The findings are the latest in a line of studies to suggest that saturated fats may not pose as much of a risk as previously believed. However, the study did find that trans-fats are associated with greater risk of death and coronary heart disease.
Led by Professor Russell de Souza at McMaster University, the researchers analysed the results of 50 observational studies assessing the association between saturated and/or trans-fats and health outcomes in adults. They found no clear association between higher intake of saturated fats and death for any reason, coronary heart disease (CHD), cardiovascular disease (CVD), ischemic stroke or type 2 diabetes.
However, consumption of industrial trans-fats was associated with a 34% increase in death for any reason, a 28% increased risk of CHD mortality, and a 21% increase in the risk of CHD.
"For years everyone has been advised to cut out fats. Trans-fats have no health benefits and pose a significant risk for heart disease, but the case for saturated fat is less clear," said de Souza.
"That said, we aren't advocating an increase of the allowance for saturated fats in dietary guidelines, as we don't see evidence that higher limits would be specifically beneficial to health."
Indeed, the researchers stressed that their results are based on observational studies, so no definitive conclusions can be drawn about cause and effect. However, they added that their analysis "confirms the findings of five previous systematic reviews of saturated and trans-fats and CHD."
de Souza and his colleagues sytematically reviewed the associations between intake of saturated fat and trans-fat and all-cause mortality, CVD and associated mortality, coronary heart CHD and associated mortality, ischemic stroke, and type 2 diabetes in the scientific literature.
For saturated fat, the team identified between three and 12 prospective cohort studies for each association, which were then pooled.
Analysis of this pooled data, showed that saturated fat intake was not associated with all-cause mortality, CVD mortality, total CHD, ischemic stroke, or type 2 diabetes.
However, the certainty of associations between saturated fat and all outcomes was ‘very low,’ they noted.
For trans-fats, between one and six prospective cohort studies were identified for each association and were pooled. Data from these studies revealed that total trans-fat intake was significantly associated with all-cause mortality, CHD mortality, and total CHD but not ischemic or type 2 diabetes.
In addition, they found that industrial, but not ruminant, trans-fats were associated with CHD mortality and CHD. Indeed, the ruminant trans-palmitoleic acid was found to be inversely associated with type 2 diabetes.
The team concluded that saturated fats are not associated with all-cause mortality, CVD, CHD, ischemic stroke, or type 2 diabetes, “but the evidence is heterogeneous with methodological limitations.”
“Trans-fats are associated with all-cause mortality, total CHD, and CHD mortality, probably because of higher levels of intake of industrial trans-fats than ruminant trans-fats,” they added – noting that dietary guidelines must carefully consider the health effects of recommendations for alternative macronutrients to replace trans-fats and saturated fats.
"If we tell people to eat less saturated or trans-fats, we need to offer a better choice,” said de Souza.
“Unfortunately, in our review we were not able to find as much evidence as we would have liked for a best replacement choice, but ours and other studies suggest replacing foods high in these fats, such as high-fat or processed meats and donuts, with vegetable oils, nuts, and whole grains."
Published online ahead of print, doi: 10.1136/bmj.h3978
“Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies”
Authors: Russell J de Souza, et al