The effects of soy vs. animal protein supplementation on muscle mass and strength

Skeletal muscle strength and mass play key roles in athletic performance, metabolism, and quality of life. Low levels of muscle mass and strength have been associated with an increased risk of falls^[1] in the elderly, lower quality of life^[2] and ability to perform activities of daily living, and the development of a number of chronic diseases^[3], such as obesity, type 2 diabetes, and cardiovascular disease. It is therefore no surprise that developing strategies for facilitating increases in muscle mass and strength are of great interest among athletes and the general aging population.

Muscle mass is regulated via changes in muscle protein synthesis (MPS) and muscle protein breakdown (MPB). Muscle protein net balance^[4] (MPS minus MPB) determines whether muscle mass will increase (net positive balance), decrease (net negative balance), or remain constant. Several factors can affect net protein balance, with the two main ones being resistance training and protein intake. More specifically, resistance training^[5] increases both MPS and MPB, while protein-containing meals^[6] ingested in the hours following a training session further increase MPS and decrease MPB, leading to a net positive balance between the two. When repeated over time, the resultant net positive protein balance is thought to^[7] increase muscle mass and contribute to the development of muscle strength.

In addition to total protein intake, research has also focused on the effects of different types of protein, mainly plant compared to animal proteins, on the development of muscle mass and strength. There are several reasons for the heightened research interest in plant proteins, such as the potential health benefits^[8] of plant sources of protein, the growing consumer market interest^[9] in plant-based foods, and some evidence that plant-based foods are advantageous over animal-based foods in terms of a global sustainability^[10] standpoint.

Despite the growing interest in plant-based proteins, several lines of evidence^[11] suggest that they are less anabolic than animal-based proteins, which could reduce their ability to promote equivalent changes in muscle mass and strength. For example, plant-derived proteins tend to have a lower essential amino acid (EAA) content, as you can see in Figure 1. Since only the EAAs^[12] stimulate MPS, with leucine playing a predominant role in regulating anabolic pathways^[13] and MPS^[14], this means that plant-based proteins have less anabolic potential.

Of the plant proteins, soy protein^[15] is regarded as one of the highest-quality^[16] options. Therefore, the study under review aimed to determine whether supplementation with soy- or animal-derived proteins differentially affects resistance training-induced changes in muscle mass and strength.

Skeletal muscle mass and strength play an important role in athletic performance and general health. There is a growing interest in plant-based proteins, but some evidence suggests that they may be less anabolic than their animal-based counterparts. The study under review sought to evaluate whether this was the case by comparing the effects of one of the best plant-based proteins, soy protein, to animal-based proteins, in the context of resistance training-induced changes in muscle mass and strength.