Nutrition has an irrefutable role in support of athletic training. Dependent on many factors Figure 1 below, sports and exercise nutritionists (SENs) can determine optimal nutrient strategies to support a specific training stimulus or competitive event. In the context of resistance training (RT) the beneficial role of protein in facilitating the accrual of lean tissue mass (LTM) is unequivocal in the extant literature (Phillips, 2005, Slater and Phillips, 2011, Camera et al., 2015, Damas et al., 2015, Koopman, 2009, Cermak et al., 2013, Cermak et al., 2012, Tieland et al., 2012, Tipton, 2004). There is a lack of consensus, however, regarding the nuances of optimal protein ingestion. Research is ongoing to determine the most potent protein source, quantity, quality, and timing (relative to training and apportioning daily) in supporting lean mass accrual.
Due to recent interest in clean eating and sustainable nutrition, and increased consumer demand, an emerging trend in performance nutrition research investigates alternative sources to more traditional protein stocks (Linneman and Dijkstra 2002; Churchward-Venne et al. 2017). Meat, fish, chicken and dairy are still the main stay of protein-providing foods among Irish adults (IUNA, 2011), with dairy derived sport supplements favoured in the recovery phase from RT (Slater and Phillips, 2011). However, alternatives including plant and even insect sources are suggested as unconventional substitutes. This blog will investigate how these plant protein sources compare in the context of supporting RT and the accrual of LTM. Will these alternative protein sources be only favoured by the margins, or is there potential for these sources to become commonplace?
Proteins foods are not created equal. They vary according to their source (plant or animal), their quality and their digestibility. Proteins of high quality are those that are readily digestible when classified by digestible indispensable amino acid score DIAAS) (Schaafsma, 2005, Mathai et al., 2017, Leser, 2013), and which contain an amino acid profile required to promote or maintain health in humans (Millward 2007).
In the context of MPS, many researchers have conducted studies on amino acids to determine which may have the more potent effects. Volpi et al. conducted experiments investigating muscle anabolism and catabolism, and amino acids in both young and elderly subjects (Volpi, 2003, Volpi, 2013, Volpi, 1999). They concluded that it is the essential amino acids (EAA) that are primarily responsible for the stimulation of muscle protein anabolism in healthy adults. This has been corroborated by others (Biola 1997, Tipton et al., 1999), whereas non-essential amino acids appear ineffective in this regard (Smith, 1998). Among EAAs, branched chain amino acids and in particular leucine, are the most efficient for protein synthesis stimulation (Kirby et al., 2012, Churchward-Venne et al., 2014, Proud, 2004). It has been demonstrated that leucine alone is able to stimulate muscle protein synthesis to the same extent as a comparable does of all other EAAs (Anthony et al. 2002a, b; Dardevet et al. 2000; Lynch et al. 2002; Crozier et al. 2005). Amino acid supplements without adequate leucine reportedly do not stimulate protein synthesis (Rieu et al, 2007, Hayes 2008, Katsanos, 2006). The ability of leucine to influence MPS can be part explained by the fact that leucine has a dual role in lean tissue mass anabolism, both as a substrate and a regulator through activation of the mammalian target of rapamycin (mTOR) pathway (Crozier, 2005, Atherton et al, 2010 and Anthony 2000).
Cognisant of the role of EAAs, BCAAs and leucine in regulating MPS, an examination of relative EAA and leucine content in both traditional and novel protein sources is necessary. van Vliet et al. (2015) reviewed skeletal muscle anabolic responses to plant-versus animal-based protein consumption (Figure 2 EAA and leucine content (as % of total protein) for proposed alternatives. In order that an alternative be consider a viable substitute for dairy based proteins in support of muscle mass accrual with resistance training, it should have a high relative EAA and leucine content. With the exception of quinoa, soy and hemp, plant sources of protein will always be limited when considered in isolation, as plant protein sources are deficient in one or more amino acids, usually methionine or lysine (e.g. wheat is low in lysine yet high in methionine, whereas lentil is high in lysine, yet low in methionine). This is referred to as the limiting / absent EAA. These plant based protein sources also contain lower quantities of leucine.
While no single plant source contains EAAs or leucine in the same quantity as is found in animal sources, and in particular whey protein, van Vliet et al. (2015) hypothesises on the merit in combining different plant sources to formulate a meal or eating occasion that provides the complement of EAAs to optimally stimulate MPS. It may be possible to overcome the absence of all EAAs in plant proteins by combining different plant sources. Examples (Figure 3 below) include the addition of lentil to quinoa (A) , lentil and soy (B) pea and wheat (C), or lentil and quinoa (D). Many recipes for suitable dishes are available from vegetarian cookbooks e.g. Anita Bean’s Vegetarian Athlete.
A more controversial consideration is insects as potential sources of dietary protein. Entomophagy, i.e., the consumption of insects, is traditionally practiced in many parts of the world. A compilation of nutrient compositions of edible insects in addition to amino acid of edible insects was published by Rumpold and Schlüter (2013). Protein represents the main nutrient constituent of insects, with cockroaches, grasshoppers, crickets and locusts (Orthoptera species) topping the poll with protein content amounting to circa. 60 – 80%. The quality of insect proteins in comparison to plant or animal sources is yet to be assessed in feeding trials; however, the amino acid profile of the Orthoptera species looks promising (Churchward-Venne et al. 2017)!
Future research should investigate the response of MPS from animal and plant-derived protein sources, or blends of plant-derived proteins (Joy et al., 2013). To date, few acute human metabolic studies have compared other animal-derived protein-rich foods, such as eggs, yoghurts, meat and fish with other plant-derived protein-rich foods, such as lentil, maize, pea, rice and wheat. These findings would be pertinent for food industry and may facilitate the production of protein-based products that are economic, environmentally sustainable, but competitive functionally with traditional dairy-based products (Macdiarmid et al., 2012). The acceptability of edible insects and the economic and environmental implications are also worthy of further investigation.
Dr Catherine Norton is a performance nutritionist and registered dietitian. Dr Norton works at the University of Limerick, Human Research Institute. She teaches on the BSc Sport & Exercise Sciences and MSc Sports Performance courses as well as researching in both food for health and performance nutrition. Contact Catherine via email on firstname.lastname@example.org or follow Catherine on Twitter