Soy protein effects on serum lipoproteins: a quality assessment and meta-analysis of randomized, controlled studies

Many randomized controlled trials (RCTs) have assessed the effects of soy protein on serum lipoprotein risk factors for coronary heart disease (CHD). This review and meta-analysis assessed the quality of these RCTs and estimated the effects of soy protein consumption on serum lipoproteins.

A comprehensive search using multiple databases was conducted for the years 1996 through 2008 to identify clinical trials related to soy protein intake and serum lipoprotein changes.

RCTs were assessed that met these requirements: soy protein intake compared with nonsoy protein, provided information on serum low-density lipoprotein (LDL)-cholesterol values, provided no more than 65 g of soy protein daily, and obtained LDL-cholesterol measurements between 4 and 18 weeks of treatment. Randomized parallel and crossover studies were evaluated.

Studies were graded for quality using 12 criteria with a possible maximum grade of 24. Net changes in lipoproteins with soy protein consumption compared with nonsoy control diets were analyzed by meta-analyses and funnel plots. Confidence intervals were constructed using inverse weighting. Analyses compared parallel to crossover studies and studies with lower and higher grades.

Analyses included 20 parallel-design studies and 23 crossover studies. Parallel studies scored significantly higher (p < 0.001) in study quality, with a mean grade of 15.8 (95% confidence interval [CI], 14.3 to 17.3) compared with 10.1 (95% CI, 8.2 to 11.9) for crossover trials. Soy protein intake was associated with net changes in serum LDL-cholesterol values of -0.23 mmol/l (95% CI, -0.28 to -0.18 mmol/l) or a 5.5% reduction in parallel studies and -0.16 mmol/l (95% CI, -0.22 to -0.11 mmol/l) or a reduction of 4.2% with crossover studies (p < 0.001 for parallel vs crossover). In parallel studies, net serum HDL-cholesterol values were 3.2% higher (p < 0.007) with soy vs control, and fasting serum triacylglycerol values were 10.7% lower (p < 0.008) for soy vs control.

Soy protein consumption with a median of 30 g/d was associated with a significant improvement in lipoprotein risk factors for CHD. Compared with crossover RCTs, parallel RCTs had significantly higher quality grades and were associated with significantly greater improvements in serum LDL-cholesterol values. Regular consumption of 1 to 2 servings of soy protein daily (15 to 30 g) has a significant favorable impact on serum lipoprotein risk factors for CHD.