Sunday, February 17, 2013

Should we really substitute saturated fat?

Remember all those years people were saying to stay clear of saturated fat? Remember being told to use margarine instead of butter? Remember hearing you should use vegetable oil? Remember being told to watch saturated fat intake, and use polyunsaturated fats (like margarine and vegetable oils) as a substitute if you were going to eat fat?

The current U.S. dietary guidelines call for people to substitute saturated fat in favor of polyunsaturated fat. The World Health Organization's new global action plan aimed at preventing chronic diseases, which is currently being developed, similarly calls for policy measures to decrease the levels of saturated fat in the diet and also eliminate trans fat, in favor of polyunsaturated fats. (The trans fat piece is a good thing).

A recent study published in the British Medical Journal provides further evidence as to why everything I just wrote is probably wrong.


The study recovered data on 458 middle-aged men (aged 30-59 years) from a large randomized controlled trial, called the Sydney Heart Diet Study, which was conducted between 1966 and 1973.
These men who were specifically enrolled had, on average, a coronary event in the past 11 weeks. The goal of the study was to essentially evaluate the effectiveness of replacing dietary saturated fat with polyunsaturated fat in the form of safflower oil and safflower oil polyunsaturated margarine, or a similar recommendation often heard today. (Second to canola oil, safflower oil has the highest unsaturated to saturated fat ratio of many of the most common oils and fats and is also quite high in inflammatory omega-6 fatty acid content.)

The men were allocated into two groups, a control group, which was not given any specific dietary advise (though some of those in the control group had already begun substituting margarine for butter after their coronary event on advice from their doctor), and saturated fat intake remained about 15% of total dietary calories throughout the study. The intervention group was instructed to increase their polyunsaturated fat intake to about 15%, reduce their intake of saturated fat and to less than 10% and reduce their dietary cholesterol to 300 mg per day. To achieve these targets men in the intervention group were provided with liquid safflower oil and safflower oil polyunsaturated margarine (high polyunsaturated fat group). The overall diet for both groups consisted of about 40% fat, 40% carbohydrate, and 15% protein, and 3-4% alcohol (doesn't add too 100% because of rounding).


Now what did the study find? First, the study found that after 12 months the cholesterol levels of the men on the high polyunsaturated fat diet were 8.5% lower than the men on the high saturated fat diet. At first glance this may seem like a good thing - oh, cholesterol went down when eating more polyunsaturated fat. But, the study measured only total serum cholesterol, which really isn't a good predictor of poor health outcomes in the future. Just think (and this is over-simplified, I know), you have your LDL cholesterol (the bad kind) and HDL cholesterol (the good kind). There can easily be a situation where total cholesterol decreases, but it's predominantly HDL, which isn't necessarily a good thing. Not to mention, the indicator says nothing about cholesterol particle size (large and fluffy vs. small and dense), which many think is a much better predictor of adverse health events. (If you're looking for a much more in-depth explanation, I recommend reading Dr. Peter Attia's "straight dope" blog series on cholesterol over at The Eating Academy. It's well worth the time.)

The below series of graphs really tells the story.
  • (Graph 1) Men who consumed the high polyunsaturated fat diet (the blue line in all three graphs) had a 62% increased risk of death compared to the men who consumed the high saturated fat diet (the dashed red line in all the graphs).
  • (Graph 2) Men who consumed the high polyunsaturated fat diet had a 70% increased risk of death from cardiovascular disease compared to the men who consumed the high saturated fat diet.
  • (Graph 3) Men who consumed the high polyunsaturated fat diet had a 62% increased risk of death from heart disease compared to the men who consumed the high saturated fat diet.

Source: Ramsden, et al (2013). BMJ;346:e8707
So, when it comes down to it, the study essentially shows that increasing dietary polyunsaturated fats from sources like vegetables oils (safflower, canola, sunflower, corn and soybean) and decreasing saturated fats is associated with an increase in both overall death rates and heart disease death rates.

The study does have some limitations (the study didn't control for some healthy lifestyle changes, such as smoking cessation, and it's a correlation study, indicating nothing about causality), but I think it does add to the growing body of evidence to suggest what we commonly hear about what's good in our diet is often contrary to what the research shows (a few examples here, here, and here). It's also a clear case-in-point regarding what I've written about before: not all calories, and especially not all fats, are created equal. So, when you can, take a look at the most important part of a nutrition label on foods: the ingredients list!

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