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Monounsaturated fatty acids, olive oil and health status: a systematic review and meta-analysis of cohort studies

Background: The aim of the present meta-analysis of cohort studies was to focus on monounsaturated fat (MUFA) and cardiovascular disease, cardiovascular mortality as well as all-cause mortality, and to distinguish between the different dietary sources of MUFA.

Methods: Literature search was performed using the electronic databases PUBMED, and EMBASE until June 2nd, 2014. Study specific risk ratios and hazard ratios were pooled using a inverse variance random effect model.

Results: Thirty-two cohort studies (42 reports) including 841,211 subjects met the objectives and were included. The comparison of the top versus bottom third of the distribution of a combination of MUFA (of both plant and animal origin), olive oil, oleic acid, and MUFA:SFA ratio in each study resulted in a significant risk reduction for: all-cause mortality (RR: 0.89, 95% CI 0.83, 0.96, p?=?0.001; I2?=?64%), cardiovascular mortality (RR: 0.88, 95% CI 0.80, 0.96, p?=?0.004; I2?=?50%), cardiovascular events (RR: 0.91, 95% CI 0.86, 0.96, p?=?0.001; I2?=?58%), and stroke (RR: 0.83, 95% CI 0.71, 0.97, p?=?0.02; I2?=?70%). Following subgroup analyses, significant associations could only be found between higher intakes of olive oil and reduced risk of all-cause mortality, cardiovascular events, and stroke, respectively. The MUFA subgroup analyses did not reveal any significant risk reduction.

Conclusion: The results indicate an overall risk reduction of all-cause mortality (11%), cardiovascular mortality (12%), cardiovascular events (9%), and stroke (17%) when comparing the top versus bottom third of MUFA, olive oil, oleic acid, and MUFA:SFA ratio. MUFA of mixed animal and vegetable sources per se did not yield any significant effects on these outcome parameters. However, only olive oil seems to be associated with reduced risk. Further research is necessary to evaluate specific sources of MUFA (i.e. plant vs. animal) and cardiovascular risk.

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Alex’s Notes: The most common monounsaturated fatty acid (MUFA) consumed in the diet is oleic acid, named after its most famed source – the olive (or olive oil). Interestingly, despite its prevalence in all human diets,

“No dietary recommendations for MUFA are given by the National Institute of Medicine, the United States Department of Agriculture, the European Food and Safety Authority and the American Diabetes Association. In contrast, the Academy of Nutrition and Dietetics as well as the Canadian Dietetic Association both promote <20% MUFA of daily total energy consumption, while the American Heart Association sets a limit of 20% MUFA in their respective guidelines [1-3].”

The above really struck me as surprising since even the AHA acknowledges there are heart healthy benefits of MUFAs. Perhaps it has to do with the fact that while oleic acid and other MUFAs are chemically identical, the food sources containing them are vast. Consider the Mediterranean diet rich in MUFA from olive oil, avocadoes, nuts, and other plant sources. Now consider the Western diet rich in MUFA from industrial seed oils and processed animal meats (note that about 40% of animal fat is MUFA).

The meta-analysis at hand took data from 32 cohort studies and 841,211 total subjects to examine the relationship between MUFAs and various health outcomes. I want to emphasize that the inclusion criteria was exclusive to cohort studies, meaning this meta-analysis is based on observational correlations, not controlled trial results. I summarized the statistically significant findings in the table below (bold), and also added results that approached significance as well (not bold).

 

All-cause mortality

CVD mortality

Combined CVD events

Stroke

Coronary Heart Disease (CHD)

Overall

11%

12%

9%

17%

 

Just MUFA (any source)

     

15%

 

MUFA:SFA

 

9%

7%

   

Olive oil

23%

30%

28%

40%

 

Oleic acid

 

19%

13%

 

13%

Table 1: Risk reduction in outcome measure by the MUFA intake type

Overall, MUFA intake in general was significantly associated with reduced risk of all outcomes except CHD. With the exception of CVD mortality, however, these reductions appear to be primarily driven by olive oil consumption, which actually demonstrated greater and more significant reductions in most the outcomes compared to the overall MUFA intake. Additionally, MUFA from any source and the MUFA:SFA ratio had minor benefits, suggesting that animal-based MUFA and MUFA intake itself may have some benefit, but nothing as pronounced as that seen with the consumption of olive oil.

Thus, coming back to my initial thoughts it does appear prudent to consider the food source of the fatty acid. It is well known that olive oil contains a number of bioactive compounds (e.g. polyphenols) that could easily explain its role in health promotion. For instance, oleuropein accounts for roughly 80% of the polyphenols in olive oil and has been shown to be a potent scavenger of superoxide radicals and inhibit LDL oxidation. Moreover, a meta-analysis of intervention trials provides evidence that a Mediterranean diet decreases inflammation and improves endothelial function.

The above really hits home when looking to the sub-group analysis in the present study. When the cohorts were analyzed according to European or non-European subjects, only the European cohorts maintained the benefits. This strongly suggests that it is European dietary choices confounding the results of MUFA consumption.

And now of course it must be mentioned that all outcomes with the exception of stroke and CHD showed significant evidence of bias. Thus, a safe takeaway from this study would be that food matters, and olive oil consumption may reduce risk of all-cause mortality and CVD events. All this warrants skepticism, however, but seeing how no bias was detected for stroke, the 40% reduced risk is a solid outcome. So eat your olives or enjoy your olive oil, as it may indeed be protective.

 
 

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