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Evidence-based eating


Replacing animal protein with vegetable protein is one of those suggestions which is often encouraged, in part because eating less meat has been suggested to be reduce coronary heart disease risk. A meta-analysis examined the effects of soy protein intake on serum lipids [ 1 ], so that we can judge for ourselves what the benefits of dietary change may be for us.

Studies


Studies were selected for analysis if they had used isolated or textured soy protein, if they were controlled and had either a crossover design and if they provided baseline values so that the changes in each study group could be calculated. Excluded studies were those without a control group, those which used several sources of vegetable protein, and those which used whole soybeans rather than soy protein.

Twenty-nine articles had information from 38 clinical studies; 34 were in adults and four in children. Most studies used random assignment. Twenty used isolated soy protein, 15 textured soy protein and three used a combination of the two. All except one were designed to maintain weight and diets were predominantly those similar to Western diets in fat and cholesterol content. Soy protein intake averaged 47 g/day (range 17 to 124 g/day).

Results


There were significant reductions in total cholesterol, LDL cholesterol and serum triglycerides (Table).
Mean change in serum lipids from baseline with soy diets
Variable Number of studies Number of subjects Change mmol/L (95%CI) Percent change
         
Total cholesterol 38 730 -0.60 (-0.35 to -0.85) -9.3
LDL cholesterol 31 564 -0.56 (-0.30 to -0.82) -12.9
HDL cholesterol 30 551 +0.03 (-0.08 to +0.14) +2.4
VLDL cholesterol 20 255 -0.01 (-0.12 to +0.10) -2.6
Triglycerides 30 628 -0.15 (-0.003 to -0.29) -10.5
The most important variable that predicted the extent of the reduction in serum cholesterol was the initial cholesterol level. The higher it was, the bigger the fall, as the depiction of results by quartiles shows (Figure).
Significant falls occurred in the highest two quartiles of cholesterol, where the initial value was greater than 6.7 mmol/L.

Comment


The amounts of soy protein in single servings of various soy products is given below.
Soy protein equivalents in various soy products
Soy product Amount Soy protein (g)
     
Soy milk 225 mL 4 to 10
Tofu 113 g 8 to 13
Soy flour 28 g 10 to 13
Isolated soy protein 28 g 23
Textured soy protein 113 g 11
Soy meat analogue 91 g 18
It is clear, though, that benefits to health from reductions in serum lipids are most likely to accrue in those with high initial levels.

Reference:

  1. JW Anderson, BM Johnstone, ME Cook-Newell. Meta-analysis of the effects of soy protein intake on serum lipids. New England Journal of Medicine 1995 333: 276-82.

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