Is olive oil or seed oil, such as sunflower, more healthful?
Dr. Lukas Schwingshackl — from the German Institute of Human Nutrition Potsdam-Rehbruecke — led the new study.
This study was the first to carry out an analysis that allows the impact of several oils and solid fats on blood lipids to be assessed in a single model.
Many have compared the effect of replacing a food rich in saturated fat, such as butter or lard, with one rich in unsaturated fat, such as plant-based olive oil and sunflower. However, the evidence makes it difficult to find out which of the many plant-derived oils have the greatest benefit.
To this end, Dr. Schwingshackl and team used a statistical technique called network meta-analysis, which is gaining ground in health research as a way to glean evidence from enormous amounts of data through the use of "direct and indirect comparisons."
Investigators use network meta-analysis to find answers that could otherwise only be addressed in "giant studies" that compare the effect of many different interventions on a single result.
"The beauty of this method," Dr. Schwingshackl explains, "is that you can compare a lot of different interventions simultaneously."
The end result, he adds, is that "you can say" which of the oils is best for the "specific outcome."
The method, for instance, allows a comparison of butter with sunflower oil to be inferred indirectly by analyzing the results of two trials: one that tested butter against olive oil directly, and another that tested sunflower against olive oil directly.
The new findings feature in a paper that is now published in the Journal of Lipid Research.
A modifiable cardiovascular risk factor
According to the World Health Organization (WHO), heart disease and stroke are the biggest killers worldwide and have been so for 15 years. In 2016, 15.2 million people died because of these cardiovascular diseases.
In their study background, the investigators explain that dyslipidemia, or abnormal blood levels of lipids such as cholesterol, is a major cardiovascular risk factor that people can modify.
"It is well-established," they point out, that replacing saturated fatty acids with either mono- or polyunsaturated fatty acids reduces low-density lipoprotein (LDL) cholesterol, the "bad kind," which is a "strong risk factor" for cardiovascular disease.
For their network meta-analysis, they searched databases going back to 1980 for studies that had compared the effect of different types of dietary fats on blood lipids.
The researchers found 55 studies that matched their criteria for inclusion. These had assessed the impact on blood levels of various lipids of consuming the "same amount of calories" from two or more types of solid fats or oils over a minimum of 3 weeks.
Their analysis compared the effect of 13 oils and solid fats: safflower oil, sunflower oil, rapeseed oil, flaxseed oil, olive oil, hempseed oil, corn oil, coconut oil, palm oil, soybean oil, butter, beef fat, and lard.
Seed oils were the 'best performers'
Dr. Schwingshackl reports that the "best performers" were safflower oil, sunflower oil, rapeseed oil, and flaxseed oil. In contrast, "solid fats like butter and lard are the worst choice for LDL," he adds.
He and his colleagues point out that their approach has limitations, and that these should be borne in mind when interpreting the results. For one, they focused on lipid levels and not disease outcomes.
"This is not a hard clinical outcome," Dr. Schwingshackl goes on to warn. "LDL is a causal risk factor for coronary heart disease, but it's not coronary heart disease."
In addition, the authors feel that the method was not robust enough to pick a "winner" from the list of seed oils.
The fact that the oils that showed greatest benefit on LDL cholesterol levels were not necessarily the ones that showed a similar impact on other types of lipid, such as HDL cholesterol and triglycerides, also complicated matters.
Nevertheless, as it would be almost impossible to carry out a trial in which people are required to consume just one type of dietary fat for several years, such methods offer the next best thing.
"Some people from Mediterranean countries probably are not so happy with this result, because they would prefer to see olive oil at the top. But this is not the case."
Dr. Lukas Schwingshackl