Paper for review:

Zhu Y, Bo Y, Liu Y. Dietary total fat, fatty acids intake, and risk of cardiovascular disease: a dose-response meta-analysis of cohort studies. Lipids in Health and Disease 2019 Apr 6;18(1):91. doi: 10.1186/s12944-019-1035-2. Review.

Authors and their affiliations:

Zhu Y, Bo Y, Liu Y.

Author information

1 Department of Cardiology, The first affiliated hospital of Zhengzhou University, Zhengzhou, China.

2 Department of Nutrition, The first affiliated hospital of Zhengzhou University, No. 1 Eastern Jianshe road, Zhengzhou, 450052, Henan, China.

3 Jockey Club School of Public Health and Primary Care, the Chinese University of Hong Kong, Hong Kong, China.
4 Department of Nutrition, The first affiliated hospital of Zhengzhou University, No. 1 Eastern Jianshe road, Zhengzhou, 450052, Henan, China. [email protected].

The journal article we’re going to review in this video was published on the 6th April 2019 in the Lipids in Health and Disease journal, under the BMC Springer Nature banner. The authors are Yongjian Zhu, Yacong Bo and Yanhua Liu from Zhengzhou University Hospital and the Chinese University of Hong Kong.

The study is a particular type of meta-analysis, a meta-analysis being a study of studies in which all the included studies are combined and analysed together. What’s important about this study is it’s a meta-analysis of cohort studies – in other words studies of population groups – that aims to better understand the effects of dosage of dietary fats on the risk of cardiovascular disease.

Randomised controlled studies or RCTs are widely viewed as the gold standard in medicine, but they’re not as easily applicable to nutrition studies in the real world. That means cohort studies, which done properly, give you some of the best real world data. Then, if the meta-analysis has the right kinds of inclusion criteria – you’ve got a pretty powerful research tool to look at studies that involve very large numbers of people. And that’s really what this Chinese study is all about.

What the authors were looking for is ALL of the available research up until 1st July 2018 that included data on total fat intake or intake of particular fats, namely monounsaturated fats, polyunsaturated fats, trans fats or saturated fats – to see how doses of these different types of fats affected cardiovascular disease outcomes like heart disease, heart attacks, arrhythmia, fibrillation strokes and other cardiovascular conditions.

The authors found over 110,000 research papers – and of these only 43 studies met their inclusion criteria which needed to have cardiovascular risk estimates for people consuming different amounts of total fats or major fat subclasses.   All pretty reasonable – and given the tendency for studies to be biased towards Mediterranean diets with quite low saturated fat intakes – such as in the PREDIMED study – it was good that the studies covered different geographical regions of the world. Of these 43 were in America, 14 from Europe and 6 from Asia.

The findings are interesting because they tell us about the relationships between the amounts of total fat and different classes of fat – and cardiovascular disease or CVD risk. What a study like this doesn’t of course show us is the effect of different quality of fats. So – for example – where you see high saturated fats, those saturated fats are probably more likely to be coming from junk food diets than they are from grass-fed, marbled meats.

The first take-home is only trans fats have got an altogether negative impact on CVD risk. That means zero is best as the percentage of energy from trans fats in your diet goes up, so does your CVD risk.

For total fats, saturated fats – generally from animal sources but also plant sources like coconut oil, monounsaturated fats – such as olive and rapeseed oils, and polyunsaturated fats – mainly from vegetable oils – most of these being highly refined – the relationships are more complex – in other words they’re not straight lines.

The second big take-home is that less isn’t always better when it comes to fat intake. You’ll see here that the CVD risk is actually higher when you only consume a little over 20 grams of fat per day – as compared with when you consume around 20 grams more.

Just as interestingly – you’ll see that risk goes down quite considerably between around 70 grams and 90 grams a day of total fat per day. All of this quite simply drives a coach and horses through governments’ low fat policies which continue to meter out the false message to the public that when it comes to fats, less is best. This is simply wrong – and this kind of public health messaging, rather than helping to save lives, actually could be contributing to more deaths. So – next time you’re in the supermarket, lay off those 0% fat products – they should carry health warnings!

We do however see from these data a fairly negative relationship with saturated fats that shows that the more that’s consumed, the greater the CVD risk.  That’s probably linked to the fact that most of the data on people consuming high saturated fat diets comes from people eating large amounts of cheap animal based foods like processed meats and burgers – given most of these data are from studies carried out in the USA.

The apparently lower CVD risk that comes from consuming more polyunsaturated fats from vegetable and seed oils is well known. It may be as much to do with other factors such as people consuming more vegetables and less refined carbohydrates, rather than being directly attributable to the fats themselves.

Finally – another somewhat complex relationship is with monounsaturated fats. It has this kind of double dip. The apparent risk goes up when you consume less than around 15g per day. But above 15g it also goes up, but goes down again over around 30g per day.

All of this shows you the relationships between fats and heart disease risk aren’t simple ones like governments try to make out. The data aren’t that reliable because they come mainly from recall questionnaires – and there are two other big factors that confuse the picture. One is the quality of the fats you consume, the other is that as your fat intake increases or decreases, the amounts of other macronutrients, especially carbohydrate changes too. It’s what we call the substitution effect.

I just want to finish on what we think is the biggest single take home: the evidence from all the available cohort studies from around the world does not show that less fat is better – which is a widely held belief created largely by 30 years of fat reduction public health policies pushed out by governments and health authorities. More fat can indeed be better for us – and given that we now know a lot more about what constitutes healthy fats, it’s really important that you consider the quality of the fats you’re eating.

In terms of monounsaturated fats, that means fats like unfiltered, cold pressed olive, avocado, macademia and non-GMO rapeseed oils. For polyunsaturated fats, get them directly from seeds if you can or consume minimally processed seed oils like hemp or walnut.  For plant-derived saturated fats, virgin coconut oil has got to be the pick for most of us.

Finally – don’t forget your Omega 3 polyunsaturated fats. For veggies and vegans that means flaxseed or algal oils. For the rest, oily fish or quality fish oils from sustainable sources are your best bets.