Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids

Author: Anderson JW, Johnstone BM, Cook-Newell ME
Publication: New England Journal of Medicine. 1995 Aug 3;333(5):276-82

The consumption of vegetable protein in place of animal protein seems to be reducing the risk of coronary heart disease and seems to lower cholesterol levels. The cholesterol lowering effect of the replacement of animal protein with vegetable protein has long been recognized with animals. In recent years, there have been many studies about the influence of soy protein and serum lipids, but the results have not always been consistent. This is because investigators used different soy products as a source of soy protein and different protocols. The aim of this meta-analysis was to investigate 38 controlled studies in order to make an overall conclusion about this influence of soy protein. Only those studies were selected which used soy proteins (and not whole soybeans), had a control group and had baseline values for serum lipids. Twenty studies used isolated soy protein, 15 used textured soy protein and 3 used a combination of isolated soy protein and textured soy protein. The daily soy protein intake ranged from 17 gram to 124 gram, with and average of 47 gram. The consumption of 30 gram soy protein can easily be obtained from two or three servings of soy products.

The low-density lipoprotein (LDL) cholesterol (or good cholesterol) decreased on average by 21.7 mg per deciliter. The higher the LDL cholesterol concentration was at the start of the study the higher the decrease in LDL cholesterol. Twenty-six studies showed a decrease, four studies showed an increase and one study showed to change in LDL cholesterol. The four studies that showed an increase in LDL cholesterol used subjects with fairly low initial cholesterol levels. The HDL cholesterol levels were not significantly influenced by soy protein intake. Serum triglyceride levels were reduced on average by 13.3 mg per deciliter.
The type of soy protein had no significant influence on the change of cholesterol concentrations. The mechanism responsible for the cholesterol lowering property of soy protein is still unknown. One theory is that soy protein contains a high amount of the amino acid arginine, which affects serum cholesterol concentrations. Another theory is that soy isoflavones, which are present in soy protein, act as weak estrogens. It is know that oral estrogens decrease serum cholesterol and LDL cholesterol.

The investigators concluded that the replacement of animal protein with soy protein significantly decreases total cholesterol, LDL cholesterol and triglycerides. The concentration of HDL cholesterol remained unchanged. The decrease in cholesterol was strongly related to the initial serum cholesterol concentration. Soy isoflavones may be responsible for the cholesterol lowering effect of soy protein.