The effect of soy protein with or without isoflavones relative to milk protein on plasma lipids in hypercholesterolemic postmenopausal women.

This was a 3-arm, randomized dietary supplement trial in postmenopausal women with moderate hypercholesterolemia. Eligibility included LDL cholesterol 3.37-4.92 mmol/L, triglycerides under 2.82 mmol/L, BMI 20-31 kg/m2, and age under 80 years; women with recent smoking, hormone therapy, lipid-lowering medication use, major cardiovascular disease, diabetes, or recent hormone-related cancer were excluded.

Participants received 42 g/day of protein by mouth for 12 weeks, given as 2 packets/day (21 g protein per packet) mixed with 240 mL liquid. The active regimens were isolated soy protein with trace isoflavones (Soy-) or isolated soy protein containing 80 mg aglycone isoflavones (Soy+); the control comparator was milk protein after a 4-week milk-protein run-in. Each supplement also contained 30 g carbohydrate and 500 mg calcium phosphate.

The isoflavone-containing soy protein arm produced greater lipid lowering than the soy protein with trace isoflavones arm, but neither soy regimen showed a significant advantage over the milk-protein control. At 12 weeks, the between-soy-arm differences were -0.02 mmol/L and -0.27 mmol/L for total cholesterol and -0.09 mmol/L and -0.38 mmol/L for LDL cholesterol, with pairwise comparisons significant at P = 0.01. End-of-study LDL cholesterol was 3.8 ± 0.8 mmol/L in Soy- and 3.5 ± 0.5 mmol/L in Soy+, compared with baseline values of 3.9 ± 0.6 mmol/L in both groups. Overall, all groups showed lipid reductions, suggesting the changes may reflect study participation or dietary control rather than a clear independent benefit of soy protein with or without isoflavones.

The trial was short, and the available summary does not report arm-specific sample sizes, limiting precision assessment. Group differences in age and years since menopause may have introduced baseline imbalance, and the absence of a significant difference versus milk protein weakens causal interpretation. Dietary restriction and a run-in phase may also limit generalizability to usual eating conditions.