however add significant details to the discussion, like which levels of processing increase the likelihood of inflammatory compounds or how to best store these oils to minimize such compounds. You can choose to only used cold pressed oils or oils that use tinted bottles so that the potential oxidation is reduced.

One study is not enough to back up claims on health outcomes but it can be a good start. If you see health practitioners using single studies to vilify a food or ingredient you should be weary. The quality of the study should also be looked at. This goes for meta-analyses as well. A study review is only as good as the studies it is reviewing and so I like to err on the side of caution and look for a vast amount of data that points in a similar direction.

There are a few studies commonly cited to back up claims that polyunsaturated fatty acids are bad for our health, one of which is the Minnesota Coronary Experiment. This study, while well designed had some serious flaws and thus the results are not of much use. One of the problems is that approximately 75% of the participants left the study before the first follow up at 12 months. This is a huge drop in participation and lessens the strength of the data. Another issue is that the cohort who was given unsaturated fats was unknowingly given trans fats through the margarine they were fed. We are all very well aware of the danger of trans fats and this probably skewed the results significantly.

We should all expect a higher degree of certainty before making any conclusions, especially when it comes to our health. But if we did want to play at their game, we could look at the Veterans study which followed less people but for a much longer period of time and had more reliable data. The people in this study eating more seed oil instead of saturated fats had 30% less occurrence of heart disease.

Better yet something like The Cochrane Review which is a meta analysis of 10 trials covering over 53000 participants also shows a reduction in the risk of cardiovascular events with people who eat more unsaturated fats vs those who eat more saturated fats. This health outcome data is very valuable for making better food decisions, particularly if you are at risk for heart disease or metabolic disorders.

Stories are intriguing but they don’t tell us much about truth. Just like mechanisms, you can find anecdotes pointing in every direction with regards to what might be an optimal way to eat. I recently saw a very popular youtube health guru try and debate for the consumption of raw milk based on his and his families personal history with it. Just because you have not experienced harm in something does not mean it does not cause harm. How many people have smoked cigarettes and lived to their 90s? Does that point towards cigarettes being good for everyone? Of course not.

having a high intake of omega 6 through ultra processed food is also not ideal. Not so much because of the fats but because of the tendencies to overeat these foods and the other ingredients that come along with it. Other issues can compound the problem such as not getting enough omega 3 fatty acids. It might be that high quantities of omega 6 without sufficient EPA and DHA may obstruct the building of these fatty acids from ALA. So this would be another compounding issue for those who eat a predominately vegan diet without omega 3 supplementation. In this particular case you may want to avoid an excess of linoleic acid intake.

The general conclusion though is that polyunsaturated fats have benefits to human health. Including them in the form of nuts and oils can help reduce fatty liver, cardiovascular disease and metabolic issues. But to stay on the safe side you can avoid oils in clear containers and reheating these oils at very high temperatures. Chances are if the oils have been oxidized you will be able to smell and taste it so you can avoid ingesting it if you notice a rancid scent. Make sure you eat enough omega 3 fatty acids regardless of your omega 6 intake as they are very good for you but if for some reason you are eating a lot of foods containing linoleic acid you can add more omega 3s to fend off on a build up of potential inflammatory compounds.

Thats it! Even the most complicated situations call for simple choices when it comes to food. If for some reason you are still not convinced then obviously it is your choice to avoid seed oils. It won’t necessarily do harm as long as you are not replacing the calories with saturated fats and refined carbs. I on the other hand feel fine occasionally using canola oil to cook with and no longer put a particular food item down if I see seed oils as an ingredient. The bigger picture of my complete diet is most important.

Joey