To my knowledge there exist no controlled dietary trials lasting long enough for us to know how swapping monounsaturated fats (e.g. olive oil) and saturated fats (e.g. butter) affects death from and incidence of various diseases. So once again let’s scope some suggestive short term findings:
In some studies monounsaturated fat appears to increase thermogenesis (1,2) and facilitate greater weight loss (3) than saturated fat. However, these studies used primarily olive oil as their monounsaturated source. This is important because animal studies suggest that olive oil may uniquely increase thermogenesis and metabolic rate via the effect of phenolic compounds it contains (4).
No significant differences have been found between monounsaturated fat and saturated fat on insulin sensitivity (5,6,7,8) although one of these studies requires further discussion; the KANWU study (5) was a three month long diet trial that is commonly cited as evidence that saturated fat is worse for insulin sensitivity than monounsaturated fat. Although this trend was in fact seen, the difference in insulin sensitivity between the two groups never achieved statistical significance.
Two studies found that consuming monounsaturated fat produced LDL that was more resistant to oxidation in vitro (9,12). One of the studies found that consuming saturated fat produced lower TBARS levels compared to monounsaturated fat (12) and the other study noting no difference in TBARS levels (9). Since olive oil was used, the effect on LDL oxidation resistance may have been mediated by antioxidants in the olive oil. This is supported by studies in which virgin olive oil consumption was more beneficial to LDL oxidation resistance than refined olive oil (10) and high oleic sunflower oil (11). At this point it is unclear whether monounsaturated fat itself would produce more oxidation resistant LDL than saturated fat.
There is mixed evidence regarding the effect of MUFA and SFA on post prandial blood triglyceride levels. Some studies have found that saturated fat produces lower serum triglyceride levels than monounsaturated fat (13), while other studies have found the opposite (14,15). Once again, because these studies used olive oil as their source of monounsaturated fat, this could have affected the outcome as olive oil has been shown to produce lower triglyceride levels than other monounsaturated fat sources in some studies (16).
Several studies have found no significant differences when examining the effects of saturated and monounsaturated fat on platelet aggregation (12,17).
A major meta analysis of 60 controlled trials found that generally, saturated fat raises both LDL and HDL compared to monounsaturated fat (18).
Some studies have found no difference between saturated and monounsaturated fat on fibrinogen levels (19) while other studies have reported higher levels after consuming saturated fat (17). Fibrinogen is a marker of inflammation and blood clotting potential. Elevated fibrinogen is correlated with cardiovascular disease (20).
Monounsaturated appears to be healthier than saturated fat, if only to a small degree. Some evidence suggests this difference may be due in part to the fact that olive oil, the oil used to represent monounsaturated fat in many of these trials, contains phytonutrients with beneficial effects on various health parameters. More evidence is likely needed to eliminate this variable and determine for sure that monounsaturated fat itself is the healthier fat.