New Review of Controlled Trials Replacing Saturated fat with Industrial Seed Oils

Readers Stanley and JBG just informed me of a new review paper by Dr. Dariush Mozaffarian and colleagues.  The authors report that overall, the controlled trials show that replacing saturated fat with polyunsaturated fat from seed oils, but not carbohydrate or monounsaturated fat (as in olive oil), modestly reduces the risk of having a heart attack:

These findings provide evidence that consuming PUFA in place of SFA reduces CHD events in RCTs. This suggests that rather than trying to lower PUFA consumption, a shift toward greater population PUFA consumption in place of SFA would significantly reduce rates of CHD.

Looking at the studies they included in their analysis (and at those they excluded), it’s hard to feel confident in their selection criteria and how they applied them. For example:

• They included the Finnish Mental Hospital trial, which is a poor quality trial for a number of reasons. It wasn’t randomized, properly controlled, or blinded*. Thus, it doesn’t fit the authors’ stated inclusion criteria**. Besides, the magnitude of the result has never been replicated by better trials– not even close.
• They included two trials that changed more than just the proportion of SFA to PUFA. For example, the Oslo Diet-heart trial replaced animal fat with seed oils, but also increased fruit, nut, vegetable and fish intake, while reducing trans fat margarine intake. The STARS trial increased both omega-6 and omega-3, reduced processed food intake, and increased fruit and vegetable intake. These obviously aren’t controlled trials isolating the issue of dietary fat substitution. If you subtract the four inappropriate trials from their analysis, the result they reported would no longer exist. 
• They excluded the Rose et al. corn oil trial and the Sydney Diet-heart trial, both of which suggested harmful effects from replacing animal fat with seed oils.

The authors suggest, based on their analysis, that replacing 5% of calories as saturated fat with polyunsaturated fat would reduce the risk of having a heart attack by 10%. For the average American, that means cutting saturated fat nearly in half to 6% of energy, which is challenging. It also means nearly doubling PUFA intake.

So basically, assuming the finding is correct, overhauling your diet would reduce your 10-year risk of having a heart attack from 10 percent to 9 percent. Together with the fact that the 10% reduction they report relies on a questionable cross-section of studies, I don’t think this adds up to a very compelling argument in favor of dietary fat modification.


* Not blinded.  Autopsies were not conducted in a blinded manner. Physicians knew which hospital the cadavers came from, because autopsies were done on-site. There is some confusion about this point because the second paper states that physicians interpreted the autopsy reports in a blinded manner. But that doesn’t make it blinded, since the autopsies weren’t blinded. The patients were also not blinded, though this is hard to accomplish with a study like this.

** They refer to it as “cluster randomized”, but I think most statisticians would dispute their use of that term.  There were only two hospitals, so “cluster randomization” in this case would just refer to deciding which hospital got the intervention first. I don’t think this counts as cluster randomization.  An example of cluster randomization would be if you had 10 hospitals, and you randomized which hospital received which treatment first.  It’s analogous to individual randomization but on a group scale.