Vegetable oils: are they healthy?
Vegetable oils — those shiny modern elixirs — have seeped their way into all the nooks and crannies of our food supply.
If you eat out, chances are your food is cooked in — or doused with — some type of vegetable oil. If you buy packaged goods like crackers, chips or cookies, there’s a very good chance that vegetable oils are in the ingredients list. If you buy spreads, dips, dressings, margarine, shortening or mayo, can you guess the likely star ingredient? Yup — vegetable oils.
Is this a good thing? To find out, let’s review what we know, and what we don’t know, about these plant-based fats.
Ready for a deep dive into this controversial issue? Click any of the links to jump to that section, or simply keep reading!
Because of inconsistent evidence of health benefits, potential negative health effects, concerns about the ultra-processed and unstable nature of many of these oils and due to evolutionary considerations, we believe that the conventional recommendations about vegetable oils deserve a more detailed analysis. Full disclaimer
What are vegetable oils?
In a technical sense, vegetable oils include all fats from plants. However, in common usage, “vegetable oil” refers to the oil extracted from crops like soy, canola (rapeseed), corn and cotton.
Is olive oil a vegetable oil? What about palm oil and coconut oil? Technically, yes, these oils come from plants, so they are vegetable oils. But they originate from the fruit or nut rather than the seed and are easier to extract.1 These oils have been a part of the food supply for thousands of years. Together, these three traditional oils account for less than 15% of today’s vegetable oil consumption in the US.2 More than half — about 53% — of the vegetable oils consumed in the US comes from just one crop: soybeans.3
For purposes of this post, we will narrow the meaning of vegetable oils to include only oils from the industrial oilseed crops: soybean, canola (rapeseed), corn, sunflower, cottonseed and safflower oils.
In addition, we will assume the vegetable oils we are discussing haven’t been hydrogenated. Partially hydrogenated vegetable oil products, like Crisco and margarine, were once marketed to Americans as “heart healthy.” Now we call them “trans fats,” which we are in the process of eliminating from our food supply due to their negative health effects.4
How are vegetable oils made?
Unlike olive oil that has been pressed for centuries, most vegetable oils require significant industrial processing.5
Heat, cold, high-speed spinning, solvents like hexane, degumming agents, deodorizers and bleaching agents are typically used to process the seeds into a palatable oil.6
For a visual rendition of this industrial process, check out this video that documents the production of canola oil.
Given the level of industrial work required to extract the oil, many modern vegetable oils are rightly classified as ultra-processed food products.
How much vegetable oil did we eat historically, and how much are we eating now?
Refined vegetable oils are the ‘new kid on the block’ in human diets.
Millions of years ago, the only vegetable fats our ancestors consumed likely came from wild plants.7 Around 4000 BC or earlier, pressed olive oil became a staple in the diets of people living in Italy, Greece, and other Mediterranean countries.8
The vegetable oils we know today were developed at the end of the 19th century, when technological advances allowed oils to be extracted from other crops.9
Around 100 years ago, it’s estimated that the average daily vegetable oil intake in the US was roughly 20 grams — about 4 teaspoons — per day. Yet that amount has steadily increased over the decades.10
By 2014, the average American was consuming about 50 g, or 11 teaspoons of vegetable oils each day.11 That’s 450 calories, or about 20% of a 2,250 calorie diet. This is roughly double the amount consumed in 1970. Indeed, it appears no other source contributed as much to Americans’ increased calorie intake between 1970 and 2014.12
What types of fatty acids are in vegetable oils?
All fats contain a blend of saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids (learn more), and vegetable oils are no exception. Each type of seed has its own signature blend of the dozens of possible fatty acids that occur in nature, and each fatty acid is either a saturated, monounsaturated or polyunsaturated fatty acid.
Take a look at the percentage makeup of the three main vegetable oils in our food supply, compared to coconut oil, a traditional plant fat:
Polyunsaturated fats are made up of omega-6 and omega-3 fat. These are the two types of essential fatty acids you need to consume because your body can’t make them.
However, most vegetable oils contain much higher levels of omega-6 fatty acids and contribute to the abundance of omega-6 fatty acids (over omega-3 fatty acids) in the standard American diet. 13
This is a decidedly modern pattern. Until relatively recently, it’s estimated that humans consumed omega-6 and omega-3 fats in a roughly 1:1 ratio. Today, that ratio is estimated to be around 16:1, on average.14
For more on the origin and structure of different types of fat, check out our guide below.
Are vegetable oils healthy?
Whether vegetable oils are beneficial or harmful is a topic of intense debate. Let’s take a look at the potential concerns about them.
What happens to the vegetable oils that we eat?
Fatty acids can be burned for energy, so vegetable oils are a source of fuel. If we do not need that energy immediately, our bodies store it in our fat cells.
But fatty acids are also used to build and repair body parts, create internal signaling molecules, and build cell membranes. And the selection of fatty acids in the food you eat provides the array of building blocks available to the body.15 So the vegetable oils you eat literally become a part of you. Your mother was right — you really are what you eat!
The fatty acids vegetable oils provide are not identical to those found in more traditional fats. Some evidence, albeit weak evidence, suggests the fatty acids from vegetable oils may be less stable that those from more traditional fats. Therefore incorporating them into our cell membranes could potentially negatively affect membrane fluidity and cellular function.16
While this is not conclusive, it does beg the question: Do vegetable oils improve our health in other ways that would offset this potential concern? At this point, we don’t know, but read on for more details.
Are vegetable oils inflammatory?
There are theories and mechanistic studies suggesting that a high absolute intake of omega-6 fatty acids and a high ratio of omega-6 to omega-3 fatty acids can increase inflammation.17 Despite a number of suggestive mechanistic studies, when repeatedly tested in randomized trials, this hasn’t been shown to be true.18
For example, a systematic review found no evidence that linoleic acid, the main omega-6 fatty acid in vegetable oils, increases inflammatory markers, at least in healthy people.19
Additionally, randomized trials, one in people with non-alcoholic fatty liver disease and one in overweight men, found that meals high in omega-6 fatty acids led to lower levels of inflammatory markers than meals high in saturated fat. Of note, both groups consumed 40% of their calories from carbohydrates.20
That doesn’t mean we recommend consuming these oils to treat fatty liver or to lose weight. But it shows clinical evidence that high omega-6 intake can actually decrease clinical inflammation in this given scenario, thus providing higher-quality evidence against the proposed mechanistic studies. While it does not completely invalidate the proposed mechanistic effect, it calls into question the degree of its application.
On the other hand, the amount of polyunsaturated fats we consume today is at odds with our evolutionary past. Our distant ancestors consumed small amounts of these essential fatty acids in fish, game, and wild plants.21 Today, most of our PUFAs come from vegetable oils, and we get far more omega-6 than we need.22
Bottom line: There’s no clear high quality evidence that vegetable oils are inflammatory. However, questions remain about the consequences of such a profound dietary shift in a relatively short period of time, and if mechanistic studies of inflammation will prove to be a long-term clinical concern.
What happens when we cook with vegetable oils?
Vegetable oils contain mostly polyunsaturated and monounsaturated fatty acids, which means they are liquid at room temperature. It also means that they are typically less stable than a predominately saturated fat. This is because unsaturated fatty acids have one or more double chemical bonds that react with oxygen more easily than the single bonds in a saturated fatty acid.
Even if vegetable oils can be stabilized during production to achieve a reasonable shelf life, adding heat can oxidize (damage) them.23
Some animal studies have shown that consuming repeatedly heated vegetable oils may increase blood pressure and cause other adverse health effects due to the formation of aldehydes and other potentially toxic compounds.24 However, this hasn’t yet been tested in humans.
At this time, research suggests that vegetable oils are probably safe for cooking as long as they aren’t exposed to very high temperatures for long time periods. However, the quality of the data is not strong and more research is needed as questions still exist. 25
However, to minimize any risk, it could still be a good idea to mainly use more stable oils for cooking. Fats that contain higher levels of saturated fatty acids, like clarified butter and coconut oil, may be safer for cooking because they remain stable at high heat.26 These mostly saturated fats are solid at room temperature, do not become rancid when stored, and resist oxidization when heated.27
Lard and extra virgin olive oil, both made up of predominately monounsaturated fatty acids, are also quite heat stable.28
Are vegetable oils “heart healthy”?
For decades public health officials have recommended that we replace butter and other saturated fats with vegetable oils for better heart health. But is there strong evidence that this reduces heart disease risk?
Observational studies are mixed, but typically find that people who eat more polyunsaturated fat and less saturated fat have slightly fewer heart events.29
Observational data like this can’t prove that eating more PUFAs protects heart health, though; it only suggests a possible relationship between the two. This is because people who tend to eat more PUFAs may have a greater tendency for other healthy behaviors. In other words, the position that vegetable oils are superior and “heart healthy” is largely based on very weak observational associations.
If you’re interested in learning more about the problems with inferring causation from observational studies, have a look at our guide.
Before replacing butter and other natural fats with vegetable oils, we recommend looking for better evidence from carefully designed clinical trials!
Randomized clinical trials (RCTs) are considered the “gold standard” for evidence. And meta-analyses that pool the evidence from several RCTs provide the strongest, best quality of evidence.
At least five of these recent meta-analyses of RCTs find no link between vegetable oils and death due to heart disease.30
However, a few other recent meta-analyses suggest that vegetable oils might reduce heart disease risk, though the effect is fairly small.31
How can different meta-analyses come up with slightly different answers like this? It all depends upon which underlying clinical trials the authors of the review include. And this can be influenced by point of view.
In some of the RCTs not included in the beneficial meta-analyses (like the Minnesota Coronary Experiment and the Sydney Diet Heart Study) diets higher in vegetable oils and lower in saturated fat did indeed lower total blood cholesterol. Yet the lower cholesterol level did not improve mortality rates.32
In fact, in these two trials, the opposite is true. The groups who consumed more vegetable oil actually had higher death rates, despite their lower blood cholesterol levels.
It’s a bit confusing, isn’t it? The American Heart Association (AHA) bases its endorsements of vegetable oils on the purported link between lower LDL cholesterol levels and better heart health.33 But as shown, many of the clinical trials do not support that final step.
Do vegetable oils lower LDL cholesterol? Yes.34 Does that improve health outcomes that matter most to patients, i.e. does this prolong life, or save anyone from dying of heart disease? The answer from all these trials appears to be that the effect is either small or non-existent.
If vegetable oils are truly good for our heart health, why aren’t the RCTs more consistently showing that people who consume them consistently live longer lives?
Some scientists and doctors think that the unstable, easily oxidized PUFAs are part of the reason vegetable oils are a problem, not a solution.35Although this also remains to be proven in high quality experimental studies.
What’s the bottom line? The science is still inconclusive. The lack of consistent evidence proving vegetable oils meaningfully improve heart health, all-cause mortality and healthspan is concerning, especially given the strength of recommendations to increase them.
Do vegetable oils increase cancer risk?
As we struggle to understand cancer risk, diet is often identified as a key driver. But studies are all over the map. One observational trial (and the splashy headlines that follow) shows a particular food is protective; the next trial of the same food shows the opposite effect.36
There are some observational studies showing associations between high omega-6 polyunsaturated fatty acid consumption and cancer.37 However, analyzing all observational studies together generally show no link between the two.38
But remember, this is observational data, and we cannot assume causation from this weak evidence.
Ultimately, we must look to well-designed clinical trials for answers. There is little of this rigorous science available. An older trial, conducted in a Los Angeles Veterans’ home in the late 60s, revealed increased cancer mortality among men eating the experimental diet that was heavy in vegetable oils.39 (This trial also showed lower CVD mortality for those replacing dairy fats with vegetable oils.)
But there were problems with the study, such as more heavy smokers in the control group, and subjects eating many meals away from the controlled institutional setting. We simply need more data to carefully evaluate vegetable oils and cancer risk.
Bottom line: We don’t yet know for sure if vegetable oils have any significant effect on cancer risk. There is no high-level evidence to suggest there is a significant risk.
Do vegetable oils affect mental health?
Some psychiatry experts have suggested that high omega-6 intake contributes to ADHD and depression.40 Psychiatrist Georgia Ede writes about this potential link, and the mechanisms she believes are behind it, on her website.
However, until clinical trials are completed, we just don’t know whether or not vegetable oils have contributed to the escalating rates of anxiety, attention disorders, and depression in recent decades.
Are vegetable oils contributing to the obesity and diabetes epidemics?
The rise of obesity and diabetes that we have seen over the last 40 years has been matched by an equally dramatic increase in vegetable oil consumption. Is there a connection, or is this just a coincidence?
Animal studies are numerous and mixed. But in humans, the science is scant.
A Cochrane review of clinical trials concluded that consuming more polyunsaturated fats likely has little or no effect on body weight. Overall, researchers found that increasing polyunsaturated fat intake (omega-3, omega-6, or both) led to a slight but significant gain of 0.76 kg (1.7 pounds), on average over one-to-eight years.41
The question of a causal link between vegetable oils and diabetes has yet to be carefully explored. A recent systematic review of clinical trials found that high intake of either omega-6 or omega-3 fatty acids failed to improve blood sugar control or reduce diabetes risk.42 Other than this, there’s very little research on the topic.
Bottom line: It’s still unclear exactly what role vegetable oils play in obesity and diabetes, but increasing the intake may slightly increase body weight.
Our top sauce and dressing recipes
At this point there’s still quite a bit we don’t know about vegetable oils. Consuming small amounts of vegetable oils might make sense for some of us, depending on lifestyle, food preferences, and other factors.
However, if your goals include eating less processed food — as ours do — the best course may be to avoid these newcomers and return to traditional dietary fat sources. Get your fats from whole foods, traditional oils and, of course, butter.
Five step action plan: How to reduce vegetable oils in your diet
Here are five steps to help you reduce potentially excessive amounts of omega-6-rich vegetable oils in your diet.
- Stop buying vegetable oils. The most obvious step is easy — stop buying them! Substitute a different fat for frying, baking and dressing. Try one of these: butter (or ghee), sour cream, lard, tallow, schmaltz, extra-virgin olive oil, cold-pressed coconut oil, and extra-virgin red palm oils. Why not check out our delicious guide dedicated to eating more healthy fat!
- Limit your vegetable oil intake to mayonnaise and salad dressing. Some of us (or our family members) just don’t love — or have the time to prepare — homemade mayo and salad dressings. If you drizzle olive oil on vegetables and cook with butter or other heat-stable fats, using packaged salad dressing or mayo made with vegetable oil is probably okay. If you’d rather avoid vegetable oils completely (a choice we completely understand), here are some recipes for quick and easy salad dressing (see above, too) and homemade mayo!
- Substitute with quality oils when eating out. When you eat out, ask for olive oil and vinegar for your salad. (Some people even like to pack their own favorite dressing.) Another trick at restaurants is asking the waiter if the chef could fry your food in butter or coconut oil instead of the standard cooking oil. Not all establishments can accommodate this request, but it’s worth a try!
- Cut out baked goods and snacks. There are lots of reasons (hello, sugar and refined grains) to avoid commercial baked goods and snacks, and now you have one more reason — they usually contain vegetable oils. Most bakery-made or packaged cookies, pastries, cakes, breads, crackers, chips and bars are made with vegetable oils. Check out the ingredients list before you buy.
- If you have to use vegetable oil, choose high-oleic varieties. A potential downside of vegetable oils is their high omega-6 fatty acid content. However, high-oleic-versions of safflower, sunflower, and canola oils are at least 70% monounsaturated fat and only 10-15% omega-6 fat, making them more shelf-stable and suitable for high-heat cooking.43
Are you still confused about vegetable oils? Science definitely doesn’t have all the answers. Our advice? When in doubt, stick with foods that have been around for thousands of years.
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Healthy fats on a keto or low-carb diet
GuideFat is one of the three macronutrients (“macros”) found in food. On a keto or low-carb diet, fat is your primary energy source, so choosing healthy types and eating the right amount is important. Here’s a guide to everything you need to know about fat on a carb-restricted diet.
Consuming trans fats has been shown to raise LDL cholesterol, lower HDL cholesterol, and increase inflammatory markers:
Canola, sunflower, and some other vegetable oils can also be pressed and are referred to as “cold pressed” or “expeller pressed” oils. After pressing, most undergo processing similar to solvent-extracted oils. ↩
57 g of plant oil, less roughly 15% sourced from olive, coconut and palm oils.
There are exceptions; for instance, high-oleic versions of canola, safflower, and sunflower oil are at least 70% monounsaturated fat. However, these are much less commonly consumed than the higher omega 6 oils ↩
Biochimica et Biophysica Acta 2014: The effect of natural and synthetic fatty acids on membrane structure, microdomain organization, cellular functions and human health [overview article; ungraded] ↩
At Diet Doctor, we feel mechanistic studies, without correlation in human outcome trials, represents a very low level of evidence, and not one that we should automatically base clinical decisions on. Mechanistic studies should, however, serve as guidance for future trials to test the potential mechanism in human outcomes. Human physiology is a complex array of interconnected systems. Therefore, we often find that a simple mechanistic hypothesis may or may not translate to significant health considerations. ↩
Journal of the Academy of Nutrition & Dietetics 2012: Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials [strong evidence] ↩
The American Journal of Clinical Nutrition 2012: Effects of n-6 PUFAs compared with SFAs on liver fat, lipoproteins, and inflammation in abdominal obesity: a randomized controlled trial [moderate evidence]
The Journal of Nutrition 2011: Exchanging saturated fatty acids for (n-6) polyunsaturated fatty acids in a mixed meal may decrease postprandial lipemia and markers of inflammation and endothelial activity in overweight me [randomized trial; moderate evidence] ↩
Heating polyunsaturated fats to high temperatures makes them more likely to react with oxygen, leading to the formation of potentially harmful byproducts:
American Journal of Clinical Nutrition 2020: Dietary intake and biomarkers of linoleic acid and mortality: systematic review and meta-analysis of prospective cohort studies [meta-analysis of observational studies with low RR, weak evidence]
The American Journal of Clinical Nutrition 2009: Major types of dietary fat and risk of coronary heart disease: a pooled analysis of 11 cohort studies [Pooled analysis of observational studies; weak evidence]
These five meta-analyses find no link between vegetable oils and cardiovascular mortality:
The British Journal of Nutrition 2010: n-6 fatty acid-specific and mixed polyunsaturate dietary interventions have different effects on CHD risk: a meta-analysis of randomised controlled trials. [strong evidence]
Typically, meta-analyses are strong evidence; however, meta-analysis on this issue do not all agree, and the noted effects are modest.
PLOS Medicine 2010: Effects on coronary heart disease of increasing polyunsaturated fat in place of saturated fat: a systematic review and meta-analysis of randomized controlled trials [strong evidence] ↩
These recovered randomized clinical trials are presented in detail, and in addition, each one includes an updated meta-analysis:
BMJ 2013: Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis [strong evidence]
Although the Minnesota Coronary Experiment and Sydney Diet Heart Study measured total cholesterol rather than LDL cholesterol, more recent studies show that LDL-C is indeed lowered by diets high in polyunsaturated fat:
Journal of the American Heart Association 2019: Replacing saturated fat with walnuts or vegetable oils improves central blood pressure and serum lipids in adults at risk for cardiovascular disease: a randomized controlled‐feeding trial [moderate evidence]
American Journal of Clinical Nutrition 2015: Replacement of saturated with unsaturated fats had no impact on vascular function but beneficial effects on lipid biomarkers, E-selectin, and blood pressure: results from the randomized, controlled Dietary Intervention and VAScular function (DIVAS) study [randomized trial; moderate evidence]
International Journal of Food Sciences and Nutrition 2009: Small, qualitative changes in fatty acid intake decrease plasma low-density lipoprotein-cholesterol levels in mildly hypercholesterolemic outpatients on their usual high-fat diets [randomized trial; moderate evidence] ↩
This includes James DiNicolantonio, PhD, and James O’Keefe, MD, who recently published a paper linking vegetable oils high in omega-6 fat to heart disease:
A review of studies investigating 50 common ingredients concluded that almost every food was linked to both positive and negative health effects, including coffee, wine, eggs, and beef:
American Journal of Clinical Nutrition 2020: Dietary intake and biomarkers of linoleic acid and mortality: systematic review and meta-analysis of prospective cohort studies [meta-analysis of observational studies with low RR, weak evidence]
Prostate Cancer 2012: Relationship of dietary intake of omega-3 and omega-6 fatty acids with risk of prostate cancer development: a meta-analysis of prospective studies and review of literature [meta analysis of observational studies; weak evidence]
Translational Psychiatry 2017: Omega-6 to omega-3 polyunsaturated fatty acid ratio and subsequent mood disorders in young people with at-risk mental states: a 7-year longitudinal study [observational study; very weak evidence]
Importantly, this meta-analysis included trials in which people increased their consumption of polyunsaturated fats from a variety of sources. Some study participants were instructed to eat more walnuts, and several of the trials looked at the effects of boosting omega-3 fat intake via consuming fish oil or flax supplements. So the effects of vegetable oils per se on weight could be somewhat different.
Cochrane Database of Systematic Reviews 2018: Polyunsaturated fatty acids for prevention and treatment of diseases of the heart and circulation [systematic review of randomized trials; strong evidence]
British Medical Journal 2019: Omega-3, omega-6, and total dietary polyunsaturated fat for prevention and treatment of type 2 diabetes mellitus: systematic review and meta-analysis of randomised controlled trials [strong evidence] ↩