A review of more than 72 clinical studies has raised further questions over current guidelines relating to fat intake - which generally restrict the consumption of saturated fats and encourage consumption of polyunsaturated fats to prevent heart disease.
The meta-analysis, published in Annals of Internal Medicine, re-analysed data from 72 cohort studies and randomised trials investigating coronary risk and fatty acid intake - with the pooled data from more than 600,000 people showing that the current evidence does not support guidelines that restrict the consumption of saturated fats in order to prevent heart disease.
Led by Dr Rajiv Chowdhury from the University of Cambridge, UK, the team found that total saturated fatty acid, whether measured in the diet or in the bloodstream as a biomarker, was not associated with coronary disease risk in the observational studies.
Similarly, when analysing the studies that involved assessments of the consumption of total monounsaturated fatty acids, such as long-chain omega-3 and omega-6 polyunsaturated fatty acids, there were no significant associations between consumption and cardiovascular risk, they added.
"This analysis of existing data suggests there isn't enough evidence to say that a diet rich in polyunsaturated fats but low in saturated fats reduces the risk of cardiovascular disease. But large scale clinical studies are needed, as these researchers recommend, before making a conclusive judgement," commented Professor Jeremy Pearson, associate medical director at the British Heart Foundation - which helped fund the study.
Chowdhury added that the results of the study are 'interesting' and may stimulate new lines of scientific inquiry, "and encourage careful reappraisal of our current nutritional guidelines."
The team analysed data from 72 unique studies with over 600,000 participants from 18 nations. These included 32 observational studies (530,525 participants) of fatty acids from dietary intake; 17 observational studies (25,721 participants) of fatty acid biomarkers; and 27 randomized, controlled trials (103,052 participants) of fatty acid supplementation.
Chowdhury and his colleagues analysed the data from the 32 observational studies, finding that 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. The team found that those with the top intake of trans fats had a 16% higher risk than those in the third with the lowest intake.
When comparing the intake of other sub-types of fat the team reported small, non statistically significant, alterations to relative risk of heart disease, including a 2% increase in risk for saturated fat, a 1% decrease in risk for monounsaturated, a 7% decrease in risk for long-chain omega-3 polyunsaturated fatty acids, and a 1% increase in risk for omega-6 polyunsaturated fatty acids.
Data from circulating fatty acids and randomised controlled trials failed to show any significant links between any of the fat types assessed.
Sub type analysis
The team did however find that different subtypes of circulating long-chain omega-3 and omega-6 fatty acids had different associations with coronary risk, with some evidence that circulating levels of eicosapentaenoic (EPA) and docosahexaenoic acids (DHA), and arachidonic acid (AA) are each associated with lower coronary risk.
Similarly, within saturated fatty acid, the researchers found weak positive associations between circulating palmitic and stearic acids (found largely in palm oil and animal fats, respectively) and cardiovascular disease, whereas circulating margaric acid (a dairy fat) significantly reduced the risk of cardiovascular disease.
They noted that this analysis of specific fatty acid subtypes demonstrates that the effects of the fatty acids on cardiovascular risk can vary even within the same broad 'family' of fat types - and as such raises questions over the existing dietary guidelines which focus principally on the total amount of fat from saturated or unsaturated rather than the food sources of the fatty acid subtypes.
Red meat, saturated fat, and heart disease
Commenting on how these findings relate to the consumption of red meat, and its links to heart disease, the researcher said: “Last year, two seminal papers very convincingly showed that the harm observed in red meat for heart disease risk can, in fact, be attributed to another harmful chemical (L-carnitine) abundant in red meat rather than the long-supposed saturated fat."
"Unless we have more evidence, higher consumption of red meat should still be considered harmful, but it’s just that the saturates may not be the principal explanation, as is traditionally perceived, for the harmful cardiovascular effects of red meat.”
Source: Annals of Internal Medicine
Volume 160, Number 6, Pages 398-406, doi: 10.7326/M13-1788
" Association of Dietary, Circulating, and Supplement Fatty Acids With Coronary Risk: A Systematic Review and Meta-analysis"
Authors: Rajiv Chowdhury, Samantha Warnakula, et al