I just published a paper in Nutrition Journal titled ‘The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials’. Nutrition Journal is an open access journal and the paper shouldn’t be too technical so I encourage you to read it but I’ll also summarise it below:
I begin in the introduction by discussing the effect that saturated fat (SFA), monounsaturated fat (MUFA) and polyunsaturated fat (PUFA) have on cholesterol levels and how this formed the basis of the diet heart hypothesis, which is that replacing SFA with PUFA would be expected to reduce the risk of coronary heart disease (since MUFA didn’t affect total cholesterol it was largely ignored at this time). Several clinical trials were conducted, mostly in the 1960s and 1970s, to test the diet heart hypothesis.
More recently, several meta-analyses have pulled the results of the diet heart trials and most came to the conclusion that replacing SFA with PUFA would reduce the risk of coronary heart disease. However, there is a lot of controversy surrounding the diet heart hypothesis, and it is an important topic since the advice to reduce saturated fat is very influential in conventional dietary advice. So I thought it would be useful to do my own research.
I researched the clinical trials included in the previous meta-analyses and found that the dietary advice or the foods given to the participants in the trials would be expected to result in the high PUFA group having a lower intake of trans fats (to varying degrees) compared to the high SFA group. I also found other confounding variables such as two trials where the high PUFA group received a multifactorial dietary intervention, one trial where the high SFA group was given a vitamin E deficient diet, and another trial where the high SFA group received more antipsychotic medication that was found to be cardiotoxic. These issues were rarely discussed or taken into account by the previous meta-analyses.
Therefore I conducted my own meta-analysis by including the trials in previous meta-analyses and then categorising them as ‘adequately controlled’ or ‘inadequately controlled’ based on the degree of confounding variables in each trial. I considered the adequately controlled trials to be those that most accurately tested the effect of replacing SFA with PUFA, while the inadequately controlled trials to have too much of a difference between the groups to be a valid test of replacing SFA with PUFA.
The main findings are:
- The results from the adequately controlled trials suggest that replacing SFA with PUFA is unlikely to increase or decrease the risk of coronary heart disease or all-cause mortality
- This is the case regardless of whether the Sydney Diet Heart Study is included or excluded
- The suggestion of benefits reported in earlier meta-analyses is due to the inclusion of inadequately controlled trials
In the discussion, I mention that there are issues in drawing conclusions from risk factors (such as cholesterol) and observational studies, and that the issues inherent in these types of studies are likely due to them having discordant results with this meta-analysis. I also discuss that similarly there are issues in drawing conclusions from nutrients (like SFA) to foods, as foods are made up of many nutrients and other substances that may affect outcomes you’re interested in, such as the risk of coronary heart disease.
So what are the implications of this paper?
- Saturated fat is unlikely to increase the risk of coronary heart disease
- Replacing saturated fat with polyunsaturated fat (such as butter with margarine or vegetable oil) is unlikely to increase or decrease the risk of coronary heart disease
- Results from observational studies and enthusiasm from discovering a new risk factor or discovering a target for an existing risk factor should be tempered with a degree of caution and scepticism until they are tested
- Dietary guidelines and public health policy should focus on other things that are more likely to be effective at preventing coronary heart disease
If you like talks, the content of the meta-analysis is similar to a talk I have in 2015 at the Ancestral Health Society of New Zealand’s (AHSNZ) international symposium in Queenstown. You can view the talk here and the other talks here. There were a lot of great speakers there talking about a diverse range of topics such as nutrition, exercise, behaviour change, sustainability, active transport and mental health. So it shouldn't be too hard to find something that interests you.
AHSNZ is holding another international symposium in October at Queenstown which you can find out more about the symposium here. I’ll be speaking again, this time about insulin resistance, which is the focus of my PhD (so this paper is very unlikely to count towards my PhD and has mostly been a weekend project) and will be a focus on the blog over the next several months.