- A new study challenges the dietary guidelines for heart health
- MUFAs and omega 6 PUFAs not effective at reducing atherosclerosis risk
- Omega 3 fish oils reverse triglycerides and weight gain in an animal model of insulin resistance, despite increased calories
Read time: 9 minutes (1400 words)
Guidlines for prevention of heart disease have shifted in recent years away from a simplistic ‘reduce total fat’ message towards a more nuanced emphasis on the type of fat. The current American Heart Association (AHA) recommendation is to ‘replace saturated fats with monounsaturated (MUFA) and polyunsaturated (PUFA) fats.’
For the purpose of this post, I am going to put aside my objection to the demonising of saturated fats and instead focus on the MUFA / PUFA alternatives recommended by the AHA. Similarly, I am not going to challenge the cholesterol hypothesis nor debate the merits or otherwise of lowering LDL cholesterol here. Instead I am going to look at a recent paper that studied at the effects of MUFAs and PUFAs on atherosclerosis risk.
Note the terms n-3, n6- and n-9 in the title of this paper; these are just shorthand for omega-3 polyunsaturated fatty acids (ω3 PUFAs), omega-6 polyunsaturated fatty acids (ω6 PUFAs) and omega-9 monounsaturated fatty acids (ω9 MUFAs). I’ve made a quick reference guide for these below, showing typical foods high in each kind of fat:
The role of triglycerides in cardiovascular disease
Cardiovascular disease is often preceded by insulin resistance during which changes to the cholesterol delivery system increase the risk of atheroma formation in the artery wall. One of the hallmarks of insulin resistance is an increased production in very low density lipoproteins (VLDL) which are high in triglycerides.
The following is a quick explanation about triglycerides which you can skip if it’s a bit too much or you are already familiar with the subject…
The relevance of triglyceride-rich VLDL
VLDL is a carrier protein, produced by the liver, which transports triglycerides from the liver to the adipose (fat) tissue. It is important to note that these triglycerides are primarily endogenous i.e. manufactured by the body, rather than coming directly from dietary sources.
VLDL and triglycerides are raised principally by (1) Excess caloric intake from any source and (2) Carbohydrates (and especially fructose). In the former case the formation of VLDL can be seen as the body packing away and storing excess calories which are transported to the adipose tissue where they can be stored for a rainy day as fat.
In the case of carbohydrates, triglyceride rich VLDL is manufactured as a response to overwhelming surges in blood glucose that cannot be dealt with sufficiently by insulin induced uptake by the organs, especially muscle and liver glycogen stores. Indeed some have said that raised blood triglycerides are a reliable marker of carbohydrate consumption.
If cells become insulin resistant they stop taking up glucose effectively, increases the need for diversion of calories into the endogenous VLDL pathway. Hence insulin resistance is characterised by raised triglycerides.
Fructose has some unique metabolic problems as it does not trigger insulin, so cannot be taken up by cells as quickly as glucose. Instead it has to be processed by the liver, which can easily be overwhelmed, turning the excess into triglyceride rich VLDL.
It should not be surprising then, that the rats used in the study I am reviewing here, were made insulin resistant by feeding with sugar water (30% sucrose water) for 12 weeks. Sucrose is equal parts glucose and fructose, so their water contained 15% of each of these sugars.
So the aim of this study was to see how different dietary fats affect the dislipidemia associated with insulin resistance, especially the triglycerides. To do this the rats in this study were split into 5 groups: A control group on standard diet, whilst the other four were all made insulin resistant by feeding 30% sucrose water for 12 weeks. Of the four insulin resistant groups one was supplemented with n3 PUFAs, one with n6 PUFAs and one with n9 MUFAs.
It’s a pretty obvious experiment to undertake, and at this point you might rightly be asking why? Hasn’t this has all been done before? Surely the science on such a basic question is settled? With 60 years of American Heart Association advice you would think they had the science to back up their assertions and advice, wouldn’t you?
Well, shockingly, you would be wrong. As the authors note:
To our knowledge no studies have addressed the impact of dietary n-3, n-6 and n-9 fatty acids on VLDL composition and size in the [insulin resistant rodent] model.
There are several reasons why this basic question has not been answered before:
- Many previous studies looking at the effects of PUFAs on insulin resistance have applied n6 and n3 together, with only a few addressing n6 alone
- Previous studies evaluating n3 fish oils have used cod liver oil, which contains high levels of vitamin A, D and cholesterol, which could affect the findings.
- Assessment of MUFAs (n9) have usually used olive oil, which contains a broad range of phytochemicals which may be responsible for the beneficial heart effects observed in those studies, rather than the actual monounsaturated fats it contains.
To get round these problems ithe researchers used the following oils:
- n3: fish oils from pressing whole fish, hence low in vitamin A and D.
- n6: linoleic acid rich sunflower oil, low in phytonutrients
- n9: high oleic sunflower oil
Rodent diets contained 15% w/w of each oil, which represents about 35% by calories i.e. similar to a standard western diet.
What they found was striking and deserves some careful reflection.
To make the findings a little easier to appreciate I have made a graph of some of the key results, but tables with all the study data are provided at the end of the post.
Data is expressed as mean percentage differences compared to the standard rodent chow diet (Reference). All four insulin resistant diets (IR) were sucrose rich; the three intervention diets consisted of supplementing with 15% w/w with n-3: deepwater fish oil; n-6: sunflower oil; n-9: high oleic sunflower oil.
Triglycerides (■) and Liver fat (■)
The effects of the high sucrose feeding, as expected was a jump in triglycerides, which can be seen between the Reference and IR results above. Dramatically, supplementation with n3 fish oils almost completely reversed this dyslipidemia, returning VLDL particles to normal. Whilst remarkable, this is in line with previous epidemiological, human and animal studies that have shown n-3 PUFA have positive physiological effects on IR and lipid metabolism.
n6 and n9 oils, however, only weakly attenuated these harmful changes, failing to reverse the atherogenic state of the VLDL particles. This casts doubt on the validity of the American Heart Association recommendations.
In the case of the MUFA (n9), previous studies using olive oil have shown greater improvements in insulin resistance parameters, but as already noted, olive oil contains a broad range of bioactive phytochemicals (e.g. sterols and polyphenols). By using high oleic sunflower oil this study has been able to show that MUFAs do not of themselves produce these beneficial effects.
In relation to MUFAs this is particularly important as many processed food uses high MUFA oils that are low in phytonutrients as these can impart undesirable flavours.
Accumulated liver fat follows a similar pattern to plasma triglycerides. Again, n3 oils produce the best reductions in damage caused by insulin resistance.
Weight gain (■) and Calorie intake (■)
Some of the most surprising results were seen in relation to caloric intake and weight gain. All groups of rats could eat ad libitum, yet in the n3 fish oils and n9 MUFA groups caloric intake was considerably raised. Extraordinarily, despite this those fed the n3 fish oils had no weight gain during this trial, whilst those fed the n9 high oleic oil had the most weight gain.
Rats fed the n6 sunflower oil supplemented diet had lower caloric intake than the n3 and n9 groups, but still gained more weight than the n3 group.
The authors of this study conclude:
In insulin resistance, while n-3 PUFA showed expected favorable effects, supplementation with n-6 PUFA and n-9 MUFA did not prevent atherogenic alterations of VLDL. Thus, the recommendations of supplementation with these fatty acids in general diet should be revised.
The authors seem somewhat nonchalant about the n3 fish oils, but it is worth reflecting for a moment just what those fish oils did: the rats were drinking insane quantities of sugar, eating a hyper caloric diet, yet avoided most of the effects of insulin resistance and weight gain. That’s a pretty impressive feat as far as I can see!
Implications for diet
This study looked at the effects of fat choice in the context of insulin resistant animal models. The results support and extend previous research in humans and epidemiological studies. Taken together these point to certain food choices: fish, seafood and olive oil are good choices based on these results; Omega 6 vegetable oils such as sunflower, safflower, corn and soya oils are best avoided, as are low-polyphenol MUFAs like high oleic oil and possibly filtered rapeseed (Canola) oil. A high quality fish oil supplement seems prudent too.
Based on the ideas suggested by this study cold pressed rapeseed oil is potentially interesting as like extra-virgin olive oil, it contains high levels of phytonutrients, but unlike olive oil it has significant levels of alpha linolenic acid (ALA), a short chain n3 fatty acid. Based on the results above I would expect it to have beneficial metabolic effects possibly similar to or slightly better than Olive oil. Well that’s my prediction. So lets see…
I searched Pubmed for “rapeseed cardiovascular”. Sure enough, in one of the first studies I found [Baxheinrich et al, 2012] patients with metabolic syndrome (which is just one step down from full blown insulin resistance), were placed on a low calorie diet enriched either with olive oil (high n9 MUFA, low n3 ALA) or cold pressed rapeseed oil (high n9 MUFA high n3 ALA) for six months. Although both groups improved similarly on many metabolic measures (body weight, systolic blood pressure, cholesterol, and insulin levels), the cold pressed rapeseed group had significantly lower triglycerides. That said rapeseed oil is probably less suitable than olive oil for high temperature cooking as the ALA it contains is very heat sensitive. Still, its a good choice for salad dressings and mayonnaise!
For those of you who like to dig into the data here are some key tables from the study:
(1) Composition of diets; (2) Intakes and body weight; (3) Effects on adipose tissue/liver and serum parameters