Abstract
Objectives: To examine the evidence from prospective observational studies, to contribute to the evidence base required for dietary recommendations relating to intakes of saturated fats (SFA) and trans-fats (TFA).
Design: Systematic review and meta-analysis of studies from database inception to October 2020.
Data sources: Medline, Embase, the Cochrane Central Registry of Controlled Trials, Evidence Based Medicine and CINAHL.
Review methods: Eligible studies reported on either dietary or tissue measures of SFA or TFA and mortality or noncommunicable disease (NCD) incidence. We considered total SFA or TFA, specific chain lengths or isomers and food sources in extreme quantile, dose–response and replacement analyses where SFA and TFA were replaced with other macronutrients. Searches and data extraction were duplicated. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach was used to assess quality of evidence.
Results: There were 112 publications (3 696 568 participants) relating to SFA and 55 publications (2 227 241 participants) relating to TFA and prespecified health outcomes. Higher dietary intakes of SFA were associated with increased mortality. Mortality reduced when 5% of total energy from SFA was replaced with polyunsaturated fats (PUFA; relative risk [RR] 0.85; 95% confidence interval [CI]: 0.75 to 0.97), monounsaturated fats (MUFA; RR: 0.84; 95% CI: 0.75 to 0.95), plant MUFA (RR: 0.85; 95% CI: 0.82 to 0.88) and carbohydrates (RR: 0.92; 95% CI: 0.86 to 0.99). Coronary heart disease (CHD) incidence reduced with a 5% energy replacement with PUFA (RR: 0.89; 95% CI: 0.81 to 0.98), plant MUFA (RR: 0.83; 95% CI: 0.69 to 1.00) and slowly digested carbohydrates (RR: 0.94; 95% CI: 0.89 to 0.99). Higher tissue measures of total SFA were associated with increased CHD and type 2 diabetes incidence. Higher dietary intakes of TFA were associated with increased mortality, CHD and cardiovascular disease. A 2% replacement of TFA with plant MUFA reduced mortality (RR: 0.90; 95% CI: 0.85 to 0.96) and CHD (RR: 0.80; 95% CI: 0.70 to 0.92). The certainty of evidence was graded from moderate to very low, largely due to the amount of data available.
Conclusions: Consideration of the totality of evidence available from prospective observational studies provides convincing evidence that replacing SFA and TFA with other macronutrients may reduce risk of mortality and CHD. These findings reinforce guidelines that SFA and TFA in the diet should be replaced by PUFA, plant MUFA and slowly digested carbohydrates.