A rapid overview of systematic reviews on the effects of palm oil intake compared with intake of other vegetable oils on mortality and cardiovascular health in children and adults

Introduction

Cardiovascular disease (CVD) is the leading cause of death worldwide. CVD continues its decades-long rise (in number of disability-adjusted life years [DALYs] and deaths) in most low- and middle-income countries (LMIC). The CVD burden attributable to modifiable factors also continues to increase globally (Roth 2020).

Dyslipidemia – the imbalance of lipids such as cholesterol, low-density lipoprotein (LDL), triglycerides and high-density lipoprotein (HDL) – is one of the most important modifiable risk factors for the development of CVD. In line with best available evidence and current CVD guidelines, blood concentrations of non-HDL-cholesterol and LDL-cholesterol (i.e. apolipoprotein B-containing lipoproteins) are accepted as causal cardiovascular risk factors and are a cornerstone of CVD risk prediction in the general population. Current recommendations focus on maintaining low non-HDLcholesterol and LDL-cholesterol to reduce CVD risk (Brunner 2019; Grundy 2019; Mach 2020; Rabar 2014). Alarmingly, the total number of DALYs due to elevated LDL-cholesterol has risen steadily since 1990 (Roth 2020).

Diet is another key modifiable risk factor for CVD – the importance of dietary risks as a cause of global disease burden is undisputed (GBD 2015 Risk Factors Collaborators 2016). The effects of diet on CVD remain considerable when assessed as individual dietary factors or as measures of overall diet quality. Globally, the absolute burden of CVD attributable to dietary risks has risen for 30 years, regardless of how it is measured, and it is predicted to rise rapidly in LMIC over the coming years (Roth 2020). Many studies have shown that the relationships between different types of dietary oils or solid fats and CVD risk factors (e.g. blood lipids) can be predicted from the fatty acid composition of the former.

Palm oils are derived from the fruit of the oil palm tree, and over recent decades they have become one of the most widely produced and consumed oils in the world. Two types of oil are typically produced from oil palms: crude palm oil from the fleshy fruit and palm kernel oil from the kernel. Compared with other liquid oils, palm oil is relatively high in saturated fats. Myristic acid (1%), stearic acid (5%) and palmitic acid (44%) make up the saturated fatty acid component of palm oil (Sambanthamurthi 2000). A recent Cochrane review (n=15 randomized controlled trials [RCTs], n ~59 000 participants) reported that reducing saturated fat intake for at least 2  years causes important reductions in combined cardiovascular events, which did not change by study duration, sex or baseline CVD risk. Greater decreases in saturated fat intake caused greater decreases in cardiovascular events, and replacing the energy from saturated fat with polyunsaturated fat or carbohydrate appeared to be useful strategies; however, the effects of replacement with monounsaturated fat were less clear (Hooper 2020). Owing to the prevalent intake of palm oil in many countries and its potential to contribute to CVD risk, it is important to evaluate the available evidence on the effects on CVD risk of palm oil compared with other vegetable oils.

When seeking to better understand the available body of evidence on a particular topic, systematic reviews can be used to collectively examine the findings from all related or similar primary studies on the question of interest. Critical appraisal of the methodological quality of systematic reviews can help users to determine the quality of reviews based on how well they were conducted (Higgins 2021). In this overview, “palm oil” includes the various palm oils and fractions thereof available.

Who is this rapid overview for?

This rapid overview is aimed at people making decisions that require an understanding of the available research evidence on the effects of palm oil intake on cardiovascular health.

What does this rapid overview include and exclude? The rapid overview includes findings from published systematic reviews, as well as assessments of the methodological quality of those reviews. It excludes assessments of the certainty of the evidence.

What is a systematic review?

A systematic review is a research study that addresses a clearly formulated question, and uses systematic and explicit methods to identify, select, appraise (i.e. assess the risk of bias) and synthesize all the primary studies that meet prespecified eligibility criteria.

What is a meta-analysis?

A meta-analysis is a statistical process to combine data from independent primary studies focused on the same question when appropriate. This analysis generates a pooled quantitative estimate of the studied phenomenon; for example, the effectiveness of an intervention