Dariush Mozaffarian and colleagues conduct a systematic review and meta-analysis to investigate the effect of consuming polyunsaturated fats in place of saturated fats for lowering the risk of coronary heart disease.
Reduced saturated fat (SFA) consumption is recommended to reduce coronary heart disease (CHD), but there is an absence of strong supporting evidence from randomized controlled trials (RCTs) of clinical CHD events and few guidelines focus on any specific replacement nutrient. Additionally, some public health groups recommend lowering or limiting polyunsaturated fat (PUFA) consumption, a major potential replacement for SFA.
Methods and Findings
We systematically investigated and quantified the effects of increased PUFA consumption, as a replacement for SFA, on CHD endpoints in RCTs. RCTs were identified by systematic searches of multiple online databases through June 2009, grey literature sources, hand-searching related articles and citations, and direct contacts with experts to identify potentially unpublished trials. Studies were included if they randomized participants to increased PUFA for at least 1 year without major concomitant interventions, had an appropriate control group, and reported incidence of CHD (myocardial infarction and/or cardiac death). Inclusions/exclusions were adjudicated and data were extracted independently and in duplicate by two investigators and included population characteristics, control and intervention diets, follow-up duration, types of events, risk ratios, and SEs. Pooled effects were calculated using inverse-variance-weighted random effects meta-analysis. From 346 identified abstracts, eight trials met inclusion criteria, totaling 13,614 participants with 1,042 CHD events. Average weighted PUFA consumption was 14.9% energy (range 8.0%–20.7%) in intervention groups versus 5.0% energy (range 4.0%–6.4%) in controls. The overall pooled risk reduction was 19% (RR = 0.81, 95% confidence interval [CI] 0.70–0.95, p = 0.008), corresponding to 10% reduced CHD risk (RR = 0.90, 95% CI = 0.83–0.97) for each 5% energy of increased PUFA, without evidence for statistical heterogeneity (Q-statistic p = 0.13; I2 = 37%). Meta-regression identified study duration as an independent determinant of risk reduction (p = 0.017), with studies of longer duration showing greater benefits.
These findings provide evidence that consuming PUFA in place of SFA reduces CHD events in RCTs. This suggests that rather than trying to lower PUFA consumption, a shift toward greater population PUFA consumption in place of SFA would significantly reduce rates of CHD.
Please see later in the article for the Editors' Summary
Coronary heart disease (CHD) is the leading cause of death among adults in developed countries. It is caused by disease of the coronary arteries, the blood vessels that supply the heart with oxygen and nutrients. With age, inflammatory deposits (atherosclerotic plaques) coat the walls of these arteries and restrict the heart's blood supply, causing angina (chest pains that are usually relieved by rest), shortness of breath, and, if these plaques rupture or break, heart attacks (myocardial infarctions), which can reduce the heart's function or even be fatal. The key risk factors for CHD are smoking, physical inactivity, and poor diet. Blood cholesterol levels are altered by consuming dietary fats. There are three main types of dietary fats—“saturated” fatty acids (SFA) and unsaturated fatty acids; the latter can be “mono” unsaturated (MUFA) or “poly” unsaturated (PUFA). Eating SFA-rich foods (for example, meat, butter, and cheese) increases the amount of LDL-C in the blood but also increases HDL-C (the “good” cholesterol) and decreases triglycerides. Eating foods that are rich in unsaturated fatty acids (for example, vegetable oils and fatty fish) decreases the amount of LDL-C and triglycerides in the blood and also raises HDL-C.
Why Was This Study Done?
Because of the connection between eating SFA and high blood LDL-C levels, reduced SFA consumption is recommended as a way to avoid CHD. However, the evidence from individual randomized controlled trials that have studied CHD events (such as heart attacks and CHD-related deaths) have been mixed and could not support this recommendation. Furthermore, dietary recommendations to reduce SFA have generally not specified any replacement, i.e., whether SFA should be replaced with carbohydrate, protein, or unsaturated fats. Because of their beneficial effects on blood LDL-C and HDL-C levels, PUFA could be one important replacement for SFA, but, surprisingly, some experts argue that eating PUFA could actually increase CHD risk. Consequently, some guidelines recommend that PUFA consumption should be limited or even reduced. In this systematic review (a study that uses predefined criteria to identify all the research on a specific topic) and meta-analysis (a statistical method for combining the results of several studies) of randomized controlled trials, the researchers assess the impact of increased PUFA consumption as replacement for SFA on CHD events.
What Did the Researchers Do and Find?
The researchers' search of the published literature, “grey” literature (doctoral dissertations, technical reports, and other documents not printed in books and journals), and contacts with relevant experts identified eight trials in which participants were randomized to increase their PUFA intake for at least a year and in which CHD events were reported. 1,042 CHD events were recorded among the 13,614 participants enrolled in these trials. In their meta-analysis, the researchers found that on average the consumption of PUFA accounted for 14.9% of total energy intake in the intervention groups compared with only 5% of total energy intake in the control groups. Participants in the intervention groups had a 19% reduced risk of CHD events compared to participants in the control groups. Put another way, each 5% increase in the proportion of energy obtained from PUFA reduced the risk of CHD events by 10%. Finally, the researchers found that the benefits associated with PUFA consumption increased with longer duration of the trials.
What Do These Findings Mean?
These findings suggest that the replacement of some dietary SFA with PUFA reduces CHD events. Because the trials included in this study looked only at replacing SFA with PUFA, it is not possible from this evidence alone to distinguish between the benefits of reducing SFA and the benefits of increasing PUFA. Furthermore, the small number of trials identified in this study all had design faults, so the risk reductions reported here may be inaccurate. However, other lines of evidence (for example, observational studies that have examined associations between the fat intake of populations and their risk of CHD) also suggest that consumption of PUFA in place of SFA reduces CHD risk. Thus, in the light of these findings, future recommendations to reduce SFA in the diet should stress the importance of replacing SFA with PUFA rather than with other forms of energy, and the current advice to limit PUFA intake should be revised.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000252.
The American Heart Association provides information about all aspects of coronary heart disease for patients, caregivers, and professionals, including advice on dietary fats (in several languages)
The UK National Health Service Choices Web site provides information about coronary heart disease
Eatwell, a resource provided by the UK Food Standards Agency, gives advice on all aspects of healthy eating, including fat consumption
MedlinePlus provides links to further resources on coronary heart disease and on cholesterol (in English and Spanish)