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.