Consuming probiotic bacteria does not enhance the cholesterol-lowering effect of soy in postmenopausal women, nutritionists have determined.

"Numerous studies report that soy lowers cholesterol. Probiotic bacteria were also reported to lower total cholesterol (TC) and LDL cholesterol (LDL-C). We hypothesized that by altering intestinal microflora, probiotic consumption may also change phytoestrogen metabolism and enhance the effects of soy," said K.A. Greany and colleagues, University of Minnesota.

"To evaluate the independent and interactive effects of probiotic bacteria and soy on plasma TC, LDL-C, HDL cholesterol (HDL-C), and triglycerides (TG), 37 women with a baseline TC of 5.24 mmol/L were given the following four treatments for six weeks, each in a randomized crossover design: soy protein isolate (26±5 g soy protein containing 44±8 mg isoflavones/d); soy protein isolate + probiotic capsules (10 colony-forming units Lactobacillus acidophilus DDS-1 and Bifidobacterium longum); milk protein isolate (26±5 g milk protein/d); and milk protein isolate + probiotic," the researchers said.

They found that "soy consumption decreased plasma TC by 2.2% (p=0.02) and LDL-C by 3.5% (p=0.005), increased HDL-C by 4.2% (p=0.006), and tended to decrease TG (p=0.07) compared with milk protein intake."

"When divided according to initial TC concentration," Greany and associates continued, "soy effects were observed only in hypercholesterolemic women (TC>5.17 mmol/L). In this subgroup, soy treatments decreased plasma TC by 3.3% (p=0.01), LDL-C by 4.5% (p=0.004), and TG by 10.6% (p=0.02), and increased HDL-C by 4.2% (p=0.02)."

"When subjects were divided on the basis of plasma and urine concentrations of the isoflavone metabolite, equol, equol producers, and nonproducers did not differ in baseline lipids or in the effects of soy. Probiotics did not lower cholesterol or enhance the effects of soy," the team reported.

They concluded, "These results confirm a beneficial effect of soy on plasma cholesterol in mildly hypercholesterolemic postmenopausal women independent of equol production status, but do not support an independent or additive effect of these particular probiotic bacteria."

Greany and coauthors published the results of their research in the Journal of Nutrition (“Probiotic consumption does not enhance the cholesterol-lowering effect of soy in postmenopausal women.” J Nutr, 2004;134(12):3277-3283).

For additional information, contact M.S. Kurzer, University of Minnesota, Dept. of Food Science & Nutrition, St. Paul, MN 55108.