Trans fats are formed when hydrogen is added to vegetable oil and can be found in fast food, frozen pizza, margarine and packaged baked goods among other foods.
Saturated fats typically come from animal products such as meat, egg yolks, butter, milk and salmon, contributing about 10% of energy to the North American diet.
In contrast, trans fats are produced industrially from plant oils and found in margarine, packaged baked goods and snack foods. They contribute 1-2% of energy to the North American diet.
At present, dietary guidelines recommend limiting saturated fat consumption to less than 10% of daily energy intake to reduce the risk of ischemic heart disease and stroke. Guidelines also recommend that trans fat consumption should be less than 1% of energy.
However, several studies have challenged the assertion that the intake of saturated fats is associated with cardiovascular risk.
Last May, a cardiologist writing in The BMJ stated that the idea that saturated fat plays a role in heart disease is a myth, pointing out that since it had been recommended that people removed it from their diets, cardiovascular risk went up.
In another study, an international group of researchers reviewed 72 studies on heart risk and intake of fatty acids, reporting that there is no evidence to support guidelines saying that people should restrict their consumption of saturated fats to reduce their risk of heart disease.
For the new study, the researchers pooled data from studies investigating the associations of saturated fats and trans fats with all-cause mortality, coronary heart disease, cardiovascular disease deaths, ischemic stroke and type 2 diabetes.
No clear association with saturated fats and all-cause mortality
The researchers could not find any clear associations between high intake of saturated fats and with health problems in the studies involving saturated fats, although they were unable to rule out the possibility that it could increase the risk of death from coronary heart disease.
In the studies involving trans fats, however, they found trans fat consumption was associated with a 34% increase in all-cause mortality, a 21% increase in the risk of coronary heart disease and a 28% increase in the risk of death from coronary heart disease.
The researchers admit that the certainty of the estimates for saturated fats is "very low" and for trans fats it ranges from "moderate" to "very low" across the different health outcomes. They state that low precision and high inconsistency among the different studies analyzed was the reason for this and that further research is likely to improve confidence in their findings.
"Dietary guidelines must carefully consider the health effects of recommendations for alternative macronutrients to replace trans fats and saturated fats," the authors conclude.
Earlier this year, Medical News Today reported on a paper published in JAMA that called for the US government to drop the recommended restrictions on total fat consumption ahead of the publication of the 2015 Dietary Guidelines for Americans.
The Dietary Guidelines for Americans Committee had not included a recommendation for total fat consumption in their technical report this year, and the authors of the paper believe that emphasis should be on fat quality rather than total fat consumption.