Thursday, March 20, 2014

Saturated Fat Study

You may have seen news stories earlier this week about a study showing no statistically significant effect of saturated fat intake and fatty acid supplements on heart disease. I have several things to say about this; I will start with the executive summary, if you trust me and want some quick advice, and then move on to some notes about how to approach news articles and scientific papers.

Main Lesson

The main thing you should know is that this study provides more support for the general nutritional rule that focusing on one particular thing is usually a mistake and supplements are probably not helpful.

Good nutrition means eating as many calories as you burn, and making sure that you get all your essential vitamins, minerals, and amino acids. If you are eating too much, then eating too much saturated fat is not noticeably worse for your heart than eating too much of other things. If you are eating the right amount and getting your micro-nutrients, than foods high in saturated fat can be a healthy part of that diet.

However, trans fat is an exception; people who ate more trans fat were 16% more likely to have coronary disease. The fact that this was the only type of fat that caused a significant change is a big deal, and almost no news articles mentioned it. Of course, correlation is not causation. We do not know for sure that trans fat itself is what caused the problem; it could be that trans fat consumption is mainly a symptom of eating a lot of processed foods and restaurant food. 

Reading Health News

Whenever you see this kind of news story, remember that one study is just one bit of information added to a giant pile. A meta-analysis like this one is best seen as one team's report on how they searched through a giant pile of data.

Also, you should be suspicious of any news article or blog post that does not link directly to the actual scientific article, like I did. There is no excuse for not doing this. Even if the actual article is behind a paywall, you can almost always see the abstract for free. 

Most of what I am saying about this study is stuff you can read for yourself in the abstract. Yes, the language they use is often arcane and intimidating, but it does not take long to learn how to read medical studies, and the benefits of doing so are substantial.

Reading Scientific Papers

This study did not show that eating more red meat or saturated fat would be harmless. Here is what it found (the quote is directly from the abstract, edited for clarity):

"In observational studies, relative risks for coronary disease were 1.02 (95% CI, 0.97 to 1.07) for saturated...fatty acids when the top and bottom thirds of baseline dietary fatty acid intake were compared."

This is saying that they looked at people, ranked them according to how much saturated fat they ate, and divided them into three buckets: the bottom third, the middle third, and the top third. They then compared the amounts of coronary disease among the top-third bucket and the bottom-third bucket. The people in the top-third bucket had 1.02 times as much coronary disease.

This number of 1.02 had a 'margin of error' of plus or minus 0.05, which means that it could be as low as 0.97 or as high as 1.07. So while the 'best guess' is that the people who ate more saturated fat were 2% more likely to have coronary disease, this number is not known with enough precision to say for sure that the difference is real.

Notice that they did not feed saturated fats to people and see what happened. They simply looked at a population and reported correlations between saturated fat consumption and heart attacks. They did not find a very strong correlation. But remember that absence of evidence is not evidence of absence. The fact that this study did not find a 'smoking gun' does not mean that saturated fat is harmless. And also note that they did not look at longevity or any other health outcome. 

It is always a mistake to read too much into one paper; science is about learning lots of little facts, but people seem to have an instinct to make grand sweeping claims on the basis of a few little facts.

Still, this is useful information, because it tells us that saturated fat is not necessarily a major villain that we should be obsessed with. Eating too much saturated fat should be seen as one of many possible ways to eat badly, one that is not noticeably worse than the others.

Now, compare the results for trans fats:

"In observational studies, relative risks for coronary disease were ... 1.16 (CI, 1.06 to 1.27) for trans fatty acids when the top and bottom thirds of baseline dietary fatty acid intake were compared."

This is a significant finding. People in the top-third trans fat bucket had 1.16 times as much coronary disease than people in the bottom-third bucket. People who ate more trans fat were 16% more likely to have heart attacks than people who ate less, although that number could be as low as 6% and as high as 27%.

If all I know about you is that your intake of trans fats is in the top third of the population, than your risk of heart disease is 8% higher than the average person's. If all I know about you is that your intake of trans fats is in the bottom third of the population, than your risk of heart disease is 8% lower than the average person's.

Again, this was not a controlled experiment. They did not feed people trans fat to see what happened. This does not definitively say that trans fat will cause heart attacks. It says that people who eat foods with more trans fats are more likely to have heart attacks. It is possible (but unlikely) that something else about the food with lots of trans fats is the real culprit.

But in any case, we know that the kinds of foods that have historically been high in trans fat are correlated with more heart disease. It is probably the trans fat doing the damage, but it could be something else. Either way, this study provides evidence that it is a good idea to avoid trans fats and they foods that contain them.

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