A number of good protein sources exist. Historically, supplemental sources of soy protein have been of lesser quality, but improved isolation techniques have resulted in the development of some soy protein isolates that have exceptional quality. I guess the best answer depends upon what is the reason for taking the protein in the first place. If the reason is to support body composition and associated changes to your body while participating in a regular exercise program, most of the evidence suggests that milk protein sources may be more effective. For example, 17 fluid ounces (500 mL) of either skim milk or soy milk were ingested immediately after completing a single bout of resistance exercise and both forms of protein rapidly increased amino acid levels in the blood and promoted a positive net balance of protein. A greater balance of protein was promoted, however, after milk was ingested which resulted in significantly greater amounts of muscle protein synthesis . More so, when the two drinks were consumed after resistance training workouts over the course of 12 weeks, skim milk ingestion resulted in significantly greater increases in fat-free mass over the 12 week study when compared to the individuals who consumed soy protein .
Many of you may be asking what if whey protein and casein protein, the individual components that make up milk protein are compared directly against soy protein isolate. It’s important to first discuss why there might even be a difference between the two milk proteins, whey and casein. While both are proteins found in milk, casein makes up around 80% of the protein found in cow’s milk and whey makes up around 20% of it. Casein is not water soluble so when it enters the stomach, it clumps together. This results in casein being digested slowly and studies have shown it to not stimulate muscle protein synthesis to as great of an extent, but to be quite effective for minimizing muscle breakdown. In the same light, whey protein digests very quickly and rapidly increases muscle protein synthesis with little efficacy to prevent muscle protein breakdown [3-5].
This next study had young, healthy men ingest similar amounts of whey hydrolysate, casein or soy protein isolate at rest (with no influence of exercise) and immediately after a single bout of lower-body resistance exercise. At rest, muscle protein synthesis values were 93% greater than casein and 18% greater than soy. Interestingly, the same relationship was found after exercise as what was found at rest, however, there seemed to be a synergistic effect with the influence of the exercise bout. After exercise, whey protein ingestion resulted in muscle protein synthesis values that were 122% greater than casein and 31% greater than soy protein . These studies, overall, provide good scientific evidence that blends of the milk protein and whey protein, but not casein protein by itself, do a better job than soy protein of increasing muscle protein synthesis. In fact, when soy protein and casein protein were compared head-to-head, ingestion of soy protein increased muscle protein synthesis to a greater extent at both rest (by 64%) and after exercise (by 69%) .
From another perspective, if your interests are more health oriented, the recommendation may change somewhat. A recent study had overweight and obese individuals consume on a daily basis for 6 months either whey protein or soy protein and monitored weight loss and body composition changes. No exercise program was part of this study, just an assessment of how feeding with the two different proteins stimulated changes. No differences were found between whey and soy protein for weight loss and body composition suggesting that when no exercise program is included as part of the study, both forms of protein may operate similarly . Another study had college-aged men consume either whey or soy protein bars on a daily basis for 9 weeks while resistance training. In this study, lean mass gains were similar in the whey and soy protein groups while also successfully maintaining some beneficial health markers in the blood . In addition, soy may confer a number of other important health benefits. For starters, soy is a low fat, low cholesterol protein source, which has been suggested to lower serum cholesterol levels [9-12]. Moreover, soybeans are one of the best dietary sources of isoflavone phytoestrogens, which may help support healthy levels of the female sex hormone, estrogen. As women age, decreases in estrogen have been associated with increases in the risk for heart disease, obesity, loss of bone mass, and breast cancer. For these reasons, adding soy protein may confer favorable health benefits not seen with other sources of protein.
In the end, soy protein and other common forms of protein such as whey, casein, egg, etc. are to be considered high quality protein sources. For women wishing to increase their protein intake, any of these sources have high quality protein in them, but if making improvements in body composition as part of a regular exercise program is a major interest, the majority of the available research supports whey protein and blends of the milk proteins. If, however, your goals are simply to add protein to your diet and overall health benefits, soy protein does offer some additional help for women that otherwise may not be achieved with other sources of protein. My conscience, however, won’t let me write this article without saying that protein sources like whey also confer meaningful health benefits such as antioxidant protection, immune support and added growth factors and it’s not intended for the reader to gain the perspective that only soy has health-related benefits because that’s simply not true. Another important point to bring up is to any male readers and ingestion of soy protein or soy milk. While non-scientific, journalistic reports exist of men drinking or consuming soy and developing high estrogen and low testosterone levels, it’s the regular habit of drinking pretty good quantities of the stuff that you need to be aware of. So if you’re lucky enough to find yourself one morning staring into your new girlfriend’s refrigerator and all you see in soy milk, don’t be afraid, it won’t feminize you in instantaneous rates. Go ahead and drink some, even have it with your cereal, it’s actually a solid source of protein.
- Wilkinson, S.B., et al., Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am J Clin Nutr, 2007. 85(4): p. 1031-40.
- Hartman, J.W., et al., Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters. Am J Clin Nutr, 2007. 86(2): p. 373-81.
- Boirie, Y., et al., Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A, 1997. 94(26): p. 14930-5.
- Dangin, M., et al., The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am J Physiol Endocrinol Metab, 2001. 280(2): p. E340-8.
- Dangin, M., et al., Influence of the protein digestion rate on protein turnover in young and elderly subjects. J Nutr, 2002. 132(10): p. 3228S-33S.
- Tang, J.E., et al., Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men. J Appl Physiol, 2009. 107(3): p. 987-92.
- Baer, D.J., et al., Whey protein but not soy protein supplementation alters body weight and composition in free-living overweight and obese adults. The Journal of nutrition, 2011. 141(8): p. 1489-94.
- Brown, E.C., et al., Soy versus whey protein bars: effects on exercise training impact on lean body mass and antioxidant status. Nutr J, 2004. 3: p. 22.
- Dewell, A., C.B. Hollenbeck, and B. Bruce, The effects of soy-derived phytoestrogens on serum lipids and lipoproteins in moderately hypercholesterolemic postmenopausal women. J Clin Endocrinol Metab, 2002. 87(11788633): p. 118-121.
- Jenkins, D.J., et al., Effect of soy-based breakfast cereal on blood lipids and oxidized low-density lipoprotein. Metabolism, 2000. 49(11092518): p. 1496-1500.
- Potter, S.M., Overview of proposed mechanisms for the hypocholesterolemic effect of soy. J Nutr, 1995. 125(7884541): p. 611.
- Takatsuka, N., et al., Hypocholesterolemic effect of soymilk supplementation with usual diet in premenopausal normolipidemic Japanese women. Prev Med, 2000. 31(11006055): p. 308-314.