Eating nuts to prevent cardiovascular disease

Review question

This Cochrane review aims to answer the question whether or not eating nuts can prevent cardiovascular disease.

Background

Cardiovascular diseases are a group of conditions that affect the heart and blood vessels. They are a major cause of death worldwide. The food we eat may influence the risk of getting cardiovascular disease. Nuts, if consumed regularly and at relatively high doses (50 g to 100 g), are believed to reduce total cholesterol and low-density lipoprotein (LDL) cholesterol (bad cholesterol).

Study characteristics

This review includes randomised controlled trials, which lasted at least 12 weeks. Participants were between 37 to 54 years old on average. The evidence is current up to 30 July 2015.

Key results

We included five trials (435 participants), one of which had two treatment arms. All five trials investigated the effects of eating nuts. No studies were found which investigated the effect of giving advice to eat more nuts. None of the studies reported on deaths or cardiovascular events. None of the results show a clear effect on total cholesterol levels and blood pressure. One study reported one case of an allergic reaction to nuts. Three studies reported no significant weight gain with increased nut consumption. No other adverse events were reported.

Quality of the evidence

All included trials are small, with 60 to 100 participants, and have a high level of variation (heterogeneity). Therefore the results should be interpreted with caution. Overall we regarded the included trials as being at unclear risk of bias.

Authors' conclusions: 

Currently there is a lack of evidence for the effects of nut consumption on CVD clinical events in primary prevention and very limited evidence for the effects on CVD risk factors. No conclusions can be drawn and further high quality longer term and adequately powered trials are needed to answer the review question.

Read the full abstract...
Background: 

Nuts contain a number of nutritional attributes which may be cardioprotective. A number of epidemiological studies have shown that nut consumption may have a beneficial effect on people who have cardiovascular disease (CVD) risk factors. However, results from randomised controlled trials (RCTs) are less consistent.

Objectives: 

To determine the effectiveness of nut consumption for the primary prevention of CVD.

Search strategy: 

We searched the following electronic databases: the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, Web of Science Core Collection, CINAHL, Database of Abstracts of Reviews of Effects (DARE), Health Technology Assessment Database (HTA) and Health Economics Evaluations Database (HEED) up to 30 July 2015. We searched trial registers and reference lists of reviews for further studies. We did not apply any language restrictions.

Selection criteria: 

We included RCTs of dietary advice to increase nut consumption or provision of nuts to increase consumption lasting at least three months and including healthy adults or adults at moderate and high risk of CVD. The comparison group was no intervention or minimal intervention. The outcomes of interest were CVD clinical events and CVD risk factors.

Data collection and analysis: 

Two review authors independently selected trials for inclusion, abstracted the data and assessed the risk of bias in included trials.

Main results: 

We included five trials (435 participants randomised) and one ongoing trial. One study is awaiting classification. All trials examined the provision of nuts to increase consumption rather than dietary advice. None of the included trials reported on the primary outcomes, CVD clinical events, but trials were small and short term. All five trials reported on CVD risk factors. Four of these trials provided data in a useable format for meta-analyses, but heterogeneity precluded meta-analysis for most of the analyses. Overall trials were judged to be at unclear risk of bias.

There were variable and inconsistent effects of nut consumption on CVD risk factors (lipid levels and blood pressure). Three trials monitored adverse events. One trial reported an allergic reaction to nuts and three trials reported no significant weight gain with increased nut consumption. None of the included trials reported on other secondary outcomes, occurrence of type 2 diabetes as a major risk factor for CVD, health-related quality of life and costs.

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