This review concluded that 25g or less of soya protein per day was effective in reducing blood cholesterol in adults with normal or mild hypercholesterolaemia. The review was generally well conducted and the authors' conclusions appear to reflect the pooled evidence. However, limitations with the included studies should be taken into consideration when interpreting the results.

To assess the effects of introducing a daily intake of soya protein on blood lipids in adults with normal or mildly elevated blood cholesterol.

MEDLINE, EMBASE, SciSearch and Current Contents were searched from January 1995 to September 2007 for publications in any language. Search terms were reported. Key papers and publications, and references of identified articles were searched manually. Experts in the field were contacted for further publications.

Placebo-controlled randomised controlled trials (RCTs) that compared the consumption of soya protein with a non-soya control in healthy or mildly hypercholesterolaemic volunteers were eligible for inclusion. Participants were required to have a weight loss of less than 2kg/day, an average body mass index (BMI) of less than 30 and consume around 25g (range 10g to 40g) of soya protein. The minimum study duration was four weeks or one menstrual cycle.

Included studies were of male and/or female patients with an average age range between 27 and 67 years; some female patients were menopausal or hypertensive. The average intake of soya protein was 26.9g (range 15g to 40g). Study durations ranged from four to 52 weeks. The baseline total cholesterol range was 4.57mmol/L to 7.03mmol/L (mean 5.96 mmol/L). The primary outcomes measured were changes in total cholesterol, low and high density lipoprotein and blood triglycerides. Secondary outcomes included effects on apolipoprotein B (ApoB) and ApoA-I.

The authors stated neither how many reviewers screened articles for relevance nor how discrepancies were resolved.

The two authors assessed the quality of the RCTs using the Jadad scale (randomisation, allocation concealment and dropout/withdrawal rates). A score of 5 denoted the highest level of quality. Any discrepancies were resolved by consensus.

The two authors independently extracted the mean differences for primary outcomes. Where data were missing, authors were contacted for further details. Where blood lipids were reported at various intervals throughout the study, the sample reported after the longest period was extracted. It was unclear how discrepancies were resolved.

Standardised mean differences (SMDs) with 95% confidence intervals (CIs) were pooled for each outcome using a fixed-effect model and weighted by the inverse of the variance. Statistical heterogeneity was assessed using Hodge's and Cohen's coefficients. Subgroup analyses were conducted to assess the effect of study design, source of protein and gender on the SMDs for low and high density lipoprotein. Weighted meta-regression analysis was undertaken to investigate the effect of dose of soya protein on low and high density lipoprotein.

Publication bias was assessed using funnel plots and the fail-safe N-value.

Thirty randomised crossover or parallel studies (n=2,913; 42 treatment arms) were included in the review. Sample sizes ranged from 22 to 216 participants. The average Jadad score was 3.5; six studies scored 5 and 10 scored 4. Seventeen studies were double blinded; two were single blinded.

Soya protein significantly reduced low density lipoprotein cholesterol (SMD -0.23 mmol/L, 95% CI: -0.160 to -0.306, p<0.0001; 42 treatment arms, four studies showed a statistically significant difference). This translated into around a 12% reduction in risk. High density lipoprotein cholesterol was not effected significantly.

Soya protein also significantly reduced blood triglycerides (SMD -0.08mmol/L, 95% CI: -0.004 to -0158, p=0.04; 39 treatment arms, only one study showed statistically significant differences) and total cholesterol (SMD -0.22 mmol/L, 95% CI: -0.142 to -0.291, p<0.0001; 41 treatment arms, five trials showed a statistically significant difference).

There was no evidence of heterogeneity for any of the analyses.

Subgroup analyses showed no significant relationships between soya protein intake (range of 15g to 40g) and low or high density lipoprotein. Further subgroup analyses and secondary outcomes were reported in the review.

There was no evidence of publication bias using funnel plots.

The inclusion of modest amounts of soya protein (25g or less per day) was effective in reducing blood total and low density lipoprotein cholesterol, ApoB and triglycerides in adults with normal or mild hypercholesterolaemia. Certain populations may also have a small beneficial increase in high density lipoprotein cholesterol.

The review question was clear and was supported by appropriate inclusion criteria for study design, intervention/comparison and participants. A comprehensive search of the literature was undertaken and included papers in any language, which reduced the risk of language bias. Publication bias was assessed. The authors took steps to locate additional publications from experts in the field, which reduced the possibility that potentially relevant papers were missed. A reliable tool was used to assess study validity, although the quality of studies appeared to vary. The authors reported the review process for data extraction and validity assessment, which minimised the potential for reviewer error and bias; the same could not be stated for study selection, thus reviewer error and bias could not be totally ruled out. Appropriate methods were used to synthesise the data and test for statistical heterogeneity. The authors acknowledged certain limitations with the studies, including short treatment duration and small sample sizes for treatment arms. However, the SMDs for individual studies showed wide confidence intervals, with most crossing zero, which affects the reliability of the results. The authors' conclusions appear to reflect the evidence available, but the above considerations should be taken into account when interpreting the results.

Practice: The authors stated that an intake of 25g was achievable using a wide range of soya foods and the evidence supported the suggestion that modest intakes of soya protein have beneficial effects on blood lipids and can make a useful contribution to portfolio-type diets. Practitioners and patients can take advantage of these opportunities and promote public health measures that reduce blood cholesterol and coronary heart disease risk through diet.

Research: The authors stated that more treatment arms were required to assess and understand the key drivers that effect blood cholesterol.

Unconditional grant from the Soya Protein Association.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.