Soy Protein and Weight Loss

Doctor’s Advice
Ask the Expert…
Bob Blair, Ph.D. & Aaron Tabor, M.D.


Recent clinical trials have begun to describe the weight loss benefits of soy protein in obese individuals. In a randomized, controlled, clinical study (Deibert et al., 2004), 90 subjects with a body mass index between 27.5 and 35 were treated for 6 months. These subjects were randomly assigned to one of three treatment groups: (1) a lifestyle education group, (2) a high soy protein, low fat substitutional diet with a guided physical activity program group, or (3) a high soy protein, low fat substitutional diet without a guided physical activity program group. Subjects in the two groups receiving the high soy protein substitutional diets lost more weight (P = 0.048) and had a significant reduction (P = 0.053) in fat mass compared to subjects receiving lifestyle education only. The two soy protein diets also appeared to improve glycemic control even though one of the soy diets did not include guided exercise.

Two clinical trials (Allison et al., 2003; Fontaine et al., 2003) utilizing a soy-based meal-replacement plan demonstrated that the use of soy supplements as a meal replacement aided in weight loss. In a 12-week, prospective, randomized controlled clinical trial (Allison et al., 2003), 100 obese (28 < BMI = 41 kg/m2) volunteers (35 – 65 years of age) were randomly assigned to either a soy-based meal replacement group or a control group. Treatment with the soy-based meal replacement resulted in significant reductions (P < 0.01) in weight, fat mass, and waist circumference. In addition to changes in body mass and weight loss, the treatment group also showed improvements in plasma cholesterol levels. Both total cholesterol (P < 0.02) and LDL cholesterol (P < 0.01) were reduced in the treatment group compared to the control group and these changes in cholesterol remained significant even after accounting for changes associated with weight loss, suggesting an additional benefit of the soy treatment beyond weight loss. Another study (Fontaine et al., 2003) reported similar benefits of a soy-based meal replacement weight loss program and confirmed that the greatest benefits were observed in those subjects with the highest baseline measures.

In a cross-sectional study (Goodman-Gruen and Kritz-Silverstein, 2003) of 208 healthy, postmenopausal women (45-74 years of age) a standardized questionnaire was administered in order to assess dietary isoflavone intake, which was then examined for associations to total body fat, lean body mass and anthropometric measures. The results demonstrated that genistein and total isoflavone intake were inversely associated with weight (P for trend = 0.04), total body fat mass (P for trend = 0.02), and waist circumference (P for trend = 0.04). Genistein was significantly inversely associated with body mass index. The waist circumference findings of this study imply that central adiposity is affected by soy protein and or the soy isoflavones. If so, a decreased risk for heart disease, diabetes and gall bladder disease would ensue.

Fleming (2004) conducted a 4-week, randomized clinical trial evaluating the effects of Revival Soy Chips on weight loss in sixty female subjects. Body weight was recorded at the initial visit and weeks 2 and 4 of the study. Subjects were instructed to substitute all in-between meal snacks with soy chips and to avoid unhealthy “junk” food snacks. Participants consumed 1 or more servings per day to control feelings of hunger. All participants completed the study. Mean weight loss in this subject population was 7.3 lbs over the 4-week period with a median weight loss of 7.0 lbs. Of the 60 participants, 59 lost weight and one gained weight. The change in body weight ranged from 19 lbs lost to 6 lbs gained.


While the exact mechanisms by which soy enhances weight loss and reduces obesity is not clear, animal and cell culture studies suggest a number of potential mechanisms. Naaz et al. (2003) demonstrated that the soy isoflavone genistein reduces adipose deposition in mice, potentially via the reduced expression of lipoprotein lipase mRNA. It has also been reported that genistein affects lipid metabolism and adipocyte proliferation (Nogowski et al., 1998; Harmon and Harp, 2001). The effects of soy do not appear to be solely due to the isoflavones as it has been recently demonstrated that the ?-conglycinin peptone of soy suppresses food intake and gastric emptying in rats by increasing cholecystokinin production (Nishi et al., 2003).

With increasing evidence that the biological effects of soy protein and its isoflavones may aid in weight loss, retention of lean body mass, and reduction of central adipose tissue, in addition to their effects on plasma cholesterol levels and glycemic control, soy protein may also prove beneficial as part of the dietary regimens for post-bariatric surgery patients. The protein needs of bariatric surgery patients can adequately be met with dietary soy supplements (bars, shakes, etc.) and these supplements would provide the added benefits of improving plasma lipid profiles (Anderson et al., 1995) and improving insulin resistance (Jayagopal et al., 2002).


  1. Deibert P, Konig D, Schmidt-Trucksaess A, Zaenker KS, Frey I, Landmann U, Berg A. Weight loss without losing muscle mass in pre-obese and obese subjects induced by a high-soy-protein diet. International Journal of Obesity 2004; 28:1349-1352 (doi: 10.1038/sj.ijo.0802765 Published online 10 August 2004)
  2. Allison DB, Gadbury G, Schwartz LG, Murugesan R, Kraker JL, Heshka S, Fontaine KR, Heymsfield SB. A novel soy-based meal replacement formula for weight loss among obese individuals: a randomized controlled clinical trial. European Journal of Clinical Nutrition 2003; 57:514-522
  3. Fontaine KR, Yang D, Gadbury GL, Heshka S, Schwartz LG, Murugesan R, Kraker JL, Heo M, Heymsfield SB, Allison DB. Results of a soy-based meal replacement formula on weight, anthropometry, serum lipids & blood pressure during a 40-week clinical weight loss trial. BMC Nutrition Journal 2003; 2:14-20.
  4. Goodman-Gruen D, Kritz-Silverstein D. Usual dietary isoflavone intake and body composition in postmenopausal women. Menopause 2003; 10:427-432.
  5. Naaz A, Yellayi S, Zakroczymski MA, Bunick D, Doerge DR, Lubahn DB, Helferich WG, Cooke PS. The soy isoflavone genistein decreases adipose deposition in mice. Endocrinology 2003; 144:3315-3320
  6. Nishi T, Hara H, Tomita F. Soybean b-conglycinin peptone suppresses food intake and gastric emptying by increasing plasma cholecystokinin levels in rats. Journal of Nutrition 2003; 133:352-357
  7. Nogowski L, Mackowiak P, Kandulska K, Szkudelski T, Nowak KW. Genistein-induced changes in lipid metabolism of ovariectomized rats. Ann Nutr Metab 1998; 42:360-366
  8. Harmon AW, Harp JB. Differential effects of flavonoids on 3T3-L1 adipogenesis and lipolysis. Am J Physiol Cell Physiol 2001; 280:C807-C813
  9. Anderson, J, Johnstone, B, Cook-Newell, M, Meta-Analysis of the effects of soy protein intake on serum lipids, The New England Journal of Medicine, Aug 1995, 276-282
  10. Jayagopal, V, Albertazzi P, Kilpatrick ES, Howarth EM, Jennings PE, Hepburn DA, Atkin SL. Beneficial effects of soy phytoestrogen intake in postmenopausal women with Type 2 diabetes. Diabetes Care 2002; 25:1709-1714

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