Milk Protein versus Soy Protein: Which is Better for You?

Protein consists of amino acids, the building blocks of protein. There are 20 amino acids that are chained and sequenced differently to create different proteins. Each protein has a specific function in the body. Nine of the 20 amino acids are essential amino acids that are not synthesized in the body. They must come from the diet.The essential acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.Foods containing all the essential amino acids are complete proteins, eg., milk, meat, eggs, soy and quinoa.

Digestion and absorption of dietary protein puts amino acids into the bloodstream to be used by tissues to synthesize body proteins. This is called protein synthesis. In a healthy adult of stable weight, protein synthesis is balanced by protein breakdown. The amino acids released by protein breakdown end up in the bloodstream and can be re-used for protein synthesis. Protein turnover is the balance between protein synthesis and breakdown. Our body constantly break down body proteins into amino acids, and re-use the amino acids to make body proteins again.

Proteins in the body has different turnover rates. Some break down within minutes after newly synthesized, others stable for months. The rate depends on its function. Structural proteins are stable, e.g., collagen , while many enzymes for energy metabolism are unstable.

Whey Protein is a group of proteins found in milk. These include:Beta-lactoglobulin, Alpha-lactalbumin, Glycomacropeptide (GMP), Bovine serum albumin, Immunoglobulins, Lactoferrin, Lactoperoxidase, Lysozyme.

Beta-lactoglobulin is the most abundant whey protein. It makes up 50 – 55% of the whey proteins and provides a good source of the essential and branched chain amino acids (BCAAs). BCAAs help prevent muscle breakdown and spare glycogen during exercise.

Gylcogen sparing is the use of non-carbohydrates, e.g., fats, as a source of energy during exercise so that the depletion of muscle glycogen stores is delayed. BCAAs, particularly leucine, help stimulate muscle protein synthesis (MPS) to build lean muscle after resistance exercise.

Alpha-lactalbumin, the second most abundant whey protein component in cow's milk, makes up 20 – 25% of the whey protein. It is high in the essential amino acid, tryptophan, that aids in increased serotonin production, sleep regulation, and mood improvement under stress.

Immunoglobulins, which makes up 10-15% of the whey proteins, help the immune systems to fight certain pathogens.Bovine Serum Albumin is a large sized protein with a good essential amino acid profile. Lactoferrin is an iron binding globular glycoprotein. It makes up approximately 1 – 2% of the whey proteins, and inhibits bacteria and fungi growth. Lactoperoxidase is a glycoprotein, a natural antibactetial agent, makes up 0.5% of the whey protein.Lysozyme is an enzyme makes up less than 0.1% of the whey protein. It enhances the immune system.

Soy protein is the protein that is found in soybean. Soybean contains about 40% protein. It is a cholesterol-free legume low in saturated fat. Soy protein is rich in BCCA's lysine and arginine but is low in methionine and cysteine. It contains all nine essential amino acids. Isoflavones are a class of phytoestrogens — plant-derived compounds with estrogenic activity. Soybeans and soy products are the richest sources of isoflavones in the human diet.

Intake of soy protein reduces serum concentrations of total cholesterol, low-density lipoproteins (LDLs), and trigylcerides (Arliss & Biermann, 2002). The U.S. Food and Drug Administration (FDA) approved the health claim that links soy product consumption and reduced risk of coronary heart disease in 1999, based on the result of human clinical intervention trials (Hasler, 2002).

A recent meta-analysis (Messina et al., 2018) of nine randomised controlled trials compared the longer-term effects of soy protein versus animal proteins on muscle mass and strength in combination with resistance exercise.

All the studies found significant increases in muscle strength and muscle mass irrespective of protein source after resistance training of at least 6 weeks of intervention. There were no difference in muscle strength between individuals supplemented with soy protein or with animal (whey, beef, milk or dairy) protein. This suggests that protein intake together with resistance training is the factor to build muscle strength. Also, soy protein supplementation produces similar gains in strength and muscle mass compared to animal protein

A systematic review (and meta-analysis of studies involves the high leucine-containing whey protein) by Anderson et al.(2004)found a significant increase in lean body mass among studies that included both resistance exercise and whey protein(WP) intake. They found that WP as a supplement combined with resistance exercise improve body composition parameters. When compared to soy protein, whey protein did not show a greater effect(Reidy, 2016)

Volek et al.(2013), in a 9 month training study, found whey protein supplement significantly enhance gains in lean body mass over a soy protein-supplemented group by about 83 %. This shows protein quality determines exercise-induced muscle mass gains. Since leucine triggers muscle protein synthesis(MPS), findings of a greater muscle mass gain in a whey supplemented group implies that leucine stimulates MPS to promote hypertrophy. However, the soy supplemented group had a muscle mass gain that was similar to the carbohydrate group, ie., soy was no better than energy in the form of carbohydrate. Hartman et al.(2007) compared bovine skimmed milk to a soy protein-containing beverage. They found that the milk drinking group gained more muscle mass compa