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Perhaps nowhere in health has there been more confusion than the confusion caused by fat. The question of which fats are healthy and which fats are unhealthy has led to some pretty unhealthy diets, including extreme no fat diets that attempt to eliminate fat as much as possible with no attention to the differences between different fats
Recently, the confusion was placed in clear relief with the publication of dueling studies. The first exonerated saturated fat and vilified polyunsaturated fats; its opponent vilified saturated fats and championed unsaturated fats.
So, which dietary fats are good and which dietary fats are bad?
In This Corner
The first media blockbuster was a meta-analysis of studies on dietary fat and heart disease (1). It caused huge excitement in the media, which reported it as showing that the conventional guideline to decrease saturated fat and increase polyunsaturated fat had been debunked.
But, not only did the media get the study wrong, more importantly, the study got it wrong.
The study did not find that it doesn't matter what type of fat you eat. It found that:
1. People who eat the most trans fats have more heart disease than people who eat the least
2. Saturated fats and omega-6 fats have no effect--positive or negative--on heart disease
3. People who eat the most omega-3 fats have significantly less heart disease.
So, the study actually found that people who eat the most polyunsaturated omega-3 essential fatty acids have less heart disease than people who eat the most saturated fat. So the media reports on the study were seriously misleading.
But, much more importantly, and not reported at all by the media, the study itself was misleading. And, within days of its publication, the authors were forced to post a new version to correct their several errors. However, the media never informed the public of these corrections.
The paper contained an incredible number of errors. For example, it reported one study on omega-3's effect on heart disease as slightly negative, when it was, in fact, strongly positive. Two out of six of the studies on omega-6 were analyzed wrongly in the study (2,3), according to Walter Willet, Chair of Harvard School of Public Health Nutrition Department, saying they slightly increase the risk of heart disease when the studies actually showed that they significantly decreased the risk: one by 23% and the other by 61%. Willet and others also criticize the study for omitting relevant data from several other studies--including at least two on omega-6 polyunsaturated fats--that also would have changed the conclusion. He also says they ignored a pooled analysis (4) that found a significant association between polyunsaturated fat and lower risk of coronary heart disease.
Crucially, Willet points out the serious flaw that, when saturated fat was reduced, the study did not take into consideration what it was replaced with. Of course, if it is replaced with calories from another source that is bad for the heart, like refined carbohydrates or sugar, reducing saturated fat won't reduce heart disease. But if it's replaced with monounsaturated and polyunsaturated fats--and that's the relevant comparison--there is a lot of evidence that it will.
For example, a previous review of the research (4) has shown that reducing saturated fat by 5% and replacing it with polyunsaturated fats reduces the risk of coronary events by a significant 13% and coronary deaths by 26%. But replacing it with carbohydrates increases the risk of a coronary event by 7% (while reducing the risk of coronary death by 4%). The review did not take into consideration the type of carbohydrate and the authors point out that the negative result may pertain to refined carbs but not to high fibre complex carbs.
Willet says that because of multiple serious errors and omissions, the conclusions of the study are seriously misleading and should be disregarded. He says that "it has caused massive confusion and the public hasn't heard the correction."
Since science becomes useful and valuable when the public is informed about it, and since the public has been informed about the misleading conclusion and not the corrected conclusion, the public has been misled. And, in this case, since it is about heart disease, the public has been dangerously mislead. Willet says "A retraction with similar press promotion should be considered."
And in This Corner
In the other corner is a massive new study from Harvard. According to the Harvard T.H. Chan School of Public Heath, this “study is the most detailed and powerful examination to date on how dietary fats impact health." The study included 83,349 women and 42,884 men who were free from cancer, cardiovascular disease and diabetes at the beginning of the study. It followed the women for 32 years and the men for 26 years. The researchers wanted to determine what effect different types of fats would have on overall mortality and on death from specific causes.
When they looked at specific types of fat, they found that the risk of overall mortality--that is, the risk of dying from any cause over the course of the study--went up by 8% in the people who had the most saturated fat (like meat, butter and lard) in their diet. On the other hand, people who got the most polyunsaturated fat in their diets (that is, the omega-3 and omega-6 essential fatty acids) had their risk go down by 19%. The risk went down by 11% for people who got the most monounsaturated fat (like the fats found in olive oil). The risk of overall mortality went up by 13% for those who got the most trans fats (or hydrogenated or partially hydrogenated vegetable oils).
The results were the same for specific causes of death. Risk of death from cancer, cardiovascular disease, neurodegenerative disease and respiratory disease were all lower for people who ate unsaturated fats instead of saturated fats.
How effective is switching fats? The researchers found that just replacing 5% of energy from saturated fats with the equivalent amount of energy from polyunsaturated fats was associated with a whopping 27% reduction in risk of overall mortality. Replacing the same amount of saturated fat with monounsaturated fat reduced the risk of overall mortality by 13%. This analysis of what saturated fat is replaced it is precisely what the previous study failed to do.
The researchers conclude that their findings support the recommendation to replace saturated fats from animal sources and trans fatty acids with unsaturated fats. The senior author of the study, Frank Hu, says that their study “shows the importance of eliminating trans fat and replacing saturated fat with unsaturated fats, including both omega-6 and omega-3 polyunsaturated fatty acids." Co-researcher Dong Wang says that this can be accomplished, for example, by cutting back on red meat and replacing it with fish, nuts and seeds and replacing butter with vegetable oils like olive oil, canola oil and sunflower oil (5).
So, despite media misrepresentation of the first study, wide media coverage of the first study, no media coverage of the correction to the first study and lukewarm media coverage of the second study, the decision goes to unsaturated fats. The conclusion of the two massive studies is that replacing saturated and trans fats in the diet with poly and monounsaturated fats is good for your heart, good for your health and good for long life.
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- Chowdhury R, Warnakula S, Kunutsor S, et al. Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk: A Systematic Review and Meta-analysis. Ann Intern Med 2014;160(6):398-406.
- Oh K, Hu FB, Manson JE, et al. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the Nurses' Health Study. Am J Epidemiol 2005;161:672-9.
- Laaksonen DE, Nyyssonen K, Niskanen L, et al. Prediction of cardiovascular mortality in middle aged men by dietary and serum linoleic and polyunsaturated fatty acids. Arch Intern Med 2005;165:193-199.
- Jakobsen MU, O'Reilly EJ, Heitmann BL, et al. Major types of dietary fat and risk of coronary heart disease: a pooled analysis of 11 cohort studies. Am J Clin Nutr 2009:1425-32.
- Wang DD, Li Y, Chiuve SE, et al. Association of Specific Dietary Fats With Total and Cause-Specific Mortality. JAMA Intern Med 2016;176(8):1134-1145.