The main strength of the story was an independent perspective from an American Heart Association expert who provided valuable context about the problem of reading too much into the results of a single study. The major weakness was the failure to distinguish between vegetable oils that are almost entirely omega-6 linoleic acid (e.g. corn and safflower oil), about which the study raises significant concerns, and those that have a balance of linoleic acid and omega-3 fatty acids (e.g. canola and soybean oil), about which there is less worry. The treatment of the statistics was also unsatisfactory, as was the explanation of why the results are novel and different from previous research. See the detailed comments below under Evidence for additional context surrounding this research.
The American Heart Association recommends consumption of polyunsaturated vegetable oils rich in linoleic acid almost without reservation. It put out a statement a few years ago emphasizing that even very high intakes of linoleic acid appear to be safe and beneficial. Some nutrition researchers have long questioned the evidence supporting these guidelines, noting that the studies involved variables besides linoleic acid intake (e.g. increased consumption of omega-3 fatty acids) that could have affected the outcomes. The study discussed here contributes important information to this debate, because it suggests that increasing linoleic acid intake alone (without also increasing intake of omega-3 fatty acids) is not beneficial and may well cause cardiovascular harm.
In the final analysis, however, it is unclear if this nearly 50-year-old study, which also has its share of limitations, will do much to change current consensus on what fats you should eat. Important questions will remain, and the only way they will be answered definitively is with large new randomized studies that probably won’t be conducted any time soon.
ADDENDUM ON FEB. 8: Consumer Reports has just published, “Fat facts and fiction,” written by Kevin Lomangino, who was one of our reviewers of this story.
The cost of vegetable oil and other sources of linoleic acid is not really in question.
The story does not provide statistics regarding the percentage of people who died, from what causes, in each group of this study. That information would have been easy to communicate, so we’ll provide it here:
Not Applicable. The primary focus of the story was addressing the potential harm of consuming too much linoleic acid. The story should have quantified these harms better, but since we’ve already docked points above under Benefits, we’ll grade this N/A here to avoid dinging it twice for the same flaw. We can’t give it credit for a satisfactory score, though, for the same reason.
The story has some important comments about the limitations of the research, and includes expert advice not to drastically change your diet based on one study — all very good features of the coverage. However, we thought the story should have mentioned that the margarine used in this study contained trans fats, which are now known to be harmful and may have contributed to the increased mortality in the linoleic acid group. In addition, the story oversteps the evidence when it suggests that the results apply generally to polyunsaturated vegetable fats. It should have distinguished between vegetable oils that contain only omega-6 fat (safflower oil, corn oil, sunflower oil), and those that contain a combination of omega-6 and omega-3 fat (soybean oil, canola oil), as the weight of evidence still indicates a protective effect for these latter oils.
On a related note, we felt the story didn’t adequately explain why this study is different from other research and why the results are important. American Heart Association recommendations to eat more omega-6 fats are based on studies, many of which were conducted in the 1960s and 70s, that used a variety of different vegetable oils to replace saturated fat in the diet. These oils are comprised mostly of omega-6 fat, but some oils (e.g. canola, soybean oil) also contain a fair amount of omega-3 fat. In addition, some of the studies gave participants in the vegetable oil group advice to consume more fish, which is also rich in omega-3 fat. So even though the AHA advises consumers to consume plenty of omega-6 fat, it’s never been totally clear if the heart benefits observed in these studies were due to the omega-6 fat or the omega-3 fat. Only a handful of studies replaced saturated fat with omega-6 fat exclusively, and the study reported on here is one of them. For this reason, the fact that this study found an increased rate of heart disease mortality in omega-6 group is newsworthy, as it undermines the case that all vegetable oils are good for you.
There was no disease-mongering of heart disease. The story is clear that the study involved middle-aged men with a history of heart disease and might not apply to other groups.
The story includes comments from an American Heart Association expert, who provides some common-sense advice. It is worth noting, though, that the AHA spokesperson quoted in this story did not hint at the AHA “considering re-evaluating its dietary recommendations” as did the AHA spokesperson in the competing TIME story.
There was no mention of other approaches to reduce cardiovascular risk, either in the general population or men with a history of heart disease. Options include other lifestyle changes, drug therapy, surgery, and quitting smoking. This could have been accomplished in just a few more words.
This story mostly used terms like “omega-6 fat” and “linoleic acid.” But food fats and oils — things like corn oil and soybean oil that people actually look for in the grocery store — contain differing combinations of omega-6 linoleic acid, omega-3 fats, and other fatty acids. Although the story notes that the study involved safflower oil, it would have been helpful to explain how much linoleic acid is found in other types of oils, and whether these oils also include omega-3 fats.
It has long been known that the subjects in the linoleic acid/safflower oil group of this study had higher all-cause mortality. What was unclear is if they had higher cardiovascular mortality. The results are not quite as novel as the story suggests. Nonethless, we’ll give the story the benefit of the doubt on capturing the overall gist of the novelty of the study adequately.
There is enough original reporting that we can be sure the story didn’t rely on a press release.