Nutrition
Narrative Review
2018
The role of the anabolic properties of plant- versus animal-based protein sources in supporting muscle mass maintenance
By Stefan H.M. Gorissen and Luc J.C. van Loon
Nutrients, 10(10), pp. 1443
<h2>Abstract</h2>
<p>The debate between plant-based and animal-based dietary protein sources for supporting muscle mass maintenance has significant implications for the growing populations of athletes adopting vegetarian or vegan dietary patterns. This review by Gorissen and van Loon (2018) systematically evaluated the anabolic properties of plant versus animal protein sources, examining differences in amino acid profiles, digestibility, and muscle protein synthetic responses.</p>
<p>Animal-derived proteins generally demonstrated superior acute <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> (MPS) responses compared to plant protein counterparts matched for dose, attributable primarily to higher essential amino acid content, greater <a href="/terms/leucine/" class="term-link" data-slug="leucine" title="leucine">leucine</a> concentrations, and superior digestibility and bioavailability [1]. However, the review identified that these differences are not absolute barriers: plant proteins can elicit comparable MPS responses when consumed in higher amounts or when supplemented with leucine to overcome the leucine threshold deficit [2].</p>
<p>The emergence of the Digestible Indispensable Amino Acid Score (DIAAS) as an improved protein quality metric over the traditional PDCAAS (Protein Digestibility Corrected Amino Acid Score) was highlighted, with DIAAS more accurately reflecting the true digestibility of individual amino acids rather than treating protein as a homogeneous substrate. Practical strategies for plant-protein consumers — including dietary variety, dose adjustment, and strategic protein combining — were identified as effective tools for matching animal protein's anabolic efficacy.</p>
<p><strong>Keywords</strong>: plant protein, animal protein, muscle protein synthesis, leucine, DIAAS, protein quality, <a href="/terms/essential-amino-acids/" class="term-link" data-slug="essential-amino-acids" title="essential amino acids">essential amino acids</a>, muscle mass</p>
<h2>Introduction</h2>
<p>The prevalence of plant-based dietary patterns among athletes has increased substantially, driven by a convergence of health, environmental, and ethical motivations. Survey data suggest that between 5-30% of athletes in various sports follow vegetarian or vegan diets, with prevalence highest in endurance and aesthetic sports [1]. This demographic shift has intensified scientific interest in whether plant-derived protein sources can adequately substitute for animal-derived proteins in supporting the muscle mass maintenance and development that is central to athletic performance.</p>
<p>The fundamental challenge facing plant-based protein consumers seeking to maintain or build muscle mass relates to protein quality rather than protein quantity per se. Protein quality encompasses multiple dimensions: the completeness of the essential amino acid (<a href="/terms/essential-amino-acids/" class="term-link" data-slug="essential-amino-acids" title="EAA">EAA</a>) profile, the absolute concentration of key rate-limiting amino acids (particularly <a href="/terms/leucine/" class="term-link" data-slug="leucine" title="leucine">leucine</a>), and the digestibility and bioavailability of the amino acids once consumed [2].</p>
<p>Most plant protein sources are characterized by one or more nutritional limitations relative to animal proteins. These include:
- <strong>Incomplete amino acid profiles</strong>: Many plant proteins are limiting in one or more essential amino acids (e.g., lysine in grains, methionine in legumes)
- <strong>Lower leucine content</strong>: Plant proteins contain approximately 6-8% leucine by weight compared to 8-11% in animal proteins, which has implications for crossing the leucine threshold required to maximally stimulate <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="MPS">MPS</a>
- <strong>Lower digestibility</strong>: The presence of cell walls, fiber, phytates, lectins, and trypsin inhibitors in plant foods reduces the proportion of protein that is ultimately absorbed and available for muscle protein synthesis [3]</p>
<p>The concept of the leucine threshold is particularly important in this context. Research by Norton and Layman established that a minimum leucine dose (approximately 2-3g per meal) is required to maximally activate <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> signaling and initiate a robust MPS response [4]. Because plant proteins are relatively leucine-poor, achieving this threshold requires consuming larger absolute amounts of plant protein per meal, or supplementing individual plant proteins with additional leucine.</p>
<p>Gorissen and van Loon (2018) contextualized this body of evidence within the framework of aging muscle biology and long-term dietary patterns, recognizing that single-meal acute MPS responses may not fully predict the muscle mass outcomes of sustained dietary patterns.</p>
<h2>Evidence Review</h2>
<h3>Amino Acid Profiles: Animal vs. Plant</h3>
<p>A critical comparison of essential amino acid content reveals systematic differences between protein source categories:</p>
<table>
<thead>
<tr>
<th>Protein Source</th>
<th><a href="/terms/leucine/" class="term-link" data-slug="leucine" title="Leucine">Leucine</a> (%)</th>
<th><a href="/terms/branched-chain-amino-acids/" class="term-link" data-slug="branched-chain-amino-acids" title="BCAA">BCAA</a> Total (%)</th>
<th>Lysine (%)</th>
<th>DIAAS Score</th>
</tr>
</thead>
<tbody>
<tr>
<td><a href="/terms/whey-protein/" class="term-link" data-slug="whey-protein" title="Whey protein">Whey protein</a> concentrate</td>
<td>10.0-10.9</td>
<td>21-22</td>
<td>8.5-9.0</td>
<td>~1.09</td>
</tr>
<tr>
<td>Eggs (whole)</td>
<td>8.5</td>
<td>20.5</td>
<td>7.0</td>
<td>~1.13</td>
</tr>
<tr>
<td><a href="/terms/casein/" class="term-link" data-slug="casein" title="Casein">Casein</a></td>
<td>8.6</td>
<td>19.8</td>
<td>7.4</td>
<td>~1.08</td>
</tr>
<tr>
<td>Beef</td>
<td>8.1</td>
<td>19.5</td>
<td>8.2</td>
<td>~0.92</td>
</tr>
<tr>
<td>Soy protein isolate</td>
<td>7.7</td>
<td>18.1</td>
<td>6.0</td>
<td>~0.91</td>
</tr>
<tr>
<td>Pea protein</td>
<td>8.0</td>
<td>18.4</td>
<td>6.9</td>
<td>~0.82</td>
</tr>
<tr>
<td>Rice protein</td>
<td>8.5</td>
<td>19.1</td>
<td>3.3</td>
<td>~0.37</td>
</tr>
<tr>
<td>Wheat gluten</td>
<td>6.8</td>
<td>15.2</td>
<td>1.5</td>
<td>~0.25</td>
</tr>
</tbody>
</table>
<p>The data illustrate that soy protein isolate and pea protein represent the highest-quality plant proteins, with leucine content and DIAAS scores approaching (though not matching) animal protein standards [1].</p>
<h3>Acute <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="MPS">MPS</a> Response Studies</h3>
<p>The most direct comparison of plant and animal protein anabolic potential comes from isotope tracer studies measuring fractional synthetic rates (FSR) of muscle protein after protein ingestion. Tang et al. (2009) demonstrated that whey protein stimulated a greater MPS response than soy protein at an equivalent 20g dose, with soy producing a response superior to casein [2].</p>
<p>However, dose-matching studies revealed that the gap between plant and animal protein can be substantially closed by increasing the plant protein dose. Yang et al. demonstrated that 40g of soy protein produced a comparable MPS response to 20g of whey, consistent with the hypothesis that soy protein's lower leucine content requires approximately double the dose to achieve an equivalent anabolic stimulus [3].</p>
<h3>Digestibility: PDCAAS vs. DIAAS</h3>
<p>The traditional PDCAAS scoring system for protein quality has several known limitations when applied to plant proteins. PDCAAS uses fecal digestibility (total tract) rather than ileal digestibility, overestimating the true absorption of amino acids from plant proteins because bacterial fermentation in the large intestine modifies amino acid profiles after the point of actual absorption [4].</p>
<p>DIAAS corrects for this by using ileal digestibility coefficients for individual amino acids, providing a more accurate reflection of true amino acid bioavailability. Under DIAAS scoring, the quality gap between plant and animal proteins is larger than PDCAAS suggests, particularly for wheat and rice proteins.</p>
<h3>Long-Term Muscle Mass Outcomes</h3>
<p>While acute MPS data paint a clear picture of animal protein superiority at matched doses, longitudinal trials examining muscle mass outcomes paint a more nuanced picture. Systematic reviews of longer-term dietary interventions (8-24 weeks) found that when total protein intake was equated between plant and animal protein conditions, differences in lean mass gains were modest and often non-significant [5].</p>
<p>This disconnect between acute MPS responses and longer-term muscle mass outcomes suggests that adaptations in protein synthesis efficiency, feeding frequency, and dietary complementarity can partially compensate for the lower per-serving anabolic stimulus of plant proteins over time.</p>
<h2>Discussion</h2>
<h3>Bridging the Gap: Can Plant Protein Match Animal Protein?</h3>
<p>The central conclusion of this review is that the anabolic inferiority of plant proteins at equivalent doses is not an immutable physiological barrier but rather a correctable nutritional characteristic. The primary strategies for bridging the protein quality gap include increasing total plant protein intake, optimizing protein distribution, ensuring dietary variety to complement amino acid profiles, and selectively using <a href="/terms/leucine/" class="term-link" data-slug="leucine" title="leucine">leucine</a> supplementation or protein combining [1].</p>
<p>The practical importance of this conclusion is substantial. It means that a well-planned plant-based diet can support muscle maintenance and growth comparably to an animal-based diet, with somewhat greater nutritional intentionality required. The "somewhat greater intentionality" involves approximately 10-20% higher total protein targets, attention to leucine-sufficient meal construction, and strategic food combining.</p>
<h3>The Leucine Leverage Point</h3>
<p>Leucine's unique role as both an essential amino acid and a cellular signal for <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> activation makes it the critical variable in plant protein optimization. The leucine content of a protein meal, more than any other single factor, determines the magnitude and duration of the post-meal <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="MPS">MPS</a> response [2].</p>
<p>Several practical strategies exploit this knowledge:</p>
<ul>
<li><strong>Leucine fortification</strong>: Adding 2-3g of isolated leucine to a plant protein meal can reliably elevate the anabolic response to match that of a leucine-replete animal protein</li>
<li><strong>Soy/pea priority</strong>: Choosing the highest-leucine plant proteins (soy, pea) as primary protein sources minimizes the leucine deficit requiring compensation</li>
<li><strong>Larger plant protein doses</strong>: Consuming 40-50g of plant protein per meal rather than 20-25g may achieve comparable MPS to smaller animal protein doses by ensuring leucine threshold is crossed [3]</li>
</ul>
<h3>Protein Complementation: Modern Perspective</h3>
<p>The traditional concept of protein complementation — combining complementary amino acid profiles within the same meal (e.g., rice with beans) — was based on the observation that plant proteins are individually limiting in different EAAs. Contemporary evidence has refined this understanding: while same-meal complementation is beneficial, combining complementary proteins across the day is also effective, as the body maintains a free amino acid pool that can be drawn upon over longer time windows [4].</p>
<p>However, for maximizing post-meal MPS (as opposed to preventing amino acid deficiency), within-meal complementation offers a more direct benefit by ensuring complete <a href="/terms/essential-amino-acids/" class="term-link" data-slug="essential-amino-acids" title="EAA">EAA</a> availability at the moment of protein synthesis stimulation.</p>
<h3>Soy Protein: Underrated Quality</h3>
<p>Among plant proteins, soy deserves particular attention as the plant protein with the most comprehensive research base and the closest overall amino acid profile to animal protein. Concerns about soy's phytoestrogen content adversely affecting testosterone or muscle development in men have not been substantiated in the literature at normal dietary intakes [5].</p>
<p>Soy protein isolate (90%+ protein) provides near-complete EAA coverage, a DIAAS of approximately 0.91, and a leucine content comparable to <a href="/terms/casein/" class="term-link" data-slug="casein" title="casein">casein</a>. For plant-based athletes, soy protein represents the single most effective animal protein substitute from a muscle building perspective.</p>
<h3>Practical Equivalence in Real-World Diets</h3>
<p>The longitudinal evidence suggesting that muscle mass differences between plant and animal protein diets are modest when protein is equated reflects the reality that most well-informed plant-based athletes naturally employ complementation, variety, and dose adjustment strategies. The gap in acute MPS studies performed at single matched doses may overestimate real-world differences observed when individuals consume varied diets over months [6].</p>
<h2>Practical Recommendations</h2>
<h3>Total Protein Targets for Plant-Based Athletes</h3>
<ul>
<li><strong>Omnivorous baseline</strong>: 1.6-2.2g/kg body weight daily</li>
<li><strong>Plant-based adjustment</strong>: Target 1.8-2.4g/kg body weight daily (10-20% higher to account for lower digestibility and <a href="/terms/leucine/" class="term-link" data-slug="leucine" title="leucine">leucine</a> content)</li>
<li><strong>During caloric restriction</strong>: Up to 2.6-3.0g/kg to protect lean mass</li>
</ul>
<p>The additional protein target compensates for the lower DIAAS of plant proteins and ensures that leucine intake across the day is sufficient to repeatedly stimulate <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="MPS">MPS</a>.</p>
<h3>Best Plant Protein Sources</h3>
<table>
<thead>
<tr>
<th>Source</th>
<th>Protein/100g</th>
<th>Leucine Content</th>
<th>Quality Rating</th>
<th>Best Use</th>
</tr>
</thead>
<tbody>
<tr>
<td>Soy protein isolate</td>
<td>90g</td>
<td>High</td>
<td>Excellent</td>
<td>Shakes, cooking</td>
</tr>
<tr>
<td>Edamame / Tempeh</td>
<td>17-19g</td>
<td>Moderate-high</td>
<td>Very good</td>
<td>Meals</td>
</tr>
<tr>
<td>Pea protein isolate</td>
<td>80g</td>
<td>High</td>
<td>Good</td>
<td>Shakes, baking</td>
</tr>
<tr>
<td>Tofu (firm)</td>
<td>17g</td>
<td>Moderate</td>
<td>Good</td>
<td>Meals</td>
</tr>
<tr>
<td>Lentils (cooked)</td>
<td>9g</td>
<td>Moderate</td>
<td>Moderate</td>
<td>Complemented with grains</td>
</tr>
<tr>
<td>Chickpeas</td>
<td>9g</td>
<td>Moderate</td>
<td>Moderate</td>
<td>Complemented with grains</td>
</tr>
<tr>
<td>Rice protein</td>
<td>75g</td>
<td>Moderate</td>
<td>Fair (low lysine)</td>
<td>Combined with pea protein</td>
</tr>
<tr>
<td>Hemp protein</td>
<td>50g</td>
<td>Moderate</td>
<td>Good (complete)</td>
<td>Shakes</td>
</tr>
<tr>
<td>Seitan (wheat gluten)</td>
<td>25g</td>
<td>Lower</td>
<td>Poor</td>
<td>Meals (pair with legumes)</td>
</tr>
</tbody>
</table>
<h3>Strategic Protein Combining</h3>
<p>For meals using grain-based proteins (rice, wheat), combine with legumes to complete the amino acid profile:</p>
<ul>
<li><strong>Rice + Pea protein</strong>: The combination achieves an amino acid profile approaching <a href="/terms/whey-protein/" class="term-link" data-slug="whey-protein" title="whey protein">whey protein</a> [1]</li>
<li><strong>Brown rice + Lentils/beans</strong>: Traditional complementation pattern; effective for daily amino acid adequacy</li>
<li><strong>Seitan + Chickpeas</strong>: Corrects methionine excess and lysine deficit</li>
</ul>
<h3>Leucine Optimization Strategies</h3>
<ul>
<li><strong>Per-meal leucine target</strong>: Aim for 2.5-3g of leucine per protein meal</li>
<li><strong>Higher plant protein doses</strong>: 40-50g per meal rather than 20-25g when using lower-leucine sources like rice or hemp</li>
<li><strong>Leucine supplement</strong>: 2-3g of isolated leucine powder added to plant protein shakes can reliably boost the anabolic response of lower-quality plant proteins</li>
<li><strong>Prioritize soy and pea</strong>: These naturally provide higher leucine content, reducing the dose adjustment needed</li>
</ul>
<h3>Practical Meal Design for Plant-Based Athletes</h3>
<p>A 75kg plant-based athlete targeting 2.0g/kg (150g protein/day) across 4 meals:</p>
<ul>
<li><strong>Meal 1</strong>: 50g pea protein shake + soy milk (provides ~50g protein, ~3.5g leucine)</li>
<li><strong>Meal 2</strong>: Tempeh stir-fry with edamame and brown rice (provides ~40g protein)</li>
<li><strong>Meal 3</strong>: Lentil dal with quinoa and tofu (provides ~35g protein)</li>
<li><strong>Meal 4</strong>: 40g soy protein isolate shake + almond butter (provides ~45g protein)</li>
</ul>
<p>This pattern achieves ~170g daily protein with strategic leucine distribution across meals [2].</p>