Polyunsaturated or saturated fat? Old data, new conclusions

Saturated or unsaturated? That is the question.

The standard diet-heart hypothesis states that, for the sake of our heart, we should replace saturated fats with unsaturated fats, such as vegetable oils rich in linoleic acid.

Linoleic acid lowers cholesterol levels and prevents buildup on artery walls.

This, therefore, prevents heart-related health issues and extends life. At least, that is the commonly held belief.

A new study, reopening data sets collected almost half a century ago, casts doubts on this standard interpretation. The results, published in The BMJ, are guaranteed to spark lively debate.

If we are to reassess our stance on fats, where does our current understanding regarding the benefits of unsaturated fats come from?

The current diet-heart theory

Initially, randomized controlled trials demonstrated a reduction in blood cholesterol when saturated fats were replaced by linoleic acid.

Next, observational evidence showed that elevated cholesterol levels are linked to coronary heart disease events.

These two findings infer that increased intake of linoleic acid will reduce cholesterol and thus reduce heart risks. However, no randomized controlled trials have directly demonstrated that replacing saturated fats with linoleic acid reduces heart disease.

A team of researchers, led by Christopher Ramsden from the National Institutes of Health and University of North Carolina, cracked the vaults of a study that directly measured this theory. The study, called the Minnesota Coronary Experiment (MCE), was conducted 45 years ago.

Research lost to time

Data from the MCE were published at the time, but, because the results were not as expected, the investigators reported no difference between the groups, and the report was published without fanfare.

According to Ramsden, the MCE was “the largest (n=9,570) and perhaps the most rigorously executed dietary trial of cholesterol lowering by replacement of saturated fat with vegetable oil rich in linoleic acid.”

The MCE used participants from mental institutes and nursing homes for a maximum of 4.5 years. The control group ate a diet high in saturated fat while the experimental group’s saturated fats were replaced by corn oil (rich in linoleic acid).

Because the participants were living full-time in institutions, all of their meals were provided, giving the experiment an unparalleled control over their diets. Additionally, it was the only trial of this type to investigate coronary, aortic and cerebrovascular atherosclerosis after death with a post-mortem.

The real effects of linoleic acid

As expected, the group that consumed the linoleic in preference to saturated fats showed a reduction in cholesterol; this was no surprise. What was a surprise was that this drop in cholesterol did not translate into improved survival rates; quite the reverse.

Members of the linoleic group with the greatest reduction in blood cholesterol had the highest risk of death.

Although it is tempting to consider the MCE results an anomaly, they are not the first historical study that Ramsden’s team has reopened.

In 2013, the researchers looked at a similar trial – the Sydney Diet Heart Study. This earlier data-mining exercise pulled in similar findings. They found that deaths from coronary heart disease were higher in the group that replaced saturated fat with linoleic acid.

As the authors state:

“Collective findings […] do not provide support for the central diet-heart tenet that the serum cholesterol lowering effects of replacing saturated fat with linoleic acid translate to reduced risks of coronary heart disease and death.”

The ramifications are two-fold. Firstly, there is the concern that current advice and thinking in regard to cholesterol, saturated fats and linoleic acid might be incorrect.

Secondly, there is the worry that unpublished data could be hiding any number of similar findings. Ramsden says the MCE results “add to growing evidence that incomplete publication has contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in linoleic acid.”

As with any research, there are limitations to the MCE; for instance, participants’ smoking status, psychiatric history and drug use were not collated.

However, limitations aside, the overall findings pose a wealth of new questions. In an editorial that accompanies the paper, Lennert Veerman, a senior lecturer at the University of Queensland, Australia, leaves us with a dash of common sense to guide us through the confusion we have been left with:

“We should continue to eat (and to advise others to eat) more fish, fruits, vegetables and whole grains. We should avoid salt, sugar, industrial trans fats and avoid overeating.”

As ever, the take-home message is to eat sensibly and never eat too much of any one thing. Medical News Today recently covered new research demonstrating the benefits on heart health of consuming fruit on a daily basis.