So Long, Soy?

Summarized by Robert W. Griffith, MD
February 13, 2006

Introduction

In 1999 the US FDA granted the manufacturers of soy products approval to label them as protective against coronary heart disease. They based this on studies showing that 25 grams of soy protein a day lowers total and LDL cholesterol; however, they added that there was no evidence that soy isoflavones ¹ had a significant role in the cholesterol-lowering effect of soy protein.

Since 1999 there have been many studies of the possible health benefits of soy protein and soy-derived isoflavones. The American Heart Association Nutritional Committee has analyzed the available published results of such studies, and posted their report online for the medical journal Circulation. This is a summary.

What was done

The Committee decided to concentrate on results of blood low-density lipoprotein cholesterol (LDL cholesterol) because it is by far the most studied risk factor for cardiovascular disease. However, the effects of soy protein and isoflavones was assessed on other factors: HDL cholesterol, triglycerides, lipoprotein-alpha, and blood pressure.

All published studies of the effects of soy protein or isoflavones on cardiovascular risk factors were examined, and those that listed soy and isoflavone content separately were analyzed fully. The possible effects of soy in other conditions - including menopausal symptoms, osteoporosis, and cancer - were also evaluated.

There were 22 randomized studies of isolated soy protein with isoflavones compared with casein or milk protein, wheat protein, or animal protein. A further 7 studies compared the effects of soy protein without isoflavones (they were removed by washing with alcohol) with those of casein, milk, or animal protein. And there were 19 studies that examined the effects of isoflavones alone, in some cases given in pill form.

What was found

In 19 of the isolated soy protein with isoflavones the LDL cholesterol was slightly reduced. This effect was statistically significant in 13 of them; however, the actual reduction in LDL levels over the 22 studies amounted to only 3%, on average. There were no significant effects on HDL cholesterol, triglycerides, lipoprotein-alpha, or blood pressure.

Soy protein without isoflavones produced small but significant decreases in LDL cholesterol in two studies, but no change in the other 5 studies. Again, the other lipids and the blood pressure were unaltered.

Of the 19 studies of isoflavones alone, only 3 showed slight but significant reductions in LDL cholesterol; others showed an increase, so that overall the average effect on cholesterol was nil. Other lipids were not influenced to any relevant extent.

The Committee found that neither soy protein nor isoflavones had been shown to decrease the flushing seen in menopause, while results for a benefit in slowing postmenopausal bone loss (osteoporosis) "are mixed". Their use in the prevention or treating cancer of the breast, uterus, and prostate is also unproven.

What this report means

Based on the results reported here, the Committee states that the use of isoflavone supplements in food or pills is not recommended. In spite of earlier reports, soy proteins and isoflavones don't have any major health benefits, although products containing them are generally good foods. In other words, they can replace other foods that are high in cholesterol, or likely to increase cholesterol levels. This is because of their high content of polyunsaturated fats, fiber, vitamins, and minerals, and their low content of saturated fat.

Source

• Sacks FM, Lichtenstein A, Van Horn L, et al. Soy protein, isoflavones, and cardiovascular health. An American Heart Association Science Advisory Panel for Professionals from the Nutritional Committee. Circulation 2006; published online before print Jan

Footnotes
1. Soy protein products contain molecules called phytoestrogens or isoflavones; these cause dilatation of blood vessels, lowering of serum cholesterol, and inhibition of atherosclerosis in animal models. The major proteins found in soy are genistin, daidzin, and glycitin; they all have estrogenic and anti-estrogenic activity. The proportion of isoflavones in soy depends on how well the soy has been purified in preparation.

Related Links
Fish or Soy?
Soy and Heart Disease
FDA: Soy - Health Claims for Soy Protein