Soy foods lower blood cholesterol levels

Although the cholesterol lowering ability of soy protein was first discovered in 1940 in rabbits ² ^, ³ hundreds of positive studies in humans have been published over the last 15-20 years.

In 1995 Dr. James Anderson and colleagues from the University of Kentucky, USA, conducted the first meta-analysis examining the relationship between soy protein and blood cholesterol levels4. They pooled together the results from 38 previously published controlled clinical trials involving some 730 subjects and found that substituting animal protein with soy protein in the diet resulted in a reduction of total cholesterol by 9.3% and LDL ('bad') cholesterol by 12.9%. The degree of decrease in the blood cholesterol level was proportional to the initial reading such that the higher the initial cholesterol level, the more that soy was able to reduce it. More recent studies however, suggest that the effectiveness of soy foods may not depend on a high starting cholesterol level ⁵ ^, ⁶ .

Recent findings on soy and cholesterol lowering

Several meta-analyses have been conducted since 1995, considering an assortment of the most recent clinical trials.

A 2003 meta-analysis published in the European Journal of Clinical Nutrition examined 10 well controlled studies where soy protein (containing isoflavones) was substituted for dairy or animal protein. A significant 4% reduction in LDL cholesterol and a 3% increase in HDL cholesterol was observed when soy protein was consumed ⁵ .

A second meta-analysis published in the American Journal of Clinical Nutrition in 2005 reviewed 23 studies and found that the consumption of soy protein with its naturally occurring level of isoflavones was associated with a significant decrease in total cholesterol (3.77%), LDL cholesterol (5.25%) and triglycerides (7.27%) as well as an increase in HDL cholesterol (3.03%)6. Similar results were found in a meta-analysis by researchers at the National Institute of Health and Nutrition in Tokyo, Japan (2007). They showed the consumption of soy protein rich in isoflavones significantly decreased LDL cholesterol by 4.98% and increased HDL cholesterol by 3.0% ⁷ .

In early 2007, Professor Cesare Sirtori - a pioneer in the field of soy and cholesterol - re-examined the studies on soy and its effect on blood cholesterol, published since the original meta-analysis by Dr Anderson, using a similar method. He found that soy protein significantly lowers total cholesterol (by 6.7%) and LDL cholesterol (by 6.4%) in individuals who have high cholesterol to start off with. A small but insignificant cholesterol reduction was also seen in individuals with normal/mild cholesterol levels. This review confirmed the validity of Dr. Anderson's original findings ⁸ .

A review by Dr. Aedin Cassidy and colleagues from the UK similarly concluded that soy protein containing isoflavones lowers LDL cholesterol by approximately 5.0% with the effects on triglycerides and HDL cholesterol being unclear ⁹ .

Based on these more recent studies, it is now generally believed that the initial findings for cholesterol reduction by soy may have been an overestimate and that soy foods probably lower LDL ('bad') cholesterol levels by around 5%. While this effect is less than that claimed by Anderson in 1995, the impact of soy on blood cholesterol levels is still considered significant for individuals with elevated cholesterol levels, and for populations overall in reducing the burden of heart disease. In addition, soy foods may protect the heart in ways other than just lowering cholesterol levels - see "More ways that soy protects the heart" section below.

Soy, heart attacks and stroke

While many studies have investigated the effect of soy on risk factors such as elevated cholesterol, few studies have looked directly at whether soy foods can reduce the risk of having a heart attack or stroke. A recent study called the Japan Public Health Centre-based Study Cohort 1 is one of the first to consider the association of soy foods in the diet and the risk of heart attack and stroke over a long time period. The study included over 40 000 men and women from the Japanese general population who were followed for more than 12 years. The key finding from this study was that women who consumed soy foods more than five times per week were 70% less likely to suffer a heart attack or stroke compared to those who had a low soy intake. Post-menopausal women with a high intake of soy were the least likely to suffer a heart attack or stroke. However, there were no indications that regular consumption of soy had the same effect in men ¹⁰ .