The effect of dietary protein source on serum lipids: Secondary data analysis from a randomized clinical trial.

Background: Plant-based diets can lower serum lipids. Whether soy foods offer additional benefits remains controversial.

Objective: To determine the effect of different protein sources on serum lipids and glucose metabolism, emphasizing comparisons between soy and nonsoy plant-based diets.

Methods: Secondary data analysis. A total of 173 postmenopausal women were randomized to 1 of 4 weighed metabolic diets for 6 weeks. Diets were equivalent in energy, protein, and fat with at least 80% of protein from either nondairy animal, dairy, nonsoy plant, or soy foods. At baseline and week 6, total cholesterol (TC), low-density lipoprotein (LDL), high-density lipoprotein (HDL), triglycerides, glucose, and insulin were measured. Changes in variables during the diet period were compared within and among groups using t tests and analysis of variance.

Results: TC decreased 38.8 mg/dL in soy group (P < .001 vs dairy and animal) and 30.5 mg/dL in nonsoy plant group (P = .003 vs dairy, .03 vs animal). LDL decreased 28.3 mg/dL in soy group (P < .001 vs dairy and animal) and 20.6 mg/dL in nonsoy plant group (P = .005 vs dairy, .06 vs animal). HDL decreased 12 mg/dL in soy group (P = .003 vs dairy, .0008 vs animal) and 10 mg/dL in nonsoy plant group (P = .05 vs dairy, .04 vs animal). There were no significant differences in lipid changes between soy and nonsoy plant-based diets. No differences among groups in changes in triglycerides, glucose, or insulin were seen.

Conclusions: Soy and nonsoy plant-based diets reduced TC and LDL with no significant difference between them. Further studies are needed to determine the specific lipid-lowering components of both soy and nonsoy plant foods.