Conventional wisdom concerning diet and heart disease has taken hard hits in recent years.
It's now clear that cardiovascular disease can't be pinned on dietary saturated fat or cholesterol.
But public health authorities still say that people should replace saturated (mostly animal) fats with polyunsaturated vegetable oils.
Importantly, most of the fats in the most widely used oils — corn, soy, safflower, sunflower, and cottonseed — are polyunsaturated omega-6 fats.
Now that saturated fat has been exonerated as a cause of heart disease, does it still make sense replace them with vegetable oils high in omega-6 fats?
Many prominent researchers would say “yes”, because a very large amount of epidemiological evidence links diets dominated by vegetable oils to lower rates of cardiovascular disease.
Clinical evidence is lacking
However, epidemiological evidence cannot prove a cause-effect relationship between a food or nutrient and risk for a disease.
So, do omega-6 vegetable oils really help reduce the risk of cardiovascular disease or its adverse outcomes, such as heart attacks?
You'd think there must be plenty of clinical evidence — which is much more reliable than epidemiological evidence — showing that these oils aid heart health.
But the conclusions of a recent evidence review from the scientifically rigorous Cochrane Collaboration reveal that's a mistaken presumption.
And, as its authors wrote, “There is conflicting evidence whether increasing or decreasing omega-6 intake results in beneficial effects.”
That statement came as a genuine surprise, given the long-standing, confident claims to the contrary by government agencies and university researchers worldwide.
And their core finding — of no good clinical evidence for the supposed heart benefits of omega-6 vegetable fats — is truly astonishing.
Review looks for proof backing advice on vegetable oils
The new evidence review comes from medical school researchers at Britain's University of Warwick.
They conducted their review under the guidelines and purview of the Cochrane Collaboration (Al-Khudairy L et al. 2015).
The Cochrane collaboration is a widely respected non-profit organization dedicated to promoting scientifically and mathematically rigorous methods of research.
Academic scientists working under the strict quality standards of the Collaboration have published hundreds of evidence reviews designed to test the validity of published papers.
This particular Cochrane review looked for evidence that either of two diet strategies actually help prevent cardiovascular disease (CVD):
• Replace calories from saturated fats, monounsaturated fats, or carbohydrates with equivalent amounts of calories from omega-6 fats.
• Replace calories from omega-6 fats with equivalent amounts of calories from carbohydrates, protein, or both.
As we noted, epidemiological (population-diet) studies can reveal promising areas for further research, but cannot prove cause-effect relationships between foods or nutrients and diseases.
So the Cochrane researchers looked for evidence from reasonably well-designed clinical trials published before September of 2014.
They narrowed their search to randomized, controlled trials (RCTs) that met three criteria:
• Lasted at least six months.
• Increased or decreased some participants' intake of omega-6 fatty acids.
• Involved either healthy adults or adults at high risk of cardiovascular disease.
Surprisingly, they found only four published RCTs, involving only 660 participants, and no ongoing, unpublished trials.
And, even more surprisingly, they found no RCTs that tested the effects of different intakes of omega-6 fats on adverse events related to cardiovascular disease.
Instead, they found only trials that tested the effects of differing intakes of omega-6 fats on key risk factors for CVD: Blood cholesterol (various forms) levels, blood triglyceride levels, and blood pressure (diastolic and systolic).
While that information is important, having one or more of these risk factors does not automatically equate to being at higher risk for adverse cardiovascular events. That's especially true of total and LDL cholesterol levels, which have been wildly oversold as predictors of heart risk.
Rigorous review finds no evidence backing omega-6 fats
Astonishingly, the Cochrane group found no solid clinical evidence that raising or lowering omega-6 intakes aided the participants' heart health.
None of these trials showed that raising or lowering omega-6 intakes either reduced adverse cardiovascular events (such as heart attacks) or reduced risk factors for cardiovascular disease.
As they wrote, “We found no studies examining the effects of either increased or decreased omega-6 [fats] on our primary outcome CVD clinical endpoints and insufficient evidence to show an effect of increased or decreased omega-6 intake on CVD risk factors such as blood lipids and blood pressure.
In other words, there is no clinical evidence — the most reliable kind by far — supporting decades of official heart-health advice to replace saturated fats with omega-6 rich vegetable oils.
The Cochrane reviewers pointed out the obvious: “There is a need for larger well-conducted RCTs assessing cardiovascular events as well as cardiovascular risk factors.”
Get your omega-6 fats from whole foods
We've long been preaching the dangers of the American diet's gross imbalance between omega-6 and omega-3 fats.
For more on that important topic, see the Omega-3/6 Balance page on our website, and the Omega-3/Omega-6 Balance section of our news archive, including the report titled Oily Omega Myths Persist.
There's no doubt that the average American needs to reduce their omega-6 intake and raise their omega-3 intake.
Most Americans get the great majority of omega-6 fats from the cheap vegetable oils mentioned above (corn, soy, safflower, cottonseed, and sunflower), which are heavily used in packaged and takeout foods.
In addition to providing huge amounts of omega-6 fats, many of those fats will been partially oxidized by the high temperatures involved in extracting and refining vegetable oils, and may undergo long storage under conditions — such as exposure to light and oxygen — that promote ongoing oxidation.
Although most of us need to cut way back on consumption of omega-6 vegetable oils, that doesn't mean you need to avoid whole foods high in omega-6 fats, such as nuts or pasture-raised meats.
After all, the richest plant-food sources of omega-6 fats provide beneficial antioxidants, fibers, B vitamins, minerals, and more.
Whole nuts and seeds are rich in omega-6 fats, but evidence from epidemiological studies and clinical trials finds them heart-healthy, while they may also reduce the risks for diabetes and cancer.
However, high intakes of omega-6 fats, even from whole foods, can be unhealthful unless they're balanced by comparable intakes omega-3 fatty acids, preferably the “long-chain” kinds (DHA and EPA) found only in seafood.
Most research suggests that human beings thrive on diets that provide about three parts plant-source omega-6 fats to one part plant-source omega-3 fats.
You can get by with fewer omega-3 fats if they are the long-chain forms found in seafood (DHA and EPA).
That's because the body has to convert (very inefficiently) the short-chain omega-3s found in plants into the long-chain forms it actually needs, whereas the body can use the omega-3s in seafood directly for essential functions.
Pick your oils carefully
Of course, vegetable oils are indispensable in food preparation, so just be sure you choose the right ones.
Compared with cheap, omega-6-rich oils, olive oil is much lower in polyunsaturated fats, and much higher in monounsaturated fats, which are considered neutral with regard to heart health.
Better yet, extra-virgin grade olive oil is rich in rare, unusually potent antioxidants (tyrosols) that are proven to reduce inflammation and improve vascular function (two important factors in cardiovascular health).
Macadamia nut oil is comparably low in omega-6s and high in monounsaturated fats, but lacks the antioxidants found in extra-virgin olive oil.
And virgin coconut oil is dominated by healthful saturated fats and medium chain triglycerides.
Compared with cheap vegetable oils, canola oil has fewer omega-6 fats, more monounsaturated fats — the same ones in olive oil – and more omega-3 fats.
The omega-3s in plant foods are certainly beneficial, but aren't as useful to the body or nearly as beneficial as the kinds found only seafood.
Plant-source omega-3s are found in canola oil, flaxseed and oil, walnuts, dark leafy greens, flaxseed, and very few other commonly consumed whole foods.
Al-Khudairy L, Hartley L, Clar C, Flowers N, Hooper L, Rees K. Omega 6 fatty acids for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2015 Nov 16;(11):CD011094. doi: 10.1002/14651858.CD011094.pub2. Review.