Effect of soy protein varying in isoflavone content on serum lipids in healthy young men.

Background: Previous research supports a role for soy protein in reducing serum lipids; however, few studies involved healthy male subjects or focused on soy isoflavones (or did both).

Objective: The objective was to ascertain the effects of soy protein varying in isoflavone content on serum lipids in healthy young men.

Design: Thirty-five males (x +/- SD age: 27.9 +/- 5.7 y) consumed milk protein isolate (MPI), low-isoflavone soy protein isolate (low-iso SPI; 1.64 +/- 0.19 mg aglycone isoflavones/d), and high-isoflavone SPI (high-iso SPI; 61.7 +/- 7.4 mg aglycone isoflavones/d) for 57 d each, separated by 4-wk washout periods, in a randomized crossover design. Blood samples were collected at the beginning and end of each treatment period, and total, LDL, and HDL cholesterol; triacylglycerols; apolipoprotein (apo) B; apo A-I; and C-reactive protein (CRP) were measured in serum. Twenty-four-hour urine samples were collected for 3 consecutive days at the end of each treatment period and analyzed for isoflavones.

Results: Urinary isoflavones were significantly greater with consumption of the high-iso SPI than with that of the low-iso SPI or MPI. The differences between the 3 treatments with respect to individual serum lipids were not significant, but the ratios of total to HDL cholesterol, LDL to HDL cholesterol, and apo B to apo A-I were significantly lower with both SPI treatments than with MPI treatment.

Conclusion: Soy protein, regardless of isoflavone content, modulates serum lipid ratios in a direction beneficial for cardiovascular disease risk in healthy young men.