The study was carried out by researchers from the University of Cambridge and Medical Research Council, University of Oxford, Imperial College London, University of Bristol, Erasmus University Medical Centre and Harvard School of Public Health. It was funded by the British Heart Foundation, Medical Research Council, Cambridge National Institute for Health Research Biomedical Research Centre and Gates Cambridge.
The study was published in the peer-reviewed journal Annals of Internal Medicine.
The results of the research were accurately reported by the UK media, though some of the headlines were a little too black and white. This study hasn’t “proved” that saturated fat isn’t bad for the heart, rather that evidence of harm does not appear to be statistically significant.
This was a systematic review and meta-analysis that aimed to summarise the evidence about associations between fatty acids and coronary disease. A systematic review is an overview of primary studies. Systematic reviews use explicit and reproducible methods to search for and assess studies for inclusion in the review. A meta-analysis is a mathematical synthesis of the results of the included studies.
This is an appropriate way of pooling and studying the body of available evidence on a specific topic.
The researchers first searched databases of published studies to identify prospective cohort studies related to fatty acid exposure that were at least one year long. They also searched for randomised control trials that had looked at the association between fatty acids exposures and coronary disease.
Fatty acid exposures included:
Coronary disease was defined as:
Once the studies had been identified, the researchers assessed if there were any biases and extracted data about the characteristics and results.
The researchers transformed the results of each study, to calculate the relative risk of coronary disease when people in the top third of the fatty acid distribution were compared with people in the bottom third.
The researchers then performed a meta-analysis to combine the results of the included studies.
The researchers identified 72 studies: 45 cohort studies and 27 randomised controlled trials. 40 studies had initially healthy populations, 10 recruited people with elevated cardiovascular risk factors and 22 recruited people with cardiovascular disease.
32 cohort studies, including 530,525 people, looked at the association between dietary fatty acid intake and coronary disease. These studies looked at the intake of:
When comparing people in the top third to those in the bottom third of dietary fatty acid intake, only trans fatty acid intake was significantly associated with a risk of coronary disease.
People in the top third of dietary intake of trans fatty acids had a 16% increased risk of coronary disease compared to people in the bottom third (relative risk [RR] 1.16, 95% confidence interval [CI] 1.06 to 1.27).
17 cohort studies, including 25,721 people, looked at the association between circulating fatty acid biomarkers (i.e. in the blood) and coronary disease. These studies looked at circulating levels of the same fatty acids listed above. Comparing the top third and the bottom third, there were no significant associations between circulating levels of any of these types of fatty acid and the risk of coronary disease.
However, there were significant associations for specific fatty acids. The saturated fatty acid margaric acid was significantly associated with lower risk (RR 0.77, 95% CI 0.63 to 0.93), as were the polyunsaturated fatty acids eicosapentaenoic (RR 0.78, 95% CI 0.65 to 0.94), docosahexaenoic (RR 0.79, 95% CI 0.67 to 0.93) and arachidonic acid (RR 0.83, 95% CI 0.74 to 0.92).
27 randomised controlled trials, including 103,052 people, looked at the effect of fatty acid supplementation on the risk of coronary disease. In these trials, people in the intervention group had been given linolenic acid, long-chain -3 polyunsaturated fatty acid or -6 polyunsaturated fatty acid supplements. No significant difference in the risk of coronary disease was seen for people in the intervention group compared to people in the control group.
The researchers concluded that “current evidence does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats”.
In contrast to current recommendations, this systematic review found no evidence that saturated fat increases the risk of coronary disease, or that polyunsaturated fats have a cardioprotective effect.
Similarly, there was no significant association between the levels of total omega-3 or omega-6 polyunsaturated fatty acids and coronary disease. This lack of association was seen in both cohort studies, which looked at dietary intake or circulating levels the in blood, and in randomised controlled trials that had looked at the effect of supplementation.
There was also no significant association between total saturated fatty acids and coronary risk, both in studies using dietary intake and in those using circulating biomarkers. In addition, there was no significant association between total monounsaturated fatty acids and coronary risk – again, both in studies using dietary intake and those studying fatty acid composition.
Dietary trans fatty acid intake was associated with increased coronary disease risk, although circulating levels were not.
There are some limitations to this study:
Despite these limitations, this was an impressively detailed and extensive piece of research, which is likely to prompt further study.
Current UK guidelines remained unchanged:
Even if saturated fats don’t directly harm your heart, eating too much can lead to obesity, which in turn can damage it.
The key to a healthy diet is “everything in moderation”. The occasional buttered scone or cream cake is not going to hurt you, but you need to be aware of your total calorie intake.