To my knowledge there exist no controlled dietary trials lasting long enough for us to know how replacing monounsaturated fats (e.g. olive oil, canola oil) with omega-6 polyunsaturated fats (e.g. corn oil, sunflower oil) affects death from and incidence of various diseases. C’mon researchers, get on that!
In the meantime, if we want to try and figure out which types of fat is healthier, we need to look at controlled trials which measured various health markers suggestive of disease risk. I went on a mad search for controlled trials on humans comparing the effects of monounsaturated fat and omega-6 (or n6) polyunsaturated fat on such markers. Here’s what I found:
Monounsaturated fat produces better insulin sensitivity (1,2) and lower blood glucose levels (2,19) than n6 polyunsaturated fat.
Consuming monounsaturated fat produces LDL that is more resistant to oxidation than when consuming n6 polyunsaturated fat (6,7,8,9,10,11,12,13). This is important because oxidized LDL appears to be a strongly predictive marker of cardiovascular disease risk (3,4,18).
Compared to n6 polyunsaturated fat, monounsaturated fat improves vascular reactivity (19).
A serum assay of TBARS (or thiobarbituric acid reactive substances) is a measure of the amount of lipid peroxides, or damaged fats, in the blood. One study found that consuming polyunsaturated fat increased LDL TBARS more than monounsaturated fat (14). This same study found that monounsaturated fat produced more oxidation resistant LDL, consistent with the numerous studies cited in the previous paragraph.
Consuming monounsaturated fat produces lower levels of DNA adducts than n6 polyunsaturated fat (5). N6 polyunsaturated fat consumption is frequently found to increase DNA adducts, while no such connection is found with monounsaturated fat (17). DNA adducts are causative agents in cancer development (16).
Finally, a meta analysis of 14 controlled trials found that monounsaturated fat and n6 polyunsaturated fat produce no significant differences on common blood lipids, though when looking closely polyunsaturated fat tended to produce lower HDL and triglycerides than monounsaturated fat, though the difference is fairly small (15).
Based on this I believe the evidence suggests that consuming monounsaturated fat instead of n6 polyunsaturated fat will reduce one’s risk of diabetes, heart disease, and cancer. So this nutrition cage match was definitely a win for monounsaturated fat, leaving n6 polyunsaturated fat beaten badly. And after our previous blog post on saturated fat versus n6 polyunsaturated fat, we can see that linoleic acid predominated vegetable oils are probably the worst type of (natural) fat we can eat.