Soy Protein or Whey Protein: What's the Difference

Soy protein is the oldest and best-studied plant protein in sports nutrition. Rumours about "female hormones" still circulate around it, while whey is regarded as the indisputable benchmark. Our editorial team has gathered the facts about how these two proteins really differ - from molecular composition to the results of clinical studies.

Origin and forms of release

Soy protein is obtained from defatted soybean meal - the residue left after the oil has been extracted from the beans. The meal is purified of soluble carbohydrates, yielding a concentrate with about 70% protein, or a deeper extraction and precipitation is carried out to produce an isolate with over 90% protein.

Whey protein is produced from a by-product of cheesemaking by means of ultra- and microfiltration. The technology for whey and soy is fundamentally different: in the first case the protein is separated from lactose and fat, in the second from fibre, oligosaccharides and oil.

Both proteins are widely used in the food industry: soy in meat products, milk substitutes, bars; whey in dairy drinks, infant formulas, confectionery. This means that their safety and properties have been tested many times over.

Soy, like milk, is on the list of main food allergens in the EU regulation on informing consumers. So neither of the two proteins is "hypoallergenic" by definition - the choice depends on what exactly a particular person reacts to.

Protein quality: amino acids and scores

Unlike most plant proteins, soy contains all the essential amino acids in sufficient quantity. On the PDCAAS scale soy isolate receives a score close to the maximum, whereas on the more accurate DIAAS it falls slightly short of milk proteins because of the lower digestibility of sulphur-containing amino acids.

The leucine content in soy is lower than in whey, which has been recorded in analyses of commercial isolates (Gorissen et al., 2018). This is one of the key factors that explains the difference in the acute anabolic response.

Soy is rich in arginine and glutamine, and also contains more lysine than grain proteins. That is why it is often used as the basis of plant protein blends or mixed with whey and casein in "multi-component" products.

So, by formal classification soy is a complete protein, but in terms of leucine concentration and the digestibility of individual amino acids whey is stronger per gram of protein.

Metabolism: where the amino acids go

The classic study by Tang and colleagues (2009) compared whey hydrolysate, casein and soy isolate in identical doses of essential amino acids. After strength training, whey stimulated muscle protein synthesis more strongly than soy, and soy more strongly than casein.

Soy is absorbed fairly quickly, but a significant proportion of its amino acids is utilised in the organs of the abdominal cavity, in particular the liver, and is oxidised or converted into urea. As a result, relatively less "building material" reaches the muscles than in the case of milk proteins.

A similar picture was shown by studies with milk: Wilkinson and colleagues (2007) found that skimmed milk after a workout provides a greater accretion balance of muscle protein than an isoenergetic soy drink. Hartman and colleagues in the same year showed a greater increase in fat-free mass in the milk group over 12 weeks.

At the same time, these differences concern identical doses. With a larger serving of soy or with a high total protein intake, the advantage of whey narrows.

Isoflavones and hormones

The main difference most argued about is the soy isoflavones (genistein, daidzein). These are plant compounds capable of binding weakly to oestrogen receptors. Hence arose the fear that soy protein lowers testosterone or "feminises" men.

Clinical data do not confirm this. The meta-analysis by Hamilton-Reeves and colleagues (2010) found no effect of soy protein or isoflavones on the levels of testosterone, free testosterone, sex-hormone-binding globulin and oestrogens in men. The expanded analysis by Reed and colleagues (2021) reached the same conclusion.

Isolated clinical cases of gynaecomastia have been described with an extremely large consumption of soy products over a long period - in amounts unattainable with the ordinary use of a protein supplement.

Whey contains no isoflavones, but the concentrate contains bioactive protein fractions - lactoferrin, alpha-lactalbumin, immunoglobulins. Their role for athletes is being studied, but there is as yet no unambiguous evidence of a noticeable effect on the result.

• Soy protein in ordinary dietary amounts does not lower testosterone in men.
• Isoflavones are not hormones but plant compounds with a weak action.
• Soy isolates may contain different amounts of isoflavones depending on the purification technology.

Long-term training results

For practice, what matters more than acute protein-synthesis studies are the results of weeks and months of training. The meta-analysis by Messina and colleagues (2018) compared soy and animal protein supplements during strength training and found no significant difference in the increase of muscle mass and strength.

This result is consistent with the broader meta-analysis by Morton and colleagues (2018), which showed: once a daily protein intake of about 1.6 g/kg is reached, further increases in amount or a change of source give a minimal additional effect.

So, the "loss" of soy in acute studies is, in the long term, largely compensated for by the overall diet and the regularity of training.

It should be noted that most studies were conducted on young people. In older individuals, in whom muscles are less sensitive to protein, the difference between sources may be more noticeable, and here whey has a justified advantage.

Editorial conclusions

Soy and whey protein are both complete proteins, but whey contains more leucine and more effectively delivers amino acids to the muscles in acute studies.

Fears about the effect of soy on male hormones have not been confirmed by meta-analyses of clinical studies.

In long-term training programmes, with a sufficient daily protein intake, the difference between soy and animal proteins is minimal.

We also recommend our articles on isoflavones and hormonal balance, on DIAAS as a modern way of assessing protein quality and on plant proteins for vegans.

References

1. Tang JE, Moore DR, Kujbida GW, Tarnopolsky MA, Phillips SM. 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):987–992.
2. Wilkinson SB, Tarnopolsky MA, MacDonald MJ, 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):1031–1040.
3. Hartman JW, Tang JE, Wilkinson SB, 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):373–381.
4. Hamilton-Reeves JM, Vazquez G, Duval SJ, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94(3):997–1007.
5. Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–67.
6. Messina M, Lynch H, Dickinson JM, Reed KE. 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;28(6):674–685.
7. Gorissen SHM, Crombag JJR, Senden JMG, et al. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids. 2018;50(12):1685–1695.
8. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376–384.