–Replacing saturated fat with vegetable oils lowered cholesterol but didn’t improve outcomes
A new analysis from a long-forgotten study dating to the 1960s adds to mounting doubts over a cornerstone of dietary advice for more than 50 years: the demonization of saturated fat in the diet. Although in recent years the nutritional establishment has retreated from recommendations to replace saturated fats with carbohydrates, current guidelines still suggest replacing saturated fat with vegetable oils.
Now the new analysis offers evidence showing no benefit— and even a possibility of harm— when replacing saturated fat with vegetable oils.
But the new analysis does not report substantially new data, as has been widely reported in the media.
The Minnesota Study
It is widely acknowledged that nutritional research is severely handicapped because of the absence of well-controlled randomized trials. Because of ethical concerns it is nearly impossible to perform these trials in settings where they might actually be feasible, such as prisons, nursing homes, mental hospitals, or the military. But in 1968 those ethical concerns did not hold sway, and researchers in Minnesota embarked on a trial in which 9,423 residents of six mental hospitals and one nursing home were randomized to a control diet high in saturated fat or an experimental diet replacing saturated fat with vegetable oil rich in linoleic acid (corn oil). The trial investigators wanted to demonstrate that the vegetable oil diet would lead to lower cholesterol levels and improved outcomes over time.
The trial didn’t work out as planned. The major findings did not support the vegetable oil diet.
Despite widespread reports in the media, however, the study results were in fact published. In 1989, 16 years after the completion of the study, the main results were published in Arteriosclerosis, Thrombosis, and Vascular Biology. According to Nina Teicholz, author of The Big Fat Surprise, Ivan Frantz Jr, the principal investigator “sat on the results for 16 years and didn’t publish, because, as he told Gary Taubes, ‘we were just so disappointed in the way they turned out’.”
Now significant portions of the original data have been recovered and analyzed by Christopher Ramsden (NIH, UNC) and colleagues in an extraordinary paper published in the BMJ. Their findings reinforce the earlier findings and show that although the trial succeeded in showing the cholesterol-lowering effect of vegetable oils on cholesterol levels it did not produce any evidence of clinical benefit with vegetable oils.
Total cholesterol dropped by 13.8% in the vegetable oil group, compared with only 1% in the control group. There was no difference in mortality between the two groups but there was a trend toward an increased risk of death in the vegetable oil group among participants who were over the age of 65.
In the 1989 paper, by contrast, the authors downplayed the overall clinical findings. They wrote that any benefits would only be expected after a long period of exposure and in a younger population. By slicing and dicing the data they reported “a favorable trend” for clinical events “in some younger age groups.”
Overall, irrespective of treatment group, there was a significant 22% increase in the risk of death associated with each 30 mg/dL reduction in total cholesterol in the entire trial population.
One dietitian, Rebecca Blake (Mount Sinai Beth Israel) cautioned that the harmful effects seen in the vegetable oil group may have been due to “extremely high trans fat intake.” She said that “trans fats should not be included in a group of allegedly polyunsaturated fats.”
Teicholz provided the following comment on the paper:
The idea that saturated fats cause heart disease has been tested more than any other hypothesis in the history of nutrition science—on more than 75,000 people in experiments that lasted for years. What we’re seeing today is an increasing number of researchers going back, re-considering this data, and coming to the conclusion that saturated fats probably do not, after all, have an effect on cardiovascular mortality. Swapping the naturally occurring saturated fats in animal foods for industrially produced vegetable oils may have been a huge mistake. And the more we know about vegetable oils—their inflammatory effects and tendency to oxidize when heated—should make us reconsider whether this has been a good swap.
James DiNicolantonio (St. Luke’s Mid America Heart Institute) sent the following comment:
The problem goes back to 1961 when the AHA first recommended to all Americans that we replace our intake of animal fat with vegetable oils. This was because the swap lead to lower cholesterol levels. The problem is that a lot more happens than just a reduction in total cholesterol when you replace animal foods (high in saturated fat but also oleic acid) with vegetable oil. What also happens (besides a reduction in cholesterol) is a reduction in HDL. Moreover, vegetable oils have consistently been found to increase the susceptibility of LDL to oxidize. And oxidized LDL is what people should care about lowering, not total cholesterol per se (as total cholesterol is also made up of HDL), and the best way to lower oxidized LDL is to avoid industrial seed oils. The take-home message (as it always should be) is “eat real food”, if the Dietary Guidelines could just keep it simple “eat real food as close to nature as possible” then all of us would be a lot better off. The consumption of industrial seed oils doesn’t fall under the category of “real food”.