A new US study provides the first conclusive evidence that shifting from saturated to polyunsaturated fat is likely to lower the risk of developing coronary heart disease by nearly 20 per cent.

You can read about the study by researchers from the Harvard School of Public Health (HSPH), Boston, Massachusetts, in the 23 March online issue of the open access journal PLoS Medicine.

Remarkably, although over the last 60 years or so we have been urged to reduce our intake of saturated fat to prevent heart disease, there has been little scientific evidence that when people actually do so it results in less heart disease, according to an HSPH statement.

Meanwhile, as the food industry has been busy reducing the amount of saturated fat in processed foods, and we have also been busy reducing our intake of butter, lard, cheese, fatty meat, and other foods high in saturated fats, there has emerged a wide variation in what has replaced them.

For example, in some cases trans fats have replaced saturated fat (which we now know are also bad for us), but in the overall American diet it appears that saturated fat has generally been replaced with higher consumption of refined carbohydrates and grains.

The situation is further confused by the absence of public health guidelines on specific nutrient replacements, and the fact that some groups advise lowering or limiting polyunsaturated fat consumption as well.

Also, as the authors wrote in their background information:

“Results from prior individual randomized controlled trials of saturated fat reduction and heart disease events were very mixed, with most showing no significant effects.”

An HSPH statement describes the new study as the “first conclusive evidence” from clinical trial data to show that compared with control groups of people who did not change their diet, people who replaced saturated fat with polyunsaturated fat reduced their risk of coronary heart disease by 19 per cent.

An important message here is that it appears it is not reducing saturated fat per se that brings this benefit, but what you replace it with. In other words, as a percentage of the calories you consume every day, if you reduce the amount contributed by saturated fat, what goes up in its place may make a significant difference to your health.

Lead author Dr Dariush Mozaffarian, assistant professor in the department of epidemiology at HSPH and the department of medicine at Harvard Medical School, said:

“The specific replacement nutrient for saturated fat may be very important.”

“Our findings suggest that polyunsaturated fats would be a preferred replacement for saturated fats for better heart health,” he added.

For the study, Mozaffarian and colleagues carried out a systematic review and meta-analysis of all randomized clinical trials up to June 2009 where participants specifically replaced saturated fat in their diet by increasing their consumption of polyunsaturated fat and where coronary heart disease events were documented.

A meta-analysis is a systematic way of pooling data from several studies and treating them as if they came from one large study. To do this you have to be careful which studies you include and exclude.

For instance, in this case, the researchers also only included studies that had randomized participants to increased polyunsaturated fat intake “for at least 1 year without major concomitant interventions, had an appropriate control group, and reported incidence of CHD (myocardial infarction and/or cardiac death)”.

Eventually they found eight trials that met their inclusion criteria, furnishing data covering 13,614 participants and 1,042 coronary heart disease events.

When they analysed the pooled data, they found that:

  • Increasing polyunsaturated fat consumption as a replacement for saturated fat reduced the risk of coronary heart disease events by 19 per cent (compared to controls who did not change their diet).
  • Coronary heart disease risk went down by 10 per cent for for every 5 per cent increase (measured as total energy) in polyunsaturated fat consumption.
  • The benefits associated with polyunsaturated fat consumption increased with longer duration of the trials.

They concluded that these results show evidence that consuming polyunsaturated fats instead of saturated fats reduces coronary heart disease events in randomized clinical trials and that if more people shifted from saturated to polyunsaturated fats we would see a drop in coronary heart disease in the population.

The HSPH statement said this is only the second dietary intervention to show a reduction in coronary heart disease events in randomized controlled trials: the first was increasing consumption of long-chain omega-3 fatty acids.

The problem with lack of evidence before this study was what the trials focused on: for instance many of them focused on levels of blood cholesterol, which is an indirect marker of heart disease risk, rather than coronary heart disease events.

Large observational studies (that follow people over time but don’t randomize them to different treatments or compare them to controls like clinical trials do) have also in general shown no link between saturated fat consumption and risk of heart disease events.

The researchers suggest that some of these mixed findings could also be because those studies did not focus specifically on the specific nutrient that replaced saturated fat: was it mostly carbohydrate, polyunsaturated fats such as in most vegetable oils, or monounsaturated fats such as in olive oil?

The current guidelines are not consistent across organizations: for example the Institute of Medicine recommends that 5 to 10 per cent of our energy should come from polyunsaturated fats, while some organzations have recently suggested that we reduce our intake of polyunsaturated fats (mostly the “omega-6” fatty acids) because of theoretical concerns that they may increase coronary heart disease risk.

HSPH suggests that the evidence from this new study shows that “polyunsaturated fats from vegetable oils may be an optimal replacement for saturated fats”. They say this is important because it helps food manufacturers and restauranteurs decide how to reduce saturated fats in the foods they prepare.

They also suggest that this study supports the idea that the current 10 per cent upper limit on energy consumption from polyunsaturated fats may be too low, as the participants in these trials who lowered their coronary heart disease risk were consuming about 15 per cent of their energy from polyunsaturated fats.

The study was sponsored by the National Heart, Lung, and Blood Institute (NIH) and a Searle Scholar Award from the Searle Funds at the Chicago Community Trust.

“Effects on Coronary Heart Disease of Increasing Polyunsaturated Fat in Place of Saturated Fat: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.”
Dariush Mozaffarian, Renata Micha, Sarah Wallace.
PLoS Medicine, 7(3): e1000252.
Published online 23 March 2010.

Sources: Harvard School of Public Health, PLoS Editors’ Summary.

Written by: Catharine Paddock, PhD