Observational studies are compatible with an association between saturated and trans fats and cardiovascular disease

Lee Hooper, Jim Mann

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Context: What is the relationship between saturated or trans fats in our food and cardiovascular risk? To find out we must examine the totality of interventional and observational evidence, and all appropriate outcomes. The World Health Organization Nutrition Guidance Advisory Group (WHO NUGAG) recently commissioned systematic reviews of randomised controlled trials (RCTs) which examined effects of modifying saturated or trans fat intakes on mortality or cardiovascular health1, 2 and controlled trials on lipid outcomes (updates of these published reviews are due to be published3, 4) as well as this systematic review of observational data (reported by de Souza et al).
Methods: De Souza’s systematic review included observational studies reporting associations between saturated or trans fat intakes and all-cause mortality, CHD or cardiovascular disease (CVD) mortality, total CHD, ischaemic stroke or type 2 diabetes. The review clearly stated the questions addressed, searched widely, one reviewer assessed titles and abstracts and a second reviewer checked potentially relevant studies. Risk of bias was assessed using the Newcastle-Ottawa scale plus risk of misclassification of exposure to trans fats, and used GRADE to assess confidence in the findings. Primary meta-analyses (reported here) presented most-adjusted results of high vs low intake in prospective cohorts.
Findings: Saturated fats: meta-analysis suggested no association between higher saturated fat intake and all-cause mortality (RR=0.99, 95%CI 0.91-1.09, 5 studies, 14090 deaths, I233%), CVD mortality, total CHD, ischaemic stroke or type 2 diabetes (all very low GRADE evidence), but suggested non-significantly increased CHD mortality (RR=1.15, 95%CI 0.97-1.36, 11 studies, 2970 deaths, I270%). Trans fats: meta-analysis suggested increased risk of CHD mortality (RR=1.28, 95%CI 1.09-1.50, 5 studies, 1234 deaths, I20%) and total CHD (RR=1.21, 95%CI 1.10-1.33, 6 studies, 4579 events, I20%) with higher total trans fat intake (moderate GRADE evidence), with similar effects for industrial, but not ruminant, trans fats. There were no consistent associations for all-cause mortality or ischaemic stroke (low to very low GRADE evidence), but the suggestion of lower diabetes risk with higher ruminant trans fat intake (RR=0.58, 95%CI 0.46-0.74, 5 studies, I230%, very low GRADE evidence).
Commentary: The observational evidence needs to be seen in the context of RCTs of effects on mortality, health outcomes and lipids.

Saturated fats: meta-analyses of RCTs found that reduced saturated fat reduced risk of CVD events by 17% (RR=0.83, 95%CI 0.72-0.96, 4377 people with events, I²65%, moderate GRADE evidence), but had no clear effects on all-cause mortality, CVD mortality, CHD deaths or events (moderate GRADE evidence). Replacement of saturated fats with polyunsaturated fats was associated with a statistically significant 27% reduction in CVD events, while replacement with carbohydrate, protein or monounsaturated fats showed no significant effects1.

Meta-analyses of controlled trials found that replacing 1% of dietary energy isoenergetically with saturated fats increased LDL cholesterol by 0.032mmol/L (95%CI 0.025-0.039, 43 studies) and the most favourable total cholesterol:HDL ratio was achieved when saturated fats were replaced by polyunsaturated fats3.

The interventional and observational systematic reviews together suggest that reducing saturated fats reduces cardiovascular events, and may have a more striking effect on cardiovascular events when replacement is by polyunsaturated fats.

Trans fats: systematic review found no RCTs assessing effects of reduction of trans fat intake on mortality, CVD or CHD2.

Combining trials in linear regression suggested that each 1% of energy from total trans fats replacing monounsaturated fats increased LDL cholesterol by 0.048mmol/L (95%CI 0.037-0.058), with similar effects of industrial trans fats4. Each 1% of energy from industrial trans fats replacing monounsaturated fats increased LDL:HDL ratio by 0.055mmol/L (95%CI 0.044-0.066), and ruminant trans fats increased LDL:HDL ratio by 0.038mmol/L (95%CI 0.012-0.074)4.

The balance of available evidence suggests that reducing total and industrial trans fats will reduce CHD mortality and events.

Implications for practice
The complete set of evidence suggests that reducing saturated and industrial trans fats, and replacing them with polyunsaturated fats, will reduce risks of CVD and CHD.

Original languageEnglish
Pages (from-to)37
JournalBMJ Evidence-Based Medicine
Issue number1
Early online date11 Nov 2015
Publication statusPublished - 1 Feb 2016


  • saturated fat
  • trans fats
  • Cardiovascular disease
  • Mortality
  • Systematic review
  • meta-analysis

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