Is butter as bad as we thought?
Recent news stories have claimed there's new evidence showing swapping butter for unsaturated fat isn't good for our heart after all. We take a look behind the headlines.
A new analysis looking at previously unpublished data from the 1960s-1970s has cast light on whether the benefits of vegetable oils were overestimated.
The researchers re-evaluated the Minnesota Coronary Experiment (MCE), a randomised controlled trial that took place from 1968-73. The MCE looked at the relationship between a diet rich in unsaturated fat from vegetable oils and risk of death from heart disease.
The MCE followed 9,423 people aged 20-97 years old, and tested whether replacing saturated fat (such as butter or lard) with vegetable oil rich in linoleic acid (an omega-6 polyunsaturated fat) reduces the risk of coronary heart disease and death.
The researchers – from the National Institute of Health in Maryland, USA – wanted to look again at the MCE evidence as some data that suggested that switching animal fats for vegetable fats didn’t protect the heart, was unpublished and excluded.
Limitations of the new analysis
The headline 'Replacing saturated fats with vegetable oils does NOTHING to cut risk of heart disease' is not an accurate reflection
The new research, published in the BMJ is a good study of the available data, but this data is still limited as it was based specifically on participants in Minnesota, USA, who were in a nursing home or state mental hospital. This means that it could be hard to apply the findings to people in the UK, and those who are not in hospital or a care home.
However, the fact that researchers used people in institutions meant that it was a well-controlled dietary study, which is very hard to do when people can make choices about their diets.
Also, the study did not take into account any wastage from participants not eating all the food they were given or any additional foods that were provided from outside the institutions that will have supplemented the diets e.g. brought in by family and friends. It also didn't look at other factors that could affect cholesterol levels or overall risk of coronary heart disease such as illness and medications prescribed, as well as whether or not participants smoked.
Another limitation is that the data that the researchers analysed is from up to 48 years ago, so could be outdated. Also, the study only looked at unsaturated fat from vegetable oil, not other types of unsaturated fats (such as those found in oily fish).
The story was widely covered, including by the BBC, Daily Mail, Evening Standard, Guardian, the Mirror, and the Telegraph. The general message was that ‘Butter is no more deadly than vegetable oil’, which is the headline of the Telegraph’s article.
However, the evidence is based on a relatively small amount of data and the follow up period was short. Much of the news coverage failed to make these limitations clear.
The Guardian, for example, described the study as of “a trial with nearly 10,000 participants”. However, while the initial trial looked at 9,570 participants, the new re-analysis only looked at the 2,355 participants who had followed the diet for over a year, had regular cholesterol measurements and had follow-up data for three years. And of these, only 149 post-mortem reports were recovered, which may not have been large enough to see any effect on cardiovascular death.
The study did look at a previously unpublished master’s degree thesis from 1981, which looked at all 517 deaths in the full study and found no reduction in risk from the unsaturated fat diet. However, that thesis may not have gone through the same peer-review process of quality control that is normal in scientific journals today.
The Mirror headline “Replacing saturated fats with vegetable oils does NOTHING to cut risk of heart disease” is not an accurate reflection of the study, which looked only at deaths from heart disease in the study period, and did not look at non-fatal heart attacks or angina.
The BHF View
We know that having too much cholesterol in your blood can increase your risk of cardiovascular disease
Associate Medical Director at the BHF
Since the 1970s, when the data for the original study was collected, large and reliable studies in the general population have shown that lowering cholesterol with statins reduces the risk of death.
Jeremy Pearson, Associate Medical Director at the BHF said: “We know that having too much cholesterol in your blood can increase your risk of cardiovascular disease, including heart attack and stroke, which is why managing our cholesterol level is crucial.
“This is an interesting study which shows that decreasing your intake of saturated fat can have a positive impact in helping lower cholesterol. However more research and longer studies are needed to assess whether or not eating less saturated fat can reduce your risk of cardiovascular death.
“In the meantime, it is advised to eat a balanced diet rich in fish, fruit, vegetables and whole grains to maintain a healthy heart,” he said.