The essential amino acids cannot be synthesised endogenously and so dietary sources are relied upon to provide them in the correct amounts to allow the synthesis of proteins. The usefulness of a dietary protein for human needs is therefore judged by its essential amino acid content. Animal proteins are considered complete proteins because they supply the essential amino acids in the correct ratios for protein synthesis in humans. However, vegetables proteins do not and are therefore considered incomplete. Soy is often touted as being a complete protein for human needs, but this is not true. As with all leguminous proteins, soy is low in methionine and cysteine, two sulphurous essential amino acids. Soy is not able to sustain adequate growth of humans in isolation and an exogenous source of methionine and cysteine must therefore be consumed. A good test of a protein’s ability to sustain growth is that of muscle accretion, and in this regard soy has been compared with animal proteins particularly casein and whey.
Comparisons of the benefits of milk versus soy protein has been undertaken. For example in one study1, novice male resistance trainers were placed on a resistance training programme and asked to consume either animal or vegetable protein following their workouts. The training consisted of a 5-day per week routine of basic bodybuilding exercises and lasted for 12 weeks. These routines were chaperoned by research assistants. Following the workout the subjects were asked to consume either 500 mL milk containing 17.5 grams of protein and 25.7 grams of carbohydrate, 500 mL soy protein which contained the same calories and same protein as the milk drink, or a control containing maltodextrin that supplied equal amounts of energy, but all as carbohydrate. The soy protein was deliberately provided devoid of isoflavones, which have been shown previously to produce an oestrogenic effect in human tissue, and so to prevent hormonal disruption isoflavones were not included with the soy.
The results of the study show that the milk protein group experienced a significant increase in the type II muscle fiber are compared to the soy and the maltodextrin group. For type I fibre area, the milk and the soy groups experienced significantly greater increases compared to the maltodextrin group. Lean mass increases and fat loss were both significantly greater in the milk group compared to the soy and the maltodextrin group. Taken as a whole, these results suggest that milk is superior to soy or an equivalent energy intake of maltodextrin for building muscle following resistance training. This is not unexpected based on the fact that milk contains all of the amino acids in the correct ratios for protein synthesis, whereas soy does not. This highlights the deficiency of pulses and legumes to supply adequate methionine and cysteine for growth. Despite contentions to the contrary, animal proteins remain the protein of choice for effective muscle growth due to their complete amino acid profiles.
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