Lest our headline mislead you, let's get one thing straight.
Some vegetable oils – especially antioxidant-rich extra virgin olive oil – promote good heart health.
But the heart-healthy reputation of America's most commonly consumed oils just took another heavy hit.
The back story: Bad advice based on bad assumptions
For decades, people have been advised to replace saturated animal fats like butter and lard with vegetable oils.
That advice rested on an early, erroneous understanding of the role of cholesterol in heart disease.
Incredibly, conventional medical advice lacked evidence from clinical trials: the gold standard of medical evidence.
The new study appeared in the British Medical Journal (BMJ).
You'd think that advice to replace saturated fats with vegetable oils rested on good clinical evidence, but it doesn't.
As as the BMJ press release said, "Although many studies support this theory, this paradigm has never been causally demonstrated in a randomized controlled trial and thus has remained uncertain for over 50 years. Further, key findings from landmark trials on this topic were not published.”
Instead, conventional wisdom has rested – shakily but persistently – on assumptions that now look flawed and or misleading.
Saturated fats scapegoated; Plant oils got a free pass
High levels of total and LDL cholesterol have long been linked to greater risk for heart disease.
Some saturated fats raise blood levels of total and LDL cholesterol, while vegetable oils lower cholesterol.
And, as they noted, research has revealed big differences in the heart-health impacts of various kinds of LDL cholesterol.
A recent evidence review found only limited evidence of small benefits from replacing saturated fats with vegetable oils (Hooper L et al. 2015).
The authors stressed that the available evidence did not show which kinds of vegetable oils might be modestly beneficial as substitutes for saturated fats.
Equally important, we now know that the omega-6 polyunsaturated fats in vegetable oils are harmful when consumed to excess.
As a result, the advice to replace saturated fats with polyunsaturated vegetable oils may well have worsened people's heart health.
At a minimum, recent evidence reveals real risks to replacing saturated fats with omega-6-rich vegetable oils.
Five vegetable oils fingered as villains
Five common vegetable oils appear to undermine cardiovascular health when overconsumed.
The quintet consists of corn oil, soybean oil, safflower oil, sunflower oil, and cottonseed oil, which is commonly used in packaged foods.
That's because these particular oils are especially high in (polyunsaturated) omega-6 fatty acids.
The exceptions are "high-oleic” versions of safflower and sunflower oil – especially high-oleic sunflower oil – which are readily available, but aren't always labeled as high-oleic.
Unlike the five problematic oils, these oils – especially high-oleic sunflower oil – are low in omega-6s and instead feature oleic acid.
Oleic acid is the monounsaturated fat that abounds in olive, avocado, canola, and macadamia oils, and it exerts neutral effects on heart and artery health.
Recent warnings on omega-6-rich vegetable oils
As we noted above, the hypothesis that diets high in saturated fat promote heart disease has never been proven.
Likewise, the idea that replacing saturated fats with omega-6-rich vegetable oils discourages heart disease also remains unproven.
Here's how the authors of a recent evidence review put it: "We found no studies examining the effects of either increased or decreased omega-6 [intake] on ... CVD [cardiovascular disease] ...". (Al-Khudairy L et al. 2015)
Three years ago, scientists at the National institutes of Health (NIH) unearthed missing data from an Australian clinical trial.
New findings cloud vegetable oils' reputation
The important new study was led by Christopher Ramsden, M.D.
The new study's senior co-author – Joseph Hibbeln, M.D. – is a renowned researcher on omega-3 and omega-6 fatty acids … especially their effects on mental, developmental, and cardiovascular health.
Dr. Ramsden's team set out to re-examine conventional wisdom by analyzing recovered data from a large randomized, controlled clinical trial that took place 45 years ago (Ramsden CE et al. 2016).
That study – called the Minnesota Coronary Experiment (MCE) – followed 9,423 participants from state mental hospitals and a nursing home for up to four and a half years.
Its goal was to see whether replacing saturated fats with omega-6-rich corn oil would lower participants' cholesterol levels and reduce the risk of cardiovascular disease and death.
The participants were divided into two groups:
- Diet high in saturated fat.
- Diet that replaced saturated fat with corn oil (naturally high in omega-6 fats)
No benefit from replacing saturated fats with omega-6 oil
As expected, the corn oil diet lowered blood levels of total and LDL cholesterol.
However, the lower cholesterol levels in the corn oil group did not produce higher survival rates.
In fact, the participants with the biggest drops in blood cholesterol suffered a higher, rather than lower, risk of death.
Cholesterol levels in the group that got the diet rich in omega-6 corn oil were about 13% lower than the levels in the saturated fat diet group.
(This part of the analysis only included the 2,403 participants who were on one of the two diets for at least a year.)
Surprisingly, lower cholesterol levels were associated with a greater risk of dying during the course of the study, among both the saturated fat and omega-6 corn oil groups.
Under current guidelines, a total cholesterol level above 200 mg/dL is considered high and expected to raise the risk for heart disease.
However, the results of this study showed the opposite effect: the risk of death rose by 22 percent (one-fifth) with every drop in cholesterol of 30 mg/dL.
Harm from omega-6 oils, or just lack of benefit?
Dr. Ramsden noted that their analysis doesn't show that omega-6 polyunsaturated fats are heart-attackers.
Instead, as he said, "This study is less of one showing harm (associated with a diet rich in unsaturated fat), and more of one saying it is surprising how little evidence there is and what there is does not show benefit [from replacing saturated fats with omega 6 rich oils].”
Many prior studies have failed to differentiate between omega-3 and omega-6 unsaturated fats, which have very different effects in the body ... a glaring, misleading flaw noted by Dr. Ramsden and his colleagues in their prior analyses of published studies.
For example, some studies that appeared to show benefit from replacing saturated fats with omega-6 vegetable oils failed to account for relatively high consumption of omega-3 fatty acids by the participants.
Limitations of Minnesota study led to a broader look
Dr. Ramsden and his colleagues acknowledged the limitations of the data available from the Minnesota study.
So his team analyzed the results of all similar randomized, controlled clinical trials: five trials in which saturated fats were replaced with omega-6-rich vegetable oils in the diets of 10,808 participants.
That analysis turned up no good evidence that replacing saturated fats with common, omega-6-rich vegetable oils reduced the risk of death from heart disease or other causes.
They concluded that their findings "add to growing evidence that incomplete publication [of data from clinical trials] has contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in [omega-6] linoleic acid.”
As Professor Lennert Veerman of Australia's University of Queensland said in an editorial about the new study, "The benefits of choosing polyunsaturated fat over saturated fat seem a little less certain than we thought.” (Veerman JL 2016)
America's omega-6 overload: Neither natural nor healthful
To be clear, there's nothing inherently unhealthful about oils that are high in omega-6 fats.
In fact, to function properly, our bodies needs omega-6 fats just as much as they need omega-3 fats.
The problem is overconsumption of these oils – a pattern found in the average American's diet – which undermines cardiovascular and overall health.
When consumed to excess, the omega-6 fats in vegetable oils create a pro-inflammatory environment in the body.
And we now know that chronic, "silent” inflammation is a major promoter of chronic conditions like heart disease, dementia, diabetes, and autoimmune disorders.
Optimize your omega balance
Excessive consumption of plant-source omega-6 fats also impairs absorption of the plant-source omega-3s in your diet.
Although plant source omega-3s are not used by the body, it converts them into the "seafood forms” (omega-3 DHA and EPA) it needs for vital functions … including moderating and ending inflammation.
To some extent, you can overcome the effects of excessive omega-6 intake by loading up on fatty fish or fish oil, whose omega-3s are not blocked by plant-source omega-6s.
The latest research shows that most Americans need to tackle both sides of the problem: reduce intake of omega-6-laden oils, and get more seafood source omega-3s.
It's critical to know the omega-6/omega-3 balance in your body, so you know whether you have a problem (most Americans do), and how big it is.
- 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.
- Hooper L, Martin N, Abdelhamid A, Davey Smith G. Reduction in saturated fat intake for cardiovascular disease. Cochrane Database Syst Rev. 2015 Jun 10;6:CD011737. doi: 10.1002/14651858.CD011737. Review.
- Hooper L, Summerbell CD, Thompson R, Sills D, Roberts FG, Moore HJ, Davey Smith G. Reduced or modified dietary fat for preventing cardiovascular disease. Cochrane Database Syst Rev. 2012 May 16;5:CD002137. doi: 10.1002/14651858.CD002137.pub3.
- Ramsden CE, Zamora D, Majchrzak-Hong S, Faurot KR, Broste SK, Frantz RP, Davis JM , Ringel A, Suchindran CM, Hibbeln JR. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73) BMJ 2016; 353 :i1246. doi: http://dx.doi.org/10.1136/bmj.i1246. Accessed at http://www.bmj.com/content/353/bmj.i1246
- BMJ. Lowering cholesterol with vegetable oils may not curb heart disease risk or help you live longer. April 13, 2016. Accessed at http://www.bmj.com/company/wp-content/uploads/2016/04/veg-oils.pdf
- Veerman JL. Dietary fats: a new look at old data challenges established wisdom. BMJ 2016; 353 doi: http://dx.doi.org/10.1136/bmj.i1512 (Published 12 April 2016). Accessed at http://www.bmj.com/content/353/bmj.i1512