There was an interesting publication in the British Medical Journal last week on the topic of saturated vs poly-unsaturated fats. Apparently, data from the Sydney Diet Heart study that has been “lost” was rediscovered, analyzed and published. The original study was a randomized controlled trial where the intervention group was asked to substitute polyunsaturated fat for saturated fat. The subjects had proven cardiovascular disease. As it turned out, the oil they were using was high in omega-6 content (linoleic acid – LA). When the newly found data were analyzed, it turned out that the intervention group had higher all-cause and cardiovascular mortality. It is noteworthy, that the n-6 LA group did, as predicted, lower their cholesterol which was supposed to lower the CVD risk yet their mortality rate went up.
The authors propose that a process independent of our paradigm of high cholesterol and heart disease can explain this finding. When linoleic acid is exposed to free radicals, the result is the production of oxidized LA metabolites (OXLAMS). It turns out that OXLAMS are highly implicated the formation of atherosclerotic plaque and in virtually every process involved in atherogenesis. OXLAMS are more abundant in oxidized LDL which is more atherogenic than normal LDL. Smoking and excess alcohol are sources of high oxidative stress and, in the study, the smokers and drinkers did have higher risk ratios.
What to take away from this new (old) evidence? It certainly reinforces what I have been telling people for awhile now, that you should focus on reducing your intake of omega-6 fatty acids. The major sources of these in our diet are the vegetable oils. Olive oil is fine. Canola has omega-6 but also has omega-3 and the balance of these two may be an important mitigating factor. Many people have a problem with canola because of the amount of processing and the risks of oxidized fatty acid consumption. My take on it is that, while not ideal, a little of this oil in your diet is not going to upset the apple cart. Olive oil is far and away the preferable oil for non-cooking applications. For cooking, I am squarely in favour of the saturated fatty acids like butter, lard, coconut oil and bacon fat. Saturates are much more tolerant of heat and are, therefore, the preferred cooking fats.
If you are convinced that omega-6 oils should be avoided, you will need to be vigilant with labels. A lot of processed and pre-prepared foods use the cheap vegetable oils like corn oil, soya, and safflower. Unfortunately, many food outlets cook with these, as well. This is one argument for avoiding the fast food outlets as much as possible.
The research paper also included information from an updated meta-analysis of trials that substituted n-6 LA for saturated fat. Again, when the newly analyzed data was included in the updated meta-analysis, there was no benefit. It appears, however, that when a combination of omega-6 and omega-3 is substituted for saturated fat there is a reduction in CVD risk. It was from these kinds of trials that the generalized advice to substitute vegetable oil for saturated fat was supported. In light of the new evidence that n-6 LA is actually harmful, it seems incredibly sloppy to have advised people to use this oil without clarity in terms of which PUFA was doing what. The evidence that omega-3 is preventative is still standing, although that is being somewhat questioned now, too.
All this suggests to me that once the anti-fat bandwagon got rolling, people weren’t too discerning in terms of the quality of the evidence as long as it was supportive of that dogma. The Sydney study was done in 1978. One wonders how much harm has been done with these cavalier recommendations since then.
The role of oxidative stress is an important factor here. All the evidence I have seen supports that idea that a very low carb diet lowers oxidative stress. So, bottom line, get your carbs as low as you can and avoid those cheap, nasty vegetable oils.