For Weight Loss, Soy No More

Summarized by Robert W. Griffith, MD
April 4, 2007

Summary

In a 12-week study, women who ate roughly 18 grams of soy-protein daily did not have greater weight loss or an improved cardiovascular risk profile compared to women eating a weight-loss diet without soy.

Introduction

While soy protein has been shown to have some cardioprotective benefits, its possible role in weight loss management is more controversial. In cardioprotective studies, a daily 'dose' of 25 grams of soy protein is used, as part of a low-saturated fat, low-cholesterol diet; the result is a lowering of total and low-density lipoprotein (LDL) cholesterol, and triglycerides. Some of these studies showed a degree of weight loss in the soy-protein group of subjects, but it is unclear if this was related to the soy-protein itself, or the accompanying dietary regimes used. To clarify this, researchers at the New York Obesity Research Center have conducted a more definitive study, and reported its findings in the Journal of the American Dietetic Association. Here's a summary of the publication.

What was done

Overweight women aged between 25 and 49 were recruited in New York through newspaper ads. Their body mass index (BMI) was between 28 and 33 kg/mm²; they had to be free of coronary artery disease, high blood pressure (i.e. below 140/90 mmHg) type 2 diabetes, or on any cholesterol-lowering or weight-control drugs.

The participants were randomly assigned to one of two groups: one with dietary counseling plus soy-protein-rich foods, and one with dietary counseling alone. At their first visit all of them had their resting metabolic rate determined so that their target calorie intake could be calculated (set at 500 calories/day less than their baseline calculated intake). This calorie target was to be maintained for 12 weeks. The purpose was to achieve a weight loss of approximately one pound a week.

Food selection and portion sizes were to be based on the Food Guide Pyramid. Those in the soy-protein-rich diet were given appropriate foods - soy burgers and hot dogs - in sufficient quantities to provide 15 grams soy protein/1,000 calories of their proposed calorie intake. Dietary counseling was given to all participants at baseline and in weeks 2, 4, 6, and 8.

Body weight, waist circumference, percent body fat, fat-free mass, blood lipids, fasting glucose, and serum insulin levels were measured at intervals throughout the study.

What was found

The women in the study averaged 38 years of age, with an average body weight of 180 pounds. Of the 75 who enrolled, 47 completed the study (soy-protein-rich diet 24, control diet 23). Reasons for the drop-outs included loss to follow-up, lack of interest, and work related problems. The average soy-protein consumed was 21 grams a day; the initial levels were higher, but trailed off to average 16.5 grams daily at the end of the study.

Both the soy-protein-rich diet and diet-alone groups lost a similar amount of weight: minus 3.2% and minus 4.0%, respectively. There was also no significant difference between the groups in percentage fat mass (minus 5.3% and minus 3.9%, respectively), or waist circumference (minus 1.7 cm and minus 1.3 cm, respectively). Moreover there were no significant differences between the groups in percentage changes in blood glucose, serum insulin, and LDL-cholesterol levels over the 12-week period, although there was a trend for greater decreases in these measurements in the soy-protein-rich diet group.

What the results mean

This study didn't come out quite as the investigators had intended. First, the participants only lost roughly ½ pound of weight weekly, not the targeted one pound weekly. Second, the women found it difficult to consume the targeted amount of soy-protein (25 grams daily). In fact, they only managed an average of 21 grams, which had fallen to 16.5 grams daily by the end of the study. Third, the groups were not equal with respect to their baseline blood chemistry - the soy-protein group had slightly higher glucose, lipids, and insulin levels that fell more during the study. The percentage difference in reductions between the groups was not statistically significant, i.e. it could have occurred by chance.

In spite of these drawbacks, the study provided useful information. Women eating roughly 18 grams of soy-protein daily for 12 weeks did not have greater weight loss or an improved cardiovascular risk profile compared with women eating a weight-loss diet without soy. There seems, therefore, little reason to eat soy-protein-rich 'functional foods'.

Source

• Supplementation with soy-protein-rich foods does not enhance weight loss. M-P. St-Onge , N. Claps, C. Wolper, SB. Heymsfield, J Am Diet Assoc , 2007, vol. 107, pp. 500--505

Related Links
Mayo Clinic: Soy
Wikipedia: Functional Foods
AHA 2006 Panel: Soy Protein et al.