For decades, the merits of consuming plant vs animal protein have been hotly debated. From an historical perspective, until fairly recently, plant protein often came up short. However, slowly over the years, mistaken beliefs about the inferiority of plant protein have been corrected. Newly published research from the University of Colorado can be credited with further highlighting that plant protein, and in particular soy protein, is not inferior to animal protein.1
The research conducted by James O. Hill and colleagues, compared the effects on weight loss of two diets, one with and the other without soy protein. Seventy-one mostly female obese adults (average BMI, 32.9) were randomly assigned to consume three servings of soy or non-soy protein foods per day for 12 months. All participants completed a group-based behavioral weight loss program lasting 4 months and follow-up assessments were completed at month 12.
Participants assigned to the soy-containing diet were provided with three pre-made, soy protein-rich food products to consume each day as part of the recommended energy restricted, high protein diet. The soy products consisted of a packet of protein powder to be made into a beverage by adding water, a protein bar, and a frozen protein patty (similar to a sausage patty). Each product provided about 20 g protein. Participants assigned to the diet lacking soy were asked to meet their recommended daily protein needs by incorporating three servings of non-soy protein products each day, as part of the recommended energy restricted high protein diet advised during the study period.
Individuals in the soy group consumed on average 2.4 servings per day of soy during weight loss (first four months) and 2.5 servings per day during the maintenance phase (months 5-12). Body weight was similarly reduced in both groups at four months (approximately 7%) and at 12 months (approximately 4%). There were no statistically significant differences between groups in the amount of weight loss or in body fat loss. Also, both groups experienced similar improvements in cholesterol, triglycerides and blood pressure.
That weight and fat loss were similar in the two diet groups is not surprising. Soy protein is similar in quality to animal protein.2 In fact, a recent meta-analysis found soy protein supplementation leads to similar gains in strength and lean tissue as whey protein supplementation among individuals undergoing resistance exercise training.3 Even more relevant, a now decade-old review by Cope and colleagues concluded: “Soyfoods are as good as other protein sources for promoting weight loss and there is a suggestive body of evidence that soyfoods may confer additional benefits”.4
One might ponder why LDL-cholesterol did not differ between the groups given that those in the soy group consumed about 50 g of soy protein per day. Both groups experienced weight loss-induced decreases in LDL-cholesterol. Perhaps weight loss masked the hypocholesterolemic effect of soy protein. Actually, at study end, only the LDL-cholesterol decrease (14.3 mg/dl) in the soy group was statistically significant when compared to baseline. Furthermore, although not statistically significant, the soy-diet decreased LDL-cholesterol by almost 5% more than the non-soy diet, which is consistent with the results of previously published meta-analyses.5,6
All in all, the work by Hill and colleagues can be added to the existing database supporting the role that soy protein can have in promoting overall health.
- Speaker, K.J., Sayer, R.D., Peters, J.C., Foley, H.N., Pan, Z., Wyatt, H.R., Flock, M.R., Mukherjea, R., and Hill, J.O. Effects of consuming a high-protein diet with or without soy protein during weight loss and maintenance: a non-inferiority, randomized clinical efficacy trial. Obesity Science Practice. 2018,
- Hughes, G.J., Ryan, D.J., Mukherjea, R., and Schasteen, C.S. Protein digestibility-corrected amino acid scores (PDCAAS) for soy protein isolates and concentrate: Criteria for evaluation. J Agric Food Chem. 2011, 59, 12707-12.
- Messina, M., Lynch, H., Dickinson, J.M., and Reed, K.E. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. Int J Sport Nutr Exerc Metab. 2018, 1-36.
- Cope, M.B., Erdman, J.W., Jr., and Allison, D.B. The potential role of soyfoods in weight and adiposity reduction: an evidence-based review. Obes Rev. 2008, 9, 219-35.
- Jenkins, D.J., Mirrahimi, A., Srichaikul, K., Berryman, C.E., Wang, L., Carleton, A., Abdulnour, S., Sievenpiper, J.L., Kendall, C.W., and Kris-Etherton, P.M. Soy protein reduces serum cholesterol by both intrinsic and food displacement mechanisms. J Nutr. 2010, 140, 2302S-2311S.
- Zhan, S. and Ho, S.C. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr. 2005, 81, 397-408.