Studies have found that soy can reduce total cholesterol, LDL ("bad") cholesterol, and triglycerides, and possibly raise HDL ("good") cholesterol as well.1,2 However, there is considerable controversy regarding what constituent in soy is responsible for these beneficial effects.
For several years, it has been assumed that substances called isoflavones are the active ingredient. Isoflavones are naturally occurring, estrogen-like substances found in soy and many other plants.
However, recent studies have begun to cast doubt on this theory.3,4,5 For example, in a double-blind study, 54 postmenopausal women were given placebo or one of two soy formulations: a high-isoflavone soy protein (providing 65 mg isoflavones daily) or a soy protein with very low isoflavone content (providing 4 mg of isoflavones daily).6 The results showed improved cholesterol levels in both soy groups as compared to placebo. However, there was no difference between the high-isoflavone and low-isoflavone forms of soy.
Considering that 4 mg of isoflavones is so low as to be almost negligible, this study strongly suggests that soy's influence on cholesterol does not depend on its isoflavone content.
More evidence comes from a double-blind placebo-controlled study that evaluated the effects of isoflavones from red clover in 76 women.7 Although red clover isoflavones are very similar to soy isoflavones, the red clover extract failed to produce any effect on cholesterol.
If it isn't the isoflavones in soy that reduce cholesterol, what is the active ingredient? The answer is that we don't know.