What You Need To Know About Monounsaturated Fat

Last week, we discussed the heart health benefits of eating at least one avocado per week. Among the many nutrients packed into this little green fruit, avocados stand out for their abundant monounsaturated fat (MUFA) content. This prompted one of our readers, who is a public health dietitian, to ask us: what’s the latest on MUFAs?

A fair enough question given the confusion in the research between MUFA and polyunsaturated fat (PUFA), especially around their effects on cardiovascular health and inflammation. Some studies have suggested that both types of unsaturated fats can lower the risk of heart disease and inflammation, while others have found that only PUFAs have these benefits.

So, what’s the scoop? How does MUFA stack up against other fats and macronutrients when it comes to heart disease risk?

Let's start with the evidence on PUFA and heart health.

Meta-analyses have sifted through mountains of data to uncover the effects of swapping out saturated fat, known for its cholesterol raising effects, for various macronutrients.

For example, a 2015 Cochrane Review of 15 controlled trials found that reducing saturated fat intake led to a significant 17% reduction in reduction in CVD events, a figure that increased to a 21% reduction when PUFA stepped in to replace saturated fat. These findings were later reaffirmed in an updated meta-analysis in 2020 by the same authors. Moreover, observational cohort studies have lent their weight to this argument, reporting significant inverse associations between PUFA consumption and risk of both coronary events (13% reduction) and coronary deaths (26% reduction).

Are the heart health benefits of PUFA similar for MUFA?

The quick answer is ‘yes’ with a ‘but’ as it is the source of MUFAs that matter.

While the evidence for PUFA is robust, the backstory for MUFA is not so clear. Meta-analyses of observational studies have found mixed results, with some showing no impact and others suggesting only slight risk reductions. And, the same Cochrane review noted above did not find any benefit for total MUFA intake. 

But along comes the game-changer. A recent meta-analysis of prospective studies has helped to demystify these inconsistencies. Rather than painting all MUFAs with the same brush, the study authors examined the sources of MUFA. This is where it gets interesting.

MUFAs are found in the usual suspects (or heroes) like olive oil, avocados, and nuts, BUT they're also present in meat and dairy products.

Around half the fat in meat is MUFA, while for dairy this is about one-fifth. Yet, both meat and dairy come hand in hand with saturated fat, which could cancel out the benefits of MUFA. So, to understand the true effect of MUFA, we need to look at where it comes from.

Replacing saturated fat and trans-fatty acids with MUFA

Here the authors examined a 5% replacement of energy intake from saturated fat with other types of fats and carbohydrates while factoring in their specific food sources. The study outcomes spanned from non-communicable diseases, including coronary heart disease (CHD) and cardiovascular disease to total mortality. 

As expected, replacing saturated fat with PUFA significantly lowered CHD incidence (by 11%).

Not surprisingly, and in line with earlier investigations, replacing saturated fat with total MUFA failed to show any tangible evidence of a reduction in CHD risk (RR=1.00; 95% CI: 0.82, 1.21).

But when looking at the source of MUFA, there was a big difference. A 5% replacement of energy intake from saturated fat with plant sources of MUFA reduced:

• CHD incidence by 17% (RR=0.83; 95% CI: 0.69, 1.00)
• Risk of death by 15% (RR=0.85; (95% CI: 0.82, 0.88) 

While the animal MUFA swap showed a 6% increase in CHD albeit non-significant (RR=1.06; 95% CI: 0.80, 1.41).

The authors also explored the impact of substituting 2% of energy intake from trans fatty acids (TFA) with plant-derived MUFA showing a 20% reduction in CHD occurrence and 10% decrease mortality.

So it's plant source MUFA for the win which has likely demystifies the inconsistencies arising in previous studies.

Slow versus fast-digested carbs

When saturated fat made way for slow-digesting carbs like whole grains or low GI foods our risk of heart disease decreased by a small but significant amount. But if we replace saturated fat with fast-digesting carbs like white bread or sugar, we won’t see any benefit. This means that the quality of carbohydrate matters for our heart health.

What underlying mechanisms might be at play here?

A review of the biomarker evidence here is a good place to start to understand the mechanisms of action. A systematic review and regression analysis of 84 intervention studies reported that every 1% of calories from SFA swapped for MUFA, resulted in significant reductions in key markers like total cholesterol, LDL-cholesterol, triglycerides, total:HDL-C and LDL-C:HDL-C. All of these change are beneficial for our heart.

Notably, a 5% energy substitution of MUFA for saturated fat saw LDL-C levels drop by 0.21 mmol/L, a figure that closely rivals the impact of PUFA at 0.28 mmol/L reduction.

A closer look at the role of monounsaturated intake compared to saturated fat on LDL-cholesterol may offer some possible explanations based on the current research.

One of the main mechanisms is thought to be related to the regulation of the LDL receptor, which is a protein on the surface of various cells such as liver cells that binds and removes LDL from the blood. As we mentioned saturated fat has been shown to decrease the expression and activity of the LDL receptor, but the good news here is that unsaturated fat has been shown to increase it.

Unsaturated fatty acids are also a preferred substrate for ACAT, an enzyme which increases the formation of cholesterol ester, and decreases the amount of free cholesterol in the liver. This decrease in hepatic free cholesterol will result in the up-regulation of LDL receptor expression leading to a decrease in LDL-C levels.

MUFA may be having some effect at reducing inflammation.

An Evidence-Based Practice Moment

Of course, no study is without its caveats. A few considerations:

1. The relationship between MUFA and heart disease risk might be influenced by other factors. For example, plant MUFA sources also contain other beneficial nutrients, like vitamins and phytonutrients, while animal MUFA sources also contain harmful substances, like sodium. These factors might explain some of the differences between plant and animal MUFA sources, not just MUFA itself.
2. The self-reported nature of dietary intake can introduce biases, but this limitation plagues most nutrition research.
3. The study used the GRADE system to assess the quality of evidence, whereby some of the associations were based on 'low quality' evidence. This system is useful, but it can be tricky to apply to nutrition research due to the unique complexities and limitations of the field (e.g. self-reported dietary intakes, challenges in isolating the effects of a single nutrient or dietary pattern accurately). GRADE starts by ranking evidence from randomised controlled trials as high quality and, because of residual confounding, observational data starts at low quality. So, we should be careful not to dismiss the findings just because they have a low GRADE rating. We should consider the nuances, and strengths and weaknesses of nutrition research in general.

The Bottomline

One of the main takeaways from this study is that we need to pay attention to not only how much fat, carbs, and protein we eat, but also where they come from.

Opting to switch out saturated fats with plant-derived MUFAs (e.g. avocados, olive oil, canola oil, nuts, and seeds) is one approach to reduce the risk of heart disease (alongside replacements with PUFA and slowly digested carbohydrates).

This nutritional strategy echoes the growing chorus of evidence that champions the importance of selecting healthier fat sources, especially of plant origin.